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diff --git a/18779.txt b/18779.txt new file mode 100644 index 0000000..fd37bb1 --- /dev/null +++ b/18779.txt @@ -0,0 +1,16298 @@ +The Project Gutenberg EBook of Physiology and Hygiene for Secondary +Schools by Francis M. Walters, A.M. + + + +This eBook is for the use of anyone anywhere at no cost and with almost no +restrictions whatsoever. You may copy it, give it away or re-use it under +the terms of the Project Gutenberg License included with this eBook or +online at http://www.gutenberg.org/license + + + +Title: Physiology and Hygiene for Secondary Schools + +Author: Francis M. Walters, A.M. + +Release Date: November 15, 2005 [Ebook #18779] + +Language: English + +Character set encoding: US-ASCII + + +***START OF THE PROJECT GUTENBERG EBOOK PHYSIOLOGY AND HYGIENE FOR SECONDARY SCHOOLS*** + + + + + +Physiology and Hygiene for Secondary Schools + + +by Francis M. Walters, A.M. + + + + +Edition 1, (November 15, 2005) + + + + + + D.C. Heath and Co. - Publishers + + Original copyright 1909 + + + "It is quite possible to give instruction in this subject in such + a manner as not only to confer knowledge which is useful in + itself, but to serve the purpose of a training in accurate + observation, and in the methods of reasoning of physical + science."--_Huxley._ + + + + + +PREFACE + + +The aim in the preparation of this treatise on the human body has been, +first, to set forth in a _teachable_ manner the actual science of +physiology; and second, to present the facts of hygiene largely as +_applied physiology_. The view is held that "right living" consists in the +harmonious adjustment of one's habits to the nature and plan of the body, +and that the best preparation for such living is a correct understanding +of the physical self. It is further held that the emphasizing of +physiology augments in no small degree the educative value of the subject, +greater opportunity being thus afforded for exercise of the reasoning +powers and for drill in the _modus operandi_ of natural forces. In the +study of physiology the facts of anatomy have a place, but in an +elementary course these should be restricted to such as are necessary for +revealing the general structure of the body. + +Although no effort has been spared to bring this work within the +comprehension of the pupil, its success in the classroom will depend +largely upon the method of handling the subject by the teacher. It is +recommended, therefore, that the _relations_ which the different organs +and processes sustain to each other, and to the body as a whole, be given +special prominence. The pupil should be impressed with the essential unity +of the body and should see in the diversity of its activities the serving +of a common purpose. In creating such an impression the introductory +paragraphs at the beginning of many of the chapters and the summaries +throughout the book, as well as the general arrangement of the +subject-matter, will be found helpful. + +Since the custom largely prevails of teaching physiology in advance of the +sciences upon which it rests--biology, physics, and chemistry--care should +be exercised to develop correct ideas of the principles and processes +derived from these sciences. Too much latitude has been taken in the past +in the use of comparisons and illustrations drawn from "everyday life." To +teach that the body is a "house," "machine," or "city"; that the nerves +carry "messages"; that the purpose of oxygen is to "burn up waste"; that +breathing is to "purify the blood," etc., may give the pupil phrases which +he can readily repeat, but teaching of this kind does not give him correct +ideas of his body. + +The method of teaching, however, that uses the pupil's experience as a +basis upon which to build has a value not to be overlooked. The fact that +such expressions as those quoted above are so easily remembered proves the +value of connecting new knowledge with the pupil's experience. But _the +inadequacy of this experience must be recognized_ and taken into account. +The concepts of the average pupil are entirely too indefinite and limited +to supply the necessary _foundation for a science_ such as physiology. +Herein lies the great value of experiments and observations. They +supplement the pupil's experience, and increase both the number and +definiteness of his concepts. No degree of success can be attained if this +phase of the study is omitted. + +The best results in physiology teaching are of course attained where +laboratory work is carried on by the pupils, but where this cannot be +arranged, class experiments and observations must suffice. The Practical +Work described at the close of most of the chapters is mainly for class +purposes. While these serve a necessary part in the development of the +subject, it is not essential that all of the experiments and observations +be made, the intention being to provide for some choice on the part of the +teacher. A note-book should be kept by the pupil. + +To adapt the book to as wide a range of usefulness as possible, more +subject-matter is introduced than is usually included in an elementary +course. Such portions, however, as are unessential to a proper +understanding of the body by the pupil are set in small type, to be used +at the discretion of the teacher. + +The use of books of reference is earnestly recommended. For this purpose +the usual high school texts may be employed to good advantage. A few more +advanced works should, however, be frequently consulted. For this purpose +Martin's _Human Body_ (Advanced Course), Rettger's _Advanced Lessons in +Physiology_, Thornton's _Human Physiology_, Huxley's _Lessons in +Elementary Physiology_, Howell's _A Text-book of Physiology_, Hough and +Sedgwick's _Hygiene and Sanitation_, and Pyle's _Personal Hygiene_ will be +found serviceable. + +In the preparation of this work valuable assistance has been rendered by +Dr. C.N. McAllister, Department of Psychology, and by Professor B.M. +Stigall, Department of Biology, along the lines of their respective +specialties, and in a more general way by President W.J. Hawkins and +others of the Warrensburg, Missouri, State Normal School. Expert advice +from Professor S.D. Magers, Instructor in Physiology and Bacteriology, +State Normal School, Ypsilanti, Michigan, has been especially helpful, and +many practical suggestions from the high school teachers of physiology of +Kansas City, Missouri, Professor C.H. Nowlin, Central High School, Dr. +John W. Scott, Westport High School, and Professor A.E. Shirling, Manual +Training High School, all of whom read both manuscript and proofs, have +been incorporated. Considerable material for the Practical Work, including +the respiration experiment (page 101) and the reaction time experiment +(page 323), were contributed by Dr. Scott. Professor Nowlin's suggestions +on subject-matter and methods of presentation deserve special mention. To +these and many others the author makes grateful acknowledgment. + + F.M.W. + +MISSOURI STATE NORMAL SCHOOL, +SECOND DISTRICT, May 1, 1909. + + + + + +CONTENTS + + +Preface +Contents +PART I: THE VITAL PROCESSES + CHAPTER I - INTRODUCTION + CHAPTER II - GENERAL VIEW OF THE BODY + CHAPTER III - THE BODY ORGANIZATION + CHAPTER IV - THE BLOOD + CHAPTER V - THE CIRCULATION + CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY + CHAPTER VII - RESPIRATION + CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY + CHAPTER IX - FOODS AND THE THEORY OF DIGESTION + CHAPTER X - ORGANS AND PROCESSES OF DIGESTION + CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION + CHAPTER XII - ENERGY SUPPLY OF THE BODY + CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION +PART II: MOTION, COORDINATION, AND SENSATION + CHAPTER XIV - THE SKELETON + CHAPTER XV - THE MUSCULAR SYSTEM + CHAPTER XVI - THE SKIN + CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM + CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM + CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM + CHAPTER XX - PRODUCTION OF SENSATIONS + CHAPTER XXI - THE LARYNX AND THE EAR + CHAPTER XXII - THE EYE + CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL +APPENDIX +INDEX + + + + + + +PHYSIOLOGY AND HYGIENE + + + + + +PART I: THE VITAL PROCESSES + + + + +CHAPTER I - INTRODUCTION + + +To derive strength equal to the daily task; to experience the advantages +of health and avoid the pain, inconvenience, and danger of disease; to +live out contentedly and usefully the natural span of life: these are +problems that concern all people. They are, however, but different phases +of one great problem--the problem of properly managing or caring for the +body. To supply knowledge necessary to the solution of this problem is the +chief reason why the body is studied in our public schools. + +*Divisions of the Subject.*--The body is studied from three standpoints: +structure, use of parts, and care or management. This causes the main +subject to be considered under three heads, known as anatomy, physiology, +and hygiene. + +_Anatomy_ treats of the construction of the body--the parts which compose +it, what they are like, and where located. Its main divisions are known as +gross anatomy and histology. _Gross anatomy_ treats of the larger +structures of the body, while _histology_ treats of the minute structures +of which these are composed--parts too small to be seen with the naked eye +and which have to be studied with the aid of the microscope. + +_Physiology_ treats of the function, or use, of the different parts of the +body--the work which the parts do and how they do it--and of their relations +to one another and to the body as a whole. + +_Hygiene_ treats of the proper care or management of the body. In a +somewhat narrower sense it treats of the "laws of health." Hygiene is said +to be _personal_, when applied by the individual to his own body; +_domestic_, when applied to a small group of people, as the family; and +public, or _general_, when applied to the community as a whole or to the +race. + +*The General Aim of Hygiene.*--There are many so-called laws of health, and +for these laws it is essential in the management of the body to find a +common basis. This basic law, suggested by the nature of the body and +conditions that affect its well-being, may be termed the _Law of Harmony: +The mode of living must harmonize with the plan of the body_. To live +properly one must supply the conditions which his body, on account of its +nature and plan, requires. On the other hand, he must avoid those things +and conditions which are injurious, _i.e._, out of harmony with the body +plan. To secure these results, it is necessary to determine what is and +what is not in harmony with the plan of the body, and to find the means of +applying this knowledge to the everyday problems of living. Such is the +general aim of hygiene. Stated in other words: Hygiene has for its general +aim the bringing about of an essential harmony between the body and the +things and conditions that affect it.(1) + +*Relation of Anatomy and Physiology to the Study of Hygiene.*--If the chief +object in studying the body is that of learning how to manage or care for +it, and hygiene supplies this information, why must we also study anatomy +and physiology? The answer to this question has already been in part +suggested. In order to determine what things and conditions are in harmony +with the plan of the body, we must know what that plan is. This knowledge +is obtained through a study of anatomy and physiology. The knowledge +gained through these subjects also renders the study of hygiene more +interesting and valuable. One is enabled to see _why_ and _how_ obedience +to hygienic laws benefits, and disobedience to them injures, the body. +This causes the teachings of hygiene to be taken more seriously and +renders them more practical. In short, anatomy and physiology supply a +necessary basis for the study of hygiene. + +*Advantages of Properly Managing the Body.*--One result following the +mismanagement of the body is loss of health. But attending the loss of +health are other results which are equally serious and far-reaching. +Without good health, people fail to accomplish their aims and ambitions in +life; they miss the joy of living; they lose their ability to work and +become burdens on their friends or society. The proper management of the +body means health, and it also means the capacity for work and for +enjoyment. Not only should one seek to preserve his health from day to +day, but he should so manage his body as to use his powers to the best +advantage and prolong as far as possible the period during which he may be +a capable and useful citizen. + + + + +CHAPTER II - GENERAL VIEW OF THE BODY + + +*External Divisions.*--Examined from the outside, the body presents certain +parts, or divisions, familiar to all. The main, or central, portion is +known as the _trunk_, and to this are attached the _head_, the _upper +extremities_, and the _lower extremities_. These in turn present smaller +divisions which are also familiar. The upper part of the trunk is known as +the _thorax_, or chest, and the lower part as the _abdomen_. The portions +of the trunk to which the arms are attached are the _shoulders_, and those +to which the legs are joined are the _hips_, while the central rear +portion between the neck and the hips is the _back_. The fingers, the +hand, the wrist, the forearm, the elbow, and the upper arm are the main +divisions of each of the upper extremities. The toes, the foot, the ankle, +the lower leg, the knee, and the thigh are the chief divisions of each of +the lower extremities. The head, which is joined to the trunk by the neck, +has such interesting parts as the eyes, the ears, the nose, the jaws, the +cheeks, and the mouth. The entire body is inclosed in a double covering, +called the _skin_, which protects it in various ways. + +*The Tissues.*--After examining the external features of the body, we +naturally inquire about its internal structures. These are not so easily +investigated, and much which is of interest to advanced students must be +omitted from an elementary course. We may, however, as a first step in +this study, determine what kinds of materials enter into the construction +of the body. For this purpose the body of some small animal should be +dissected and studied. (See observation at close of chapter.) The +different materials found by such a dissection correspond closely to the +substances, called _tissues_, which make up the human body. The main +tissues of the body, as ordinarily named, are the _muscular_ tissue, the +_osseous_ tissue, the _connective_ tissue, the _nervous_ tissue, the +_adipose_ tissue, the _cartilaginous_ tissue, and the _epithelial_ and +_glandular_ tissue. Most of these present different varieties, making all +together some fifteen different kinds of tissues that enter into the +construction of the body.(2) + +*General Purposes of the Tissues.*--The tissues, first of all, _form the +body_. As a house is constructed of wood, stone, plaster, iron, and other +building materials, so is the body made up of its various tissues. For +this reason the tissues have been called the _building materials_ of the +body. + +In addition to forming the body, the tissues supply the means through +which its work is carried on. They are thus the _working materials_ of the +body. In serving this purpose the tissues play an active role. All of them +must perform the activities of growth and repair, and certain ones (the +so-called active tissues) must do work which benefits the body as a whole. + +*Purposes of the Different Tissues.*--In the construction of the body and +also in the work which it carries on, the different tissues are made to +serve different purposes. The osseous tissue is the chief substance in the +bony framework, or skeleton, while the muscular tissue produces the +different movements of the body. The connective tissue, which is +everywhere abundant, serves the general purpose of connecting the +different parts together. Cartilaginous tissue forms smooth coverings over +the ends of the bones and, in addition to this, supplies the necessary +stiffness in organs like the larynx and the ear. The nervous tissue +controls the body and brings it into proper relations with its +surroundings, while the epithelial tissue (found upon the body surfaces +and in the glands) supplies it with protective coverings and secretes +liquids. The adipose tissue (fat) prevents the too rapid escape of heat +from the body, supplies it with nourishment in time of need, and forms +soft pads for delicate organs like the eyeball. + +*Properties of the Tissues.*--If we inquire how the tissues are able to +serve such widely different purposes, we find this answer. The tissues +differ from one another both in composition and in structure and, on this +account, differ in their properties.(3) Their different properties enable +them to serve different purposes in the body. Somewhat as glass is adapted +by its transparency, hardness, and toughness to the use made of it in +windows, the special properties of the tissues adapt them to the kinds of +service which they perform. Properties that adapt tissues to their work in +the body are called _essential_ properties. The most important of these +essential properties are as follows: + +1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular +tissue, contractility and irritability. 3. Of nervous tissue, irritability +and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5. +Of connective tissue, toughness and pliability. 6. Of epithelial tissue, +ability to resist the action of external forces and power to secrete. + + [Fig. 1] + + + Fig. 1--Hand and forearm, showing the grouping of muscular and connective + tissues in the organ for grasping. + + +*Tissue Groups.*--In the construction of the body the tissues are grouped +together to form its various divisions or parts. A group of tissues which +serves some special purpose is known as an _organ_. The hand, for example, +is an organ for grasping (Fig. 1). While the different organs of the body +do not always contain the same tissues, and never contain them in the same +proportions, they do contain such tissues as their work requires and these +have a special arrangement--one adapted to the work which the organs +perform. + +In addition to forming the organs, the tissues are also grouped in such a +manner as to provide supports for organs and to form cavities in which +organs are placed. The various cavities of the body are of particular +interest and importance. The three largest ones are the _cranial_ cavity, +containing the brain; the _thoracic_ cavity, containing the heart and the +lungs; and the _abdominal_ cavity, containing the stomach, the liver, the +intestines, and other important organs (Fig. 2). Smaller cavities serving +different purposes are also found. + + [Fig. 2] + + + Fig. 2--Diagram of a lengthwise section of the body to show its large + cavities and the organs which they contain. + + +*Organs and Systems.*--The work of the body is carried on by its various +organs. Many, in fact the majority, of these organs serve more than one +purpose. The tongue is used in talking, in masticating the food, and in +swallowing. The nose serves at least three distinct purposes. The mouth, +the arms, the hands, the feet, the legs, the liver, the lungs, and the +stomach are also organs that serve more than one purpose. This introduces +the principle of economy into the construction of the body and diminishes +the number of organs that would otherwise be required. + +The various organs also _combine_ with one another in carrying on the work +of the body. An illustration of this is seen in the digestion of the +food--a process which requires the combined action of the mouth, stomach, +liver, intestines, and other organs. A number of organs working together +for the same purpose form a _system_. The chief systems of the body are +the digestive system, the circulatory system, the respiratory system, the +muscular system, and the nervous system. + +*The Organ and its Work.*--A most interesting question relating to the work +of the organ is this: Does the organ work for its own benefit or for the +benefit of the body as a whole? Does the hand, for example, grasp for +itself or in order that the entire body may come into possession? Only +slight study is sufficient to reveal the fact that each organ performs a +work which benefits the body as a whole. In other words, just as the organ +itself is a part of the body, the work which it does is a part of the +necessary work which the body has to do. + +But in working for the general good, or for the body as a whole, each +organ becomes a sharer in the benefits of the work done by every other +organ. While the hand receives only a little of the nourishment contained +in the food which it places in the mouth or of the heat from, fuel which +it places on the fire, it is aided and supported by the work of all the +other organs of the body--eyes, feet, brain, heart, etc. The hand does not +and cannot work independently of the other organs. It is one of the +partners in a very close combination where, by doing a particular work, +it, shares in the profits of all. What is true of the hand is true of +every other organ of the body. + +*An Organization.*--The relations which the different organs sustain to +each other and to the body as a whole suggest the possibility of +classifying the body as an organization. This term is broadly applied to a +variety of combinations. An organization is properly defined as _any group +of individuals which, in working together for a common purpose, practices +the division of labor_. This definition will be better understood by +considering a few familiar examples. + +A baseball team is an organization. The team is made up of individual +players. These work together for the common purpose of winning games. They +practice the division of labor in that the different players do different +things--one catching, another pitching, and so on. A manufacturing +establishment which employs several workmen may also be an organization. +The article manufactured provides the common purpose toward which all +strive; and, in the assignment of different kinds of work to the +individual workmen, the principle of division of labor is carried out. For +the same reason a school, a railway system, an army, and a political party +are organizations. + +An organization of a lower order of individuals than these human +organizations is to be found in a hive of bees. This is made up of the +individual bees, and these, in carrying on the general work of the hive, +are known to practice the division of labor. + +*Is the Body an Organization*?--If the body is an organization, it must +fulfill the conditions of the definition. It must be made up of separate +or individual parts. These must work together for the same general +purpose, and, in the accomplishment of this purpose, must practice the +division of labor. That the body practices the division of labor is seen +in the related work of the different organs. That it is made up of minute, +but individual, parts will be shown in the chapter following. That it +carries on a _general work_ which is accomplished through the combined +action of its individual parts is revealed through an extended study of +its various activities. _The body is an organization._ Moreover, it is one +of the most complex and, at the same time, most perfect of the +organizations of which we have knowledge. + +*Summary.*--Viewed from the outside, the body is seen to be made up of +divisions which are more or less familiar. Viewed internally, it is found +to consist of different kinds of materials, called tissues. The tissues +are adapted, by their properties, to different purposes both in the +construction of the body and in carrying on its work. The working parts of +the body are called organs and these in their work combine to form +systems. The entire body, on account of the method of its construction and +the character of its work, may be classed as an organization. + +*Exercises.*--1. Name and locate the chief external divisions of the body. + +2. What tissues may be found by dissecting the leg of a chicken? + +3. Name the most important properties and the most important uses of +muscular tissue, osseous tissue, and connective tissue. + +4. Define an organ. Define a system. Name examples of each. + +5. Name the chief cavities of the body and the organs which they contain. + +6. What tissues are present in the hand? How does each of these aid in the +work of the hand? + +7. Define an organization. Show that a railway system, an army, and a +school are organizations. + +8. What is meant by the phrase "division of labor"? In what manner is the +division of labor practiced in a shoe or watch factory? What are the +advantages? + +9. What are the proofs that the body is an organization? + + + +PRACTICAL WORK + + +*Observation on the Tissues.*--Examine with care the structures in the +entire leg of a chicken, squirrel, rabbit, or other small animal used for +food. Observe, first of all, the external covering, consisting of cuticle +and hair, claws, scales, or feathers, according to the specimen. These are +similar in structure, and they form the epidermis, which is one kind of +_epithelial_ tissue. With a sharp knife lay open the skin and observe that +it is attached to the parts underneath by thin, but tough, threads and +sheaths. These represent a variety of _connective_ tissue. The reddish +material which forms the greater portion of the specimen is a variety of +_muscular_ tissue, and its divisions are called muscles. With a blunt +instrument, separate the muscles, by tearing apart the connective tissue +binding them together, and find the glistening white strips of connective +tissue (tendons) which attach them to the bones. Find near the central +part of the leg a soft, white cord (a nerve) which represents one variety +of _nervous_ tissue. The bones, which may now be examined, form the +_osseous_ tissue. At the ends of the bones will be found a layer of +smooth, white material which represents one kind of _cartilaginous_ +tissue. The _adipose_, or fatty, tissue, which is found under the skin and +between the other tissues, is easily recognized. + +*Relation of the Tissues to the Organs.*--Observe in the specimen just +studied the relation of the different tissues to the organ as a whole +(regarding the leg as an organ), _i.e._, show how each of the tissues aids +in the work which the organ accomplishes. Show in particular how the +muscles supply the foot with motion, by tracing out the tendons that +connect them with the toes. Pull on the different tendons, noting the +effect upon the different parts of the foot. + + + + +CHAPTER III - THE BODY ORGANIZATION + + +What is the nature of the body organization? What are the individual +parts, or units, that make it up? What general work do these carry on and +upon what basis do they practice the division of labor? The answers to +these questions will suggest the main problems in the study of the body. + + [Fig. 3] + + + Fig. 3--Diagram showing the relation of the cells and the intercellular + material. _C._ Cells. _I._ Intercellular material. + + +*Complex Nature of the Tissues.*--To the unaided eye the tissues have the +appearance of simple structures. The microscope, however, shows just the +reverse to be true. When any one of the tissues is suitably prepared and +carefully examined with this instrument, at least two classes of materials +can be made out. One of these consists of minute particles, called +_cells_; the other is a substance lying between the cells, known as the +_intercellular material_ (Fig. 3). The cells and the intercellular +material, though varying in their relative proportions, are present in all +the tissues. + +*The Body a Cell Group.*--The biologist has found that the bodies of all +living things, plants as well as animals, consist either of single cells +or of groups of cells. The single cells live independently of one another, +but the cells that form groups are attached to, and are more or less +dependent upon, one another. In the first condition are found the very +lowest forms of life. In the second, life reaches its greatest +development. The body of man, which represents the highest type of life, +is recognized as a group of cells. In this group each cell is usually +separate and distinct from the others, but is attached to them, and is +held in place by the intercellular material. + +*Protoplasm, the Cell Substance.*--The cell is properly regarded as an +_organized_ bit of a peculiar material, called _protoplasm_. This is a +semi-liquid and somewhat granular substance which resembles in appearance +the white of a raw egg. Its true nature and composition are unknown, +because any attempt to analyze it kills it, and dead protoplasm is +essentially different from living protoplasm. It is known, however, to be +a highly complex substance and to undergo chemical change readily. It +appears to be the only kind of matter with which life is ever associated, +and for this reason protoplasm is called the _physical basis of life_. Its +organization into separate bits, or cells, is necessary to the life +activities that take place within it. + +*Structure of the Cell.*--Though all portions of the cell are formed from +the protoplasm, this essential substance differs both in structure and in +function at different places in the cell. For this reason the cell is +looked upon as a complex body having several distinct parts. At or near +the center is a clear, rounded body, called the _nucleus_. This plays some +part in the nourishment of the cell and also in the formation of new +cells. If it be absent, as is sometimes the case, the cell is short-lived +and unable to reproduce itself. The variety of protoplasm contained in the +nucleus is called the _nucleoplasm_. + + [Fig. 4] + + +Fig. 4--Diagram of a typical cell (after Wilson). 1. Main body. 2. Nucleus. +3. Attraction sphere. 4. Food particles and waste. 5. Cell-wall. 6. Masses + of active material found in certain cells, called plastids. + + +Surrounding the nucleus is the _main body_ of the cell, sometimes referred +to as the "protoplasm." Since the protoplasm forms all parts of the cell, +this substance is more properly called the _cytoplasm_, or cell plasm. +Surrounding and inclosing the cytoplasm, in many cells, is a thin outer +layer, or membrane, which affords more or less protection to the contents +of the cell. This is usually referred to as the _cell-wall_. A fourth part +of the cell is also described, being called the _attraction sphere_. This +is a small body lying near the nucleus and cooeperating with that body in +the formation of new cells. Food particles, wastes, and other substances +may also be present in the cytoplasm. The parts of a typical cell are +shown in Fig. 4. + +*Importance of the Cells.*--The cells must be regarded as the living, +working parts of the body. They are the active agents in all of the +tissues, enabling them to serve their various purposes. Working through +the tissues, they build up the body and carry on its different activities. +They are recognized on this account as _the units of structure and of +function_, and are the "individuals" in the body organization. Among the +most important and interesting of the activities of the cells are those by +which they build up the body, or cause it to grow. + +*How the Cells enable the Body to Grow.*--Every cell is able to take new +material into itself and to add this to the protoplasm. This tends to +increase the amount of the protoplasm, thereby causing the cells to +increase in size. A general increase in the size of the cells has the +effect of increasing the size of the entire body, and this is one way by +which they cause it to grow. There is, however, a fixed limit, varying +with different cells, to the size which they attain, and this is quite +low. (The largest cells are scarcely visible to the naked eye.) Any marked +increase in the size of the body must, therefore, be brought about by +other means. Such a means is found in the formation of new cells, or _cell +reproduction_. The new cells are always formed _by_ and _from_ the old +cells, the essential process being known as _cell-division_. + + [Fig. 5] + + +Fig. 5--Steps in cell-division (after Wilson). Note that the process begins +with the division of the attraction sphere, then involves the nucleus, and + finally separates the main body. + + +*Cell-Division.*--By dividing, a single cell will, on attaining its growth, +separate into two or more new cells. The process is quite complex and is +imperfectly understood. It is known, however, that the act of separation +is preceded by a series of changes in which the attraction sphere and the +nucleus actively participate, and that, as a result of these changes, the +contents of the old cell are rearranged to form the new cells. Some of the +different stages in the process, as they have been studied under the +microscope, are indicated in Fig. 5. + +Gradually, through the formation of new cells and by the growth of these +cells after they have been formed, the body attains its full size. When +growth is complete, cell reproduction is supposed to cease except where +the tissues are injured, as in the breaking of a bone, or where cells, +like those at the surface of the skin, are subject to wear. Then new +material continues to be added to the protoplasm throughout life, but in +amount only sufficient to replace that lost from the protoplasm as waste. + + [Fig. 6] + + +Fig. 6--A tumbler partly filled with marbles covered with water, suggesting + the relations of the cells to the lymph. + + +*Cell Surroundings.*--All cells are said to be _aquatic_. This means simply +that they require water for carrying on their various activities. The +cells, in order to live, must take in and give out materials, and water is +necessary to both processes. It is also an essential part of the +protoplasm. Deprived of water, cells become inactive and usually die. +Aquatic surroundings are provided for the cells of the body through a +liquid known as the _lymph_, which is distributed throughout the +intercellular material (Fig. 6). This consists of water containing oxygen +and food substances in solution. Besides supplying these to the cells, the +lymph also receives their wastes. Through the lymph the necessary +conditions for cell life are provided in the body. + +*The General Work of Cells.*--In handling the materials derived from the +lymph, the cells carry on three well-defined processes, known as +absorption, assimilation, and excretion. + +_Absorption_ is the process of taking water, food, and oxygen into the +cells. + +_Assimilation_ is a complex process which results in the addition of the +absorbed materials to the protoplasm. Through assimilation the protoplasm +is built up or renewed. + +_Excretion_ is the throwing off of such waste materials as have been +formed in the cells. These are passed into the lymph and thence to the +surface of the body. + +Absorption, assimilation, excretion, and also reproduction are performed +by all classes of cells. They are, on this account, referred to as the +_general work of cells_. + +*The Special Work of Cells.*--In addition to the general work which all +cells do in common, each class of cells in the body is able to do some +particular kind of work--a work which the others cannot do or which they +can do only to a limited extent. This is spoken of as the _special work of +cells_. Examples of the special work of cells are found in the production +of motion by muscle cells and in the secretion of liquids by gland cells. +It may be noted that while the general work of cells benefits them +individually, their special work benefits the body as a whole. Another +example of the special work of cells is found in the + + [Fig. 7] + + + Fig. 7--Cartilage cells, surrounded by the intercellular material which + they have deposited. + + +*Production of the Intercellular Material.*--Though most of the cells of +the body deposit to a slight extent this material, the greater part of it +is produced by a single class of cells found in bone, cartilage, and +connective tissue. Cartilage, bone, and connective tissue differ greatly +from the other tissues in the amount of intercellular material which they +contain, the difference being due to these cells. In the connective tissue +they deposit the fibrous material so important in holding the different +parts of the body together. In the cartilage they produce the gristly +substance which forms by far its larger portion (Fig. 7). In the bones +they deposit a material similar to that in the cartilage, except that with +it is mixed a mineral substance which gives the bones their hardness and +stiffness.(4) The intercellular material, in addition to connecting the +cells, supplies to certain tissues important properties, such as the +elasticity of cartilage and the stiffness of the bones. + +*Nature of the Body Organization.*--The division of labor carried on by the +different organs, as shown in the preceding chapter, is in reality carried +on by the cells that form the organs. To see that this is true we have +only to observe the relation of cells to tissues and of tissues to organs. +The cells form the tissues and the tissues form the organs. This +arrangement enables the special work of different kinds of cells to be +combined in the work of the organ as a whole. This is seen in the hand +which, in grasping, uses motion supplied by the muscle cells, a +controlling influence supplied by the nerve cells, a framework supplied by +the bone cells, and so on. The cells supply the basis for the body +organization and, properly speaking, the body is _an organization of +cells_(5) (Recall the definition of an organization, page 10.) In this +organization there are to be observed: + +1. A definite arrangement of the cells to form the tissues. A tissue is a +group of like cells. + +2. A definite arrangement of the tissues in the organ. Each organ contains +the tissues needed for its work. + +3. In several instances there is a definite arrangement of organs to form +systems. + +4. The body as a whole is made up of organs and systems, together with the +structures necessary for their support and protection. + +There now remains a further question for consideration. What is the one +supreme end, or purpose, toward which all the activities of the body +organization are directed? This purpose will naturally have some relation +to the maintenance, or preservation, of the cell group which we call the +body. + +*The Maintenance of Life.*--The preservation of any cell group in its +natural condition, whether it be plant or animal, is accomplished through +keeping it alive. If life ceases, the group quickly disintegrates and its +elements become scattered, a fact which is verified through everyday +observation. Though the nature of life is unknown, it may be looked upon +as the organizer and preserver of the protoplasm. But in preserving the +protoplasm it also preserves the entire cell group, or body. Life is thus +the most essential condition of the body. _With life all portions of the +body are concerned, and toward its maintenance all the activities of the +body organization are directed_. + +*The Nutrient Fluid in its Relations to the Cells.*--The maintenance of +life within the cells requires, as we have seen, that they be supplied +with water, food, and oxygen, and that they be relieved of such wastes as +they form. This double purpose is accomplished through the agency of an +internal nutrient fluid, a portion of which has already been referred to +as the lymph. Not only does this fluid supply the means for keeping the +cells alive, but, through the cells, it is also the means of preserving +the life of the body as a whole. + +The cells, however, rapidly exhaust the nutrient fluid. They take from it +food and oxygen and they put into it their wastes. To prevent its becoming +unfit for supplying their needs, food and oxygen must be continually added +to this fluid, and waste materials must be continually removed. This is +not an easy task. As a matter of fact, the preparation, distribution, and +purification of the nutrient fluid requires the direct or indirect aid of +practically all parts of the body. It supplies for this reason a broad +basis for the division of labor on the part of the cells. + +*Relation of the Body to its Environment.*--While life is directly +dependent upon the internal nutrient fluid, it is indirectly dependent +upon the physical surroundings of the body. Herein lies the need of the +_external_ organs--the feet and legs for moving about, the hands for +handling things, the eyes for directing movements, etc. That the great +needs of the body are supplied from its surroundings are facts of common +experience. Food, shelter, air, clothing, water, and the means of +protection are external to the body and form a part of its environment. In +making the things about him contribute to his needs, man encounters a +problem which taxes all his powers. Only by toil and hardship, "by the +sweat of his brow," has he been able to wrest from his surroundings the +means of his sustenance. + +*The Main Physiological Problems.*--The study of the body is thus seen to +resolve itself naturally into the consideration of two main problems: + +1. _That of maintaining in the body a nutrient fluid for the cells._ + +2. _That of bringing the body into such relations with its surroundings as +will enable it to secure materials for the nutrient fluid and satisfy its +other needs._ + +The first problem is _internal_ and includes the so-called vital +processes, known as digestion, circulation, respiration, and excretion. +The second problem is _external_, as it were, and includes the work of the +external organs--the organs of motion and of locomotion and the organs of +special sense. These problems are closely related, since they are the two +divisions of the one problem of maintaining life. Neither can be +considered independently of the other. In the chapter following is taken +up the first of these problems. + +*Summary.*--The individual parts, or units, that form the body organization +are known as cells. These consist of minute but definitely arranged +portions of protoplasm and are held together by the intercellular +material. They build up the body and carry on its different activities. +The tissues are groups of like cells. By certain general activities the +cells maintain their existence in the tissues and by the exercise of +certain special activities they adapt the tissues to their purposes in the +body. The body, as a cell organization, has its activities directed under +normal conditions toward a single purpose--that of maintaining life. In the +accomplishment of this purpose a nutrient fluid is provided for the cells +and proper relations between the body and its surroundings are +established. + +*Exercises.*--1. If a tissue be compared to a brick wall, to what do the +separate bricks correspond? To what the mortar between the bricks? + +2. Draw an outline of a typical cell, locating and naming the main +divisions. + +3. How do the cells enable the body to grow? Describe the process of +cell-division. + +4. How does the general work of cells differ from their special work? +Define absorption, excretion, and assimilation as applied to the cells. + +5. Compare the conditions surrounding a one-celled animal, living in +water, to the conditions surrounding the cells in the body. + +6. What is meant by the term "environment"? How does man's environment +differ from that of a fish? + +7. What is the necessity for a nutrient fluid in the body? + +8. Why is the maintenance of life necessarily the chief aim of all the +activities of the body? + +9. State the two main problems in the study of the body. + + + +PRACTICAL WORK + + +*Observations.*--1. Make some scrapings from the inside of the cheek with a +dull knife and mix these with a little water on a glass slide. Place a +cover-glass on the same and examine with a compound microscope. The large +pale cells that can be seen in this way are a variety of epithelial cells. + +2. Mount in water on a glass slide some thin slices of cartilage and +examine first with a low and then with a high power of microscope. +(Suitable slices may be cut, with a sharp razor, from the cartilage found +at the end of the rib of a young animal.) Note the small groups of cells +surrounded by, and imbedded in, the intercellular material. + +3. Mount and examine with the microscope thin slices of elder pith, +potato, and the stems of growing plants. Make drawings of the cells thus +observed. + +4. Examine with the microscope a small piece of the freshly sloughed off +epidermis of a frog's skin. Examine it first in its natural condition, and +then after soaking for an hour or two in a solution of carmine. Make +drawings. + +5. Mount on a glass slide some of the scum found on stagnant water and +examine it with a compound microscope. Note the variety and relative size +of the different things moving about. The forms most frequently seen by +such an examination are one-celled plants. Many of these have the power of +motion. + +6. Examine tissues of the body, such as nervous, muscular, and glandular +tissues, which have been suitably prepared and mounted for microscopic +study, using low and high powers of the microscope. Make drawings of the +cells in the different tissues thus observed. + + + + +CHAPTER IV - THE BLOOD + + +Two liquids of similar nature are found in the body, known as the blood +and the lymph. These are closely related in function and together they +form the nutrient fluid referred to in the preceding chapter. The blood is +the more familiar of the two liquids, and the one which can best be +considered at this time. + +*The Blood: where Found.*--The blood occupies and moves through a system of +closed tubes, known as the blood vessels. By means of these vessels the +blood is made to circulate through all parts of the body, but from them it +does not escape under normal conditions. Though provisions exist whereby +liquid materials may both enter and leave the blood stream, it is only +when the blood vessels are cut or broken that the blood, as blood, is able +to escape from its inclosures. + +*Physical Properties of the Blood.*--Experiments such as those described at +the close of this chapter reveal the more important physical properties of +the blood. It may be shown to be heavier and denser than water; to have a +faint odor and a slightly salty taste; to have a bright red color when it +contains oxygen and a dark red color when oxygen is absent; and to +undergo, when exposed to certain conditions, a change called coagulation. +These properties are all accounted for through the different materials +that enter into the formation of the blood. + + [Fig. 8] + + + Fig. 8--Blood corpuscles, highly magnified. _A._ Red corpuscles as they + appear in diluted blood. _B._ Arrangement of red corpuscles in rows + between which are white corpuscles, as may be seen in undiluted blood. + _C._ Red corpuscles much enlarged to show the form. + + +*Composition of the Blood.*--To the naked eye the blood appears as a thick +but simple liquid; but when examined with a compound microscope, it is +seen to be complex in nature, consisting of at least two distinct +portions. One of these is a clear, transparent liquid; while the other is +made up of many small, round bodies that float in the liquid. The liquid +portion of the blood is called the _plasma_; the small bodies are known as +_corpuscles_. Two varieties of corpuscles are described--the _red_ +corpuscles and the _white_ corpuscles (Fig. 8). Other round particles, +smaller than the corpuscles, may also be seen under favorable conditions. +These latter are known as _blood platelets_. + +*Red Corpuscles.*--The red corpuscles are classed as cells, although, as +found in the blood of man and the other mammals (Fig. 9), they have no +nuclei.(6) Each one consists of a little mass of protoplasm, called the +_stroma_, which contains a substance having a red color, known as +_hemoglobin_. The shape of the red corpuscle is that of a circular disk +with concave sides. It has a width of about 1/3200 of an inch (7.9 +microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The +red corpuscles are exceedingly numerous, there being as many as five +millions in a small drop (one cubic millimeter) of healthy blood. But the +number varies somewhat and is greatly diminished during certain forms of +disease. + + [Fig. 9] + + +Fig. 9--Red corpuscles from various animals. Those from mammals are without + nuclei, while those from birds and cold-blooded animals have nuclei. + + +It is the _function_ of the red corpuscles to serve as _oxygen carriers_ +for the cells. They take up oxygen at the lungs and release it at the +cells in the different tissues.(8) The performance of this function +depends upon the hemoglobin. + +*Hemoglobin.*--This substance has the remarkable property of forming, under +certain conditions, a weak chemical union with oxygen and, when the +conditions are reversed, of separating from it. It forms about nine tenths +of the solid matter of the red corpuscles and to it is due the colors of +the blood. When united with the oxygen it forms a compound, called +_oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a +dark red color. These colors are the same as those of the blood as it +takes on and gives off oxygen. The stroma, which forms only about one +tenth of the solid matter of the corpuscles, serves as a contrivance for +holding the hemoglobin. The conditions which cause the hemoglobin to unite +with oxygen in the lungs and to separate from it in the tissues, will be +considered later (Chapter VIII). + +*Disappearance and Origin of Red Corpuscles.*--The red corpuscles, being +cells without nuclei, are necessarily short-lived. It has been estimated +that during a period of one to two months, all the red corpuscles in the +body at a given time will have disappeared and their places taken by new +ones. The origin of new corpuscles, however, and the manner of ridding the +blood of old ones are problems that are not as yet fully solved. The +removal of the products of broken down corpuscles is supposed to take +place both in the liver and in the spleen.(9) + +Regarding the origin of the red corpuscles, the evidence now seems +conclusive that large numbers of them are formed in the red marrow of the +bones. The red marrow is located in what is known as the spongy substance +of the bones (Chapter XIV) and consists, to a large extent, of cells +somewhat like the red corpuscles, but differing from them in having +nuclei. These appear to be constantly in a state of reproduction. The +blood, flowing through the minute cavities containing these cells, carries +those that have been loosened out into the blood stream. Nuclei appear in +the red corpuscles at the time of their formation, but these quickly +separate and, according to some authorities, form the blood platelets. + +*White Corpuscles.*--The white corpuscles, or _leucocytes_, are cells of a +general spherical shape, each containing one, two, or more nuclei. They +are much less numerous than the red, there being on the average only one +white corpuscle to about every five hundred of the red ones. On the other +hand, the white corpuscles are larger than the red, one of the former +being equal in volume to about three of the latter. + + [Fig. 10] + + +Fig. 10--*Escape of white corpuscles from a small blood vessel* (Hall). At + _A_ the conditions are normal, but at _B_ some excitation in the + surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show + different stages of the passage. + + +The white corpuscles are found, when studied under favorable conditions, +to possess the power of changing their shape and, by this means, of moving +from place to place. This property enables them to penetrate the walls of +capillaries and to pass with the lymph in between the cells of the +tissues. The white corpuscles are, therefore, not confined to the blood +vessels, as are the red corpuscles, but migrate through the intercellular +spaces (Fig. 10). If any part of the body becomes inflamed, the white +corpuscles collect there in large numbers; and, on breaking down, they +form most of the white portion of the sore, called the _pus_. + +New white corpuscles are formed from old ones, by cell-division. Their +production may occur in almost any part of the body, but usually takes +place in the lymphatic glands (Chapter VI) and in the spleen, where +conditions for their development are especially favorable. In these places +they are found in great abundance and in various stages of development. + +*Functions of White Corpuscles.*--The main use of the white corpuscles +appears to be that of a destroyer of disease germs. These consist of +minute organisms that find their way into the body and, by living upon the +tissues and fluids and by depositing toxins (poisons) in them, cause +different forms of disease. Besides destroying germs that may be present +in the blood, the white corpuscles also leave the blood and attack germs +that have invaded the cells. By forming a kind of wall around any foreign +substance, such as a splinter, that has penetrated the skin, they are able +to prevent the spread of germs through the body. In a similar manner they +also prevent the germs from boils, abscesses, and sore places in general +from getting to and infecting other parts of the body.(10) Another +function ascribed to the white corpuscles is that of aiding in the +coagulation of the blood (page 31); and still another, of aiding in the +healing of wounds. + +*Plasma.*--The plasma is a complex liquid, being made up of water and of +substances dissolved in the water. The dissolved substances consist mainly +of foods for the cells and wastes from the cells. + +1. _The foods_ represent the same classes of materials as are taken in the +daily fare, _i.e._, proteids, carbohydrates, fats, and salts (Chapter IX). +Three kinds of proteids are found in the plasma, called _serum albumin_, +_serum globulin_, and _fibrinogen_. These resemble, in a general way, the +white of raw egg, but differ from each other in the readiness with which +they coagulate. Fibrinogen coagulates more readily than the others and is +the only one that changes in the ordinary coagulation of the blood. The +others remain dissolved during this process, but are coagulated by +chemical agents and by heat. While all of the proteids probably serve as +food for the cells, the fibrinogen, in addition, is a necessary factor in +the coagulation of the blood (page 31). + +The only representative of the carbohydrates in the plasma is _dextrose_. +This is a variety of sugar, being derived from starch and the different +sugars that are eaten. The _fat_ in the plasma is in minute quantities and +appears as fine droplets--the form in which it is found in milk. While +several mineral salts are present in small quantities in the plasma, +_sodium chloride_, or common salt, is the only one found in any +considerable amount. The mineral salts serve various purposes, one of +which is to cause the proteids to dissolve in the plasma. + +2. _The wastes_ are formed at the cells, whence they are passed by the +lymph into the blood plasma. They are carried by the blood until removed +by the organs of excretion. The two waste products found in greatest +abundance in the plasma are carbon dioxide and urea. + +The substances dissolved in the plasma form about 10 per cent of the whole +amount. The remaining 90 per cent is water. Practically all the +constituents of the plasma, except the wastes, enter the blood from the +digestive organs. + +*Purposes of Water in the Blood.*--Not only is water the most abundant +constituent of the blood; it is, in some respects, the most important. It +is the liquefying portion of the blood, holding in solution the +constituents of the plasma and floating the corpuscles. Deprived of its +water, the blood becomes a solid substance. Through the movements of the +blood the water also serves the purpose of a transporting agent in the +body. The cells in all parts of the body require water and this is +supplied to them from the blood. Water is present in the corpuscles as +well as in the plasma and forms about 80 per cent of the entire volume of +the blood. + +*Coagulation of the Blood.*--If the blood is exposed to some unnatural +condition, such as occurs when it escapes from the blood vessels, it +undergoes a peculiar change known as _coagulation_.(11) In this change the +corpuscles are collected into a solid mass, known as the _clot_, thereby +separating from a liquid called the _serum_. The serum, which is similar +in appearance to the blood plasma, differs from that liquid in one +important respect as explained below. + +*Causes of Coagulation.*--Although coagulation affects all parts of the +blood, only one of its constituents is found in reality to coagulate. This +is the fibrinogen. The formation of the clot and the separation of the +serum is due almost entirely to the action of this substance. Fibrinogen +is for this reason called the _coagulable constituent of the blood_. In +the plasma the fibrinogen is in a liquid form; but during coagulation it +changes into a white, stringy solid, called _fibrin_. This appears in the +clot and is the cause of its formation. Forming as a network of +exceedingly fine and very delicate threads (Fig. 11) _throughout the mass +of blood_ that is coagulating, the fibrin first entangles the corpuscles +and then, by contracting, draws them into the solid mass or clot.(12) The +contracting of the fibrin also squeezes out the serum. This liquid +contains all the constituents of the plasma except the fibrinogen. + + [Fig. 11] + + + Fig. 11--*Fibrin threads* (after Ranvier). These by contracting draw the + corpuscles together and form the clot. + + +*Fibrin Ferment and Calcium.*--Most difficult of all to answer have been +the questions: What causes the blood to coagulate outside of the blood +vessels and what prevents its coagulation inside of these vessels? The +best explanation offered as yet upon this point is as follows: Fibrinogen +does not of itself change into fibrin, but is made to undergo this change +by the presence of another substance, called _fibrin ferment_. This +substance is not a regular constituent of the blood, but is formed as +occasion requires. It is supposed to result from the breaking down of the +white corpuscles, and perhaps also from the blood platelets, when the +blood is exposed to unnatural conditions. The formation of the ferment +leads in turn to the changing of the fibrinogen into fibrin. + +Another substance which is necessary to the process of coagulation is the +element calcium. If compounds of calcium are absent from the blood, +coagulation does not take place. These are, however, regular constituents +of healthy blood. Whether the presence of the calcium is necessary to the +formation of the ferment or to the action of the ferment upon the +fibrinogen is unknown. + +*Purpose of Coagulation.*--The purpose of coagulation is to check the flow +of blood from wounds. The fact that the blood is contained in and kept +flowing continuously through a system of _connected_ vessels causes it to +escape rapidly from the body whenever openings in these vessels are made. +Clots form at such openings and close them up, stopping in this way the +flow that would otherwise go on indefinitely. Coagulation, however, does +not stop the flow of blood from the large vessels. From these the blood +runs with too great force for the clot to form within the wound. + +*Time Required for Coagulation.*--The rate at which coagulation takes place +varies greatly under different conditions. It is influenced strongly by +temperature; heat hastens and cold retards the process. It may be +prevented entirely by lowering the temperature of the blood to near the +freezing point. The presence of a foreign substance increases the rapidity +of coagulation, and it has been observed that bleeding from small wounds +is more quickly checked by covering them with linen or cotton fibers. The +fibers in this case hasten the process of coagulation. + +*Quantity of Blood.*--The quantity of blood is estimated to be about one +thirteenth of the entire weight of the body. This for the average +individual is an amount weighing nearly twelve pounds and having a volume +of nearly one and one half gallons. About 46 per cent by volume of this +amount is made up of corpuscles and 54 per cent of plasma. Of the plasma +about 10 per cent consists of solids and 90 per cent of water, as already +stated. + +*Functions of the Blood.*--The blood is the great carrying, or +distributing, agent in the body. Through its movements (considered in the +next chapter) it carries food and oxygen to the cells and waste materials +from the cells. Much of the blood may, therefore, be regarded as _freight_ +in the process of transportation. The blood also carries, or distributes, +heat. Taking up heat in the warm parts of the body, it gives it off at +places having a lower temperature. This enables all parts of the body to +keep at about the same temperature. + +In addition to serving as a carrier, the blood has antiseptic properties, +i.e., it destroys disease germs. While this function is mainly due to the +white corpuscles, it is due in part to the plasma.(13) Through its +coagulation, the blood also closes leaks in the small blood vessels. The +blood is thus seen to be a liquid of several functions. + + [Fig. 12] + + +Fig. 12--*A balanced change* in water. The level remains constant although +the water is continually changing; suggestive of the changes in the blood. + + +*Changes in the Blood.*--In performing its functions in the body the blood +must of necessity undergo rapid and continuous change. The red corpuscles, +whose changes have already been noted, appear to be the most enduring +constituents of the blood. The plasma is the portion that changes most +rapidly. Yet in spite of these changes the quantity and character of the +blood remain practically constant.(14) This is because there is a +_balancing_ of the forces that bring about the changes. The addition of +various materials to the blood just equals the withdrawal of the same +materials from the blood. Somewhat as a vessel of water (Fig. 12) having +an inflow and an outflow which are equal in amount may keep always at the +same level, the balancing of the intake and outgo of the blood keeps its +composition about the same from time to time. + +*Hygiene of the Blood.*--The blood, being a changeable liquid, is easily +affected through our habits of living. Since it may be affected for ill as +well as for good, one should cultivate those habits that are beneficial +and avoid those that are harmful in their effects. Most of the hygiene of +the blood, however, is properly included in the hygiene of the organs that +act upon the blood--a fact which makes it unnecessary to treat this subject +fully at this time. + +From a health standpoint, the most important constituents of the blood +are, perhaps, the corpuscles. These are usually sufficient in number and +vigor in the blood of those who take plenty of physical exercise, accustom +themselves to outdoor air and sunlight, sleep sufficiently, and avoid the +use of injurious drugs. On the other hand, they are deficient in quantity +and inferior in quality in the bodies of those who pursue an opposite +course. Impurities not infrequently find their way into the blood through +the digestive organs. One should eat wholesome, well-cooked food, drink +freely of _pure_ water, and limit the quantity of food _to what can be +properly digested_. The natural purifiers of the blood are the organs of +excretion. The skin is one of these and its power to throw off impurities +depends upon its being clean and active. + +*Effect of Drugs.*--Certain drugs and medicines, including alcohol and +quinine,(15) have recently been shown to destroy the white corpuscles. The +effect of such substances, if introduced in considerable amount in the +body, is to render one less able to withstand attacks of disease. Many +patent medicines are widely advertised for purifying the blood. While +these may possibly do good in particular cases, the habit of doctoring +one's self with them is open to serious objection. Instead of taking drugs +and patent medicines for purifying the blood, one should study to live +more hygienically. We may safely rely upon wholesome food, pure water, +outdoor exercise and sunlight, plenty of sleep, and a clean skin for +keeping the blood in good condition. If these natural remedies fail, a +physician should be consulted. + +*Summary.*--The blood is the carrying or transporting agent of the body. It +consists in part of constituents, such as the red corpuscles, that enable +it to carry different substances; and in part of the materials that are +being carried. The latter, which include food and oxygen for the cells and +wastes from the cells, may be classed as freight. Certain constituents in +the blood destroy disease germs, and other constituents, by coagulating, +close small leaks in the blood vessels. Although subject to rapid and +continuous change, the blood is able--by reason of the balancing of +materials added to and withdrawn from it--to remain about the same in +quantity and composition. + +Exercises.--1. Compare blood and water with reference to weight, density, +color, odor, and complexity of composition. + +2. Show by an outline the different constituents of the blood. + +3. Compare the red and white corpuscles with reference to size, shape, +number, origin, and function. + +4. Name some use or purpose for each constituent of the blood. + +5. What constituents of the blood may be regarded as freight and what as +agents for carrying this freight? + +6. After coagulation, what portions of the blood are found in the clot? +What portions are found in the serum? + +7. What purposes are served by water in the blood? + +8. Show how the blood, though constantly changing, is kept about the same +in quantity, density, and composition. + +9. In the lungs the blood changes from a dark to a bright red color and in +the tissues it changes back to dark red. What is the cause of these +changes? + +10. If the oxygen and hemoglobin formed a strong instead of a weak +chemical union, could the hemoglobin then act as an oxygen carrier? Why? + +11. What habits of living favor the development of corpuscles in the +blood? + +12. Why will keeping the skin clean and active improve the quality of +one's blood? + + + +PRACTICAL WORK + + +*To demonstrate the Physical Properties of Blood* (Optional).--Since blood +is needed in considerable quantity in the following experiments, it is +best obtained from the butcher. To be sure of securing the blood in the +manner desired, take to the butcher three good-sized bottles bearing +labels as follows: + +*1* Fill two thirds full. While the blood is cooling, stir rapidly with +the hand or a bunch of switches to remove the clot. + +*2* Fill two thirds full and set aside without shaking or stirring. + +*3* Fill two thirds full and thoroughly mix with the liquid in the bottle. + +Label 3 must be pasted on a bottle, having a tight-fitting stopper, which +is filled one fifth full of a saturated solution of Epsom salts. The +purpose of the salts is to prevent coagulation until the blood is diluted +with water as in the experiments which follow. + +*Experiments.*--1. Let some of the defibrinated blood (bottle 1) flow (not +fall) on the surface of water in a glass vessel. Does it remain on the +surface or sink to the bottom? What does the experiment show with +reference to the relative weight of blood and water? + +2. Fill a large test tube or a small bottle one fourth full of the +defibrinated blood and thin it by adding an equal amount of water. Then +place the hand over the mouth and shake until the blood is thoroughly +mixed with the air. Compare with a portion of the blood not mixed with the +air, noting any difference in color. What substance in the air has acted +on the blood to change its color? + +3. Fill three tumblers each two thirds full of water and set them in a +warm place. Pour into one of the tumblers, and thoroughly mix with the +water, two tablespoonfuls of the blood containing the Epsom salts. After +an interval of half an hour add blood to the second tumbler in the same +manner, and after another half hour add blood to the third. The water +dilutes the salts so that coagulation is no longer prevented. Jar the +vessel occasionally as coagulation proceeds; and if the clot is slow in +forming, add a trace of some salt of calcium (calcium chloride). After the +blood has been added to the last tumbler make a comparative study of all. +Note that coagulation begins in all parts of the liquid at the same time +and that, as the process goes on, the clot shrinks and is drawn toward the +center. + +4. Place a clot from one of the tumblers in experiment 3 in a large vessel +of water. Thoroughly wash, adding fresh water, until a white, stringy +solid remains. This substance is fibrin. + +5. Examine the coagulated blood obtained from the butcher (bottle 2). +Observe the dark central mass (the clot) surrounded by a clear liquid (the +serum). Sketch the vessel and its contents, showing and naming the parts +into which the blood separates by coagulation. + +*To examine the Red Corpuscles.*--Blood for this purpose is easily obtained +from the finger. With a handkerchief, wrap one of the fingers of the left +hand from the knuckle down to the first joint. Bend this joint and give it +a sharp prick with the point of a sterilized 'needle just above the root +of the nail. Pressure applied to the under side of the finger will force +plenty of blood through a very small opening. (To prevent any possibility +of blood poisoning the needle should be sterilized. This may be done by +dipping it in alcohol or by holding it for an instant in a hot flame. It +is well also to wash the finger with soap and water, or with alcohol, +before the operation.) Place a small drop of the blood in the middle of a +glass slide, protect the same with a cover glass, and examine with a +compound microscope. At least two specimens should be examined, one of +which should be diluted with a little saliva or a physiological salt +solution.(16) In the diluted specimen the red corpuscles appear as +amber-colored, circular, disk-shaped bodies. In the undiluted specimen +they show a decided tendency to arrange themselves in rows, resembling +rows of coins. (Singly, the corpuscles do not appear red when highly +magnified.) + +A few white corpuscles may generally be found among the red ones in the +undiluted specimen. These become separated by the formation of the red +corpuscles into rows. They are easily recognized by their larger size and +by their silvery appearance, due to the light shining through them. + +*To examine White Corpuscles.*--Obtain from the butcher a small piece of +the neck sweetbread of a calf. Press it between the fingers to squeeze out +a whitish, semi-liquid substance. Dilute with physiological salt solution +on a glass slide and examine with a compound microscope. Numerous white +corpuscles of different kinds and sizes will be found. Make sketches. + +*To prepare Models of Red Corpuscles.*--Several models of red corpuscles +should be prepared for the use of the class. Clay and putty may be pressed +into the form of red corpuscles and allowed to harden, and small models +may be cut out of blackboard crayon. Excellent models can be molded from +plaster of Paris as follows: Coat the inside of the lid of a baking powder +can with oil or vaseline and fill it even full of a thick mixture of +plaster of Paris and water. After the plaster has set, remove it from the +lid and with a pocket-knife round off the edges and hollow out the sides +until the general form of the corpuscle is obtained. The models may be +colored red if it is desired to match the color as well as the form of the +corpuscle. + + + + +CHAPTER V - THE CIRCULATION + + +A Carrier must move. To enable the blood to carry food and oxygen _to_ the +cells and waste materials _from_ the cells, and also to distribute heat, +it is necessary to keep it moving, or circulating, in all parts of the +body. So closely related to the welfare of the body is the circulation(17) +of the blood, that its stoppage for only a brief interval of time results +in death. + +*Discovery of the Circulation.*--The discovery of the circulation of the +blood was made about 1616 by an English physician named Harvey. In 1619 he +announced it in his public lectures and in 1628 he published a treatise in +Latin on the circulation. The chief arguments advanced in support of his +views were the presence of valves in the heart and veins, the continuous +movement of the blood in the same direction through the blood vessels, and +the fact that the blood comes from a cut artery in jets, or spurts, that +correspond to the contractions of the heart. + +No other single discovery with reference to the human body has proved of +such great importance. A knowledge of the nature and purpose of the +circulation was the necessary first step in understanding the plan of the +body and the method of maintaining life, and physiology as a science dates +from the time of Harvey's discovery. + +*Organs of Circulation.*--The organs of circulation, or blood vessels, are +of four kinds, named the heart, the arteries, the capillaries, and the +veins. They serve as contrivances both for holding the blood and for +keeping it in motion through the body. The heart, which is the chief organ +for propelling the blood, acts as a force pump, while the arteries and +veins serve as tubes for conveying the blood from place to place. +Moreover, the blood vessels are so connected that the blood moves through +them in a regular order, performing two well-defined circuits. + + [Fig. 13] + + +Fig. 13--*Heart* in position in thoracic cavity. Dotted lines show positin + of diaphragm and of margins of lungs. + + +*The Heart.*--The human heart, roughly speaking, is about the size of the +clenched fist of the individual owner. It is situated very near the center +of the thoracic cavity and is almost completely surrounded by the lungs. +It is cone-shaped and is so suspended that the small end hangs downward, +forward, and a little to the left. When from excitement, or other cause, +one becomes conscious of the movements of the heart, these appear to be in +the left portion of the chest, a fact which accounts for the erroneous +impression that the heart is on the left side. The position of the heart +in the cavity of the chest is shown in Fig. 13. + +*The Pericardium.*--Surrounding the heart is a protective covering, called +the pericardium. This consists of a closed membranous sac so arranged as +to form a double covering around the heart. The heart does not lie inside +of the pericardial sac, as seems at first glance to be the case, but its +relation to this space is like that of the hand to the inside of an empty +sack which is laid around it (Fig. 14). The inner layer of the pericardium +is closely attached to the heart muscle, forming for it an outside +covering. The outer layer hangs loosely around the heart and is continuous +with the inner layer at the top. The outer layer also connects at certain +places with the membranes surrounding the lungs and is attached below to +the diaphragm. Between the two layers of the pericardium is secreted a +liquid which prevents friction from the movements of the heart. + + [Fig. 14] + + + Fig. 14--*Diagram of section of the pericardial sac*, heart removed. _A._ + Place occupied by the heart. _B._ Space inside of pericardial sac. _a._ +Inner layer of pericardium and outer lining of heart. _b._ Outer layer of + pericardium. _C._ Covering of lung. _D._ Diaphragm. + + +*Cavities of the Heart.*--The heart is a hollow, muscular organ which has +its interior divided by partitions into four distinct cavities. The main +partition extends from top to bottom and divides the heart into two +similar portions, named from their positions the right side and the left +side. On each side are two cavities, the one being directly above the +other. The upper cavities are called _auricles_ and the lower ones +_ventricles_. To distinguish these cavities further, they are named from +their positions the right auricle and the left auricle, and the right +ventricle and the left ventricle (Fig. 15). The auricles on each side +communicate with the ventricles below; but after birth there is no +communication between the cavities on the opposite sides of the heart. All +the cavities of the heart are lined with a smooth, delicate membrane, +called the _endocardium_. + + [Fig. 15] + + + Fig. 15--*Diagram showing plan of the heart.* 1. Semilunar valves. 2. + Tricuspid valve. 3. Mitral valve. 4. Right auricle. 5. Left auricle. 6. + Right ventricle. 7. Left ventricle. 8. Chordae tendineae. 9. Inferior vena + cava. 10. Superior vena cava. 11. Pulmonary artery. 12. Aorta. 13. + Pulmonary veins. + + +*Valves of the Heart.*--Located at suitable places in the heart are four +gate-like contrivances, called valves. The purpose of these is _to give +the blood a definite direction_ in its movements. They consist of tough, +inelastic sheets of connective tissue, and are so placed that pressure on +one side causes them to come together and shut up the passageway, while +pressure on the opposite side causes them to open. A valve is found at the +opening of each auricle into the ventricle, and at the opening of each +ventricle into the artery with which it is connected. + +The valve between the right auricle and the right ventricle is called the +_tricuspid_ valve. It is suspended from a thin ring of connective tissue +which surrounds the opening, and its free margins extend into the +ventricle (Fig. 16). It consists of three parts, as its name implies, +which are thrown together in closing the opening. Joined to the free edges +of this valve are many small, tendinous cords which connect at their lower +ends with muscular pillars in the walls of the ventricle. These are known +as the _chordae tendineae_, or heart tendons. Their purpose is to serve as +_valve stops_, to prevent the valve from being thrown, by the force of the +blood stream, back into the auricle. + +The _mitral_, or bicuspid, valve is suspended around the opening between +the left auricle and the left ventricle, with the free margins extending +into the ventricle. It is exactly similar in structure and arrangement to +the tricuspid valve, except that it is stronger and is composed of two +parts instead of three. + + [Fig. 16] + + +Fig. 16--*Right side of heart* dissected to show cavities and valves. _B._ + Right semilunar valve. The tricuspid valve and the chordae tendineae shown + in the ventricle. + + +The _right semilunar_ valve is situated around the opening of the right +ventricle into the pulmonary artery. It consists of three pocket-shaped +strips of connective tissue which hang loosely from the walls when there +is no pressure from above; but upon receiving pressure, the pockets fill +and project into the opening, closing it completely (Fig. 16). The _left +semilunar_ valve is around the opening of the left ventricle into the +aorta, and is similar in all respects to the right semilunar valve. + +*Differences in the Parts of the Heart.*--Marked differences are found in +the walls surrounding the different cavities of the heart. The walls of +the ventricles are much thicker and stronger than those of the auricles, +while the walls of the left ventricle are two or three times thicker than +those of the right. A less marked but similar difference exists between +the auricles and also between the valves on the two sides of the heart. +These differences in structure are all accounted for by the work done by +the different portions of the heart. The greater the work, the heavier the +structures that perform the work. + + [Fig. 17] + + +Fig. 17--*Diagram of the circulation*, showing in general the work done by + each part of the heart. The right ventricle forces the blood through the + lungs and into the left auricle. The left ventricle forces blood through + all parts of the body and back to the auricle. The auricles force blood + into the ventricles. + + +*Connection with Arteries and Veins.*--Though the heart is in communication +with all parts of the circulatory system, it makes actual connection with +only a few of the blood tubes. These enter the heart at its upper portion +(Fig. 15), but connect with its different cavities as follows: + +1. _With the right auricle_, the superior and the inferior venae cavae and +the coronary veins. The superior vena cava receives blood from the head +and the upper extremities; the inferior vena cava, from the trunk and the +lower extremities; and the coronary veins, from the heart itself. + +2. _With the left auricle_, the four pulmonary veins. These receive blood +from the lungs and empty it into the left auricle. + +3. _With the right ventricle_, the pulmonary artery. This receives blood +from the heart and by its branches distributes it to all parts of the +lungs. + +4. _With the left ventricle_, the aorta. The aorta receives blood from the +heart and through its branches delivers it to all parts of the body. + +*How the Heart does its Work.*--The heart is a muscular pump(18) and does +its work through the contracting and relaxing of its walls. During +contraction the cavities are closed and the blood is forced out of them. +During relaxation the cavities open and are refilled. The valves direct +the flow of the blood, being so arranged as to keep it moving always in +the same direction (Fig. 17). + +The heart, however, is not a single or a simple pump, but consists in +reality of _four_ pumps which correspond to its different cavities. These +connect with each other and with the blood vessels over the body in such a +manner that each aids in the general movement of the blood. + + [Fig. 18] + + + Fig. 18--Diagram illustrating the "cardiac cycle." + + +*Work of Auricles and Ventricles Compared.*--In the work of the heart the +two auricles contract at the same time--their contraction being followed +immediately by the contraction of both ventricles. After the contraction +of the ventricles comes a period of rest, or relaxation, about equal in +time to the period of contraction of both the auricles and the +ventricles.(19) On account of the work which they perform, the auricles +have been called the "feed pumps" of the heart; and the ventricles, the +"force pumps."(20) It is the function of the auricles to collect the blood +from the veins, to let this run slowly into the ventricles when both the +auricles and ventricles are relaxed, and finally, by contracting, _to +force an excess of blood into the ventricles_, thereby distending their +walls. The ventricles, having in this way been fully charged by the +auricles, now contract and force their contents into the large arteries. + +*Sounds of the Heart.*--Two distinct sounds are given out by the heart as +it pumps the blood. One of them is a dull and rather heavy sound, while +the other is a short, sharp sound. The short sound follows quickly after +the dull sound and the two are fairly imitated by the words "lubb, dup." +While the cause of the first sound is not fully understood, most +authorities believe it to be due to the contraction of the heart muscle +and the sudden tension on the valve flaps. The second sound is due to the +closing of the semilunar valves. These sounds are easily heard by placing +an ear against the chest wall. They are of great value to the physician in +determining the condition of the heart. + +*Arteries and Veins.*--These form two systems of tubes which reach from the +heart to all parts of the body. The arteries receive blood from the heart +and distribute it to the capillaries. The veins receive the blood from the +capillaries and return it to the heart. The arteries and veins are similar +in structure, both having the form of tubes and both having three distinct +layers, or coats, in their walls. The corresponding coats in the arteries +and veins are made up of similar materials, as follows: + +1. _The inner coat_ consists of a delicate lining of flat cells resting +upon a thin layer of connective tissue. The inner coat is continuous with +the lining of the heart and provides a smooth surface over which the blood +glides with little friction. + +2. _The middle coat_ consists mainly of non-striated, or involuntary, +muscular fibers. This coat is quite thin in the veins, but in the arteries +it is rather thick and strong. + +3. _The outer coat_ is made up of a variety of connective tissue and is +also much thicker and stronger in the arteries than in the veins. + + [Fig. 19] + + + Fig. 19--Artery dissected to show the coats. + + +Marked differences exist between the arteries and the veins, and these +vessels are readily distinguished from each other. The walls of the +arteries are much thicker and heavier than those of the veins (Fig. 19). +As a result these tubes stand open when empty, whereas the veins collapse. +The arteries also are highly elastic, while the veins are but slightly +elastic. On the other hand, many of the veins contain valves, formed by +folds in the inner coat (Fig. 20), while the arteries have no valves. The +blood flows more rapidly through the arteries than through the veins, the +difference being due to the fact that the system of veins has a greater +capacity than the system of arteries. + + [Fig. 20] + + + Fig. 20--Vein split open to show the valves. + + +*Why the Arteries are Elastic.*--The elasticity of the arteries serves a +twofold purpose. It keeps the arteries from bursting when the blood is +forced into them from the ventricles, and it is a means of _supplying +pressure to the blood while the ventricles are in a condition of +relaxation._ The latter purpose is accomplished as follows: + +Contraction of the ventricles fills the arteries overfull, causing them to +swell out and make room for the excess of blood. Then while the ventricles +are resting and filling, the stretched arteries press upon the blood to +keep it flowing into the capillaries. In this way _they cause the +intermittent flow from, the heart to become a steady stream in the +capillaries_. + +The swelling of the arteries at each contraction of the ventricle is +easily felt at certain places in the body, such as the wrist. This +expansion, known as the "pulse," is the chief means employed by the +physician in determining the force and rapidity of the heart's action. + +*Purpose of the Valves in the Veins.*--The valves in the veins are not used +for directing the _general_ flow of the blood, the valves of the heart +being sufficient for this purpose. Their presence is necessary because of +the pressure to which the veins are subjected in different parts of the +body. The contraction of a muscle will, for example, close the small veins +in its vicinity and diminish the capacity of the larger ones. The natural +tendency of such pressure is to empty the veins in two directions--one in +the same direction as the regular movement of the blood, but the other in +the opposite direction. The valves by closing cause the contracting muscle +to push the blood in one direction only--toward the heart. The valves in +the veins are, therefore, an economical device for _enabling variable +pressure_ in different parts of the body _to assist in the circulation_. +Veins like the inferior vena cava and the veins of the brain, which are +not compressed by movements of the body, do not have valves. + +*Purposes of the Muscular Coat.*--The muscular coat, which is thicker in +the arteries than in the veins and is more marked in small arteries than +in large ones, serves two important purposes. In the first place it, +together with the elastic tissue, keeps the capacity of the blood vessels +_equal to the volume of the blood_. Since the blood vessels are capable of +holding more blood than may be present at a given time in the body, there +is a liability of empty spaces occurring in these tubes. Such spaces would +seriously interfere with the circulation, since the heart pressure could +not then reach all parts of the blood stream. This is prevented by the +contracted state, or "tone," of the blood vessels, due to the muscular +coat. + +In the second place, the muscular coat serves the purpose of _regulating_ +the amount of blood which any given organ or part of the body receives. +This it does by varying the caliber of the arteries going to the organ in +question. To increase the blood supply, the muscular coat relaxes. The +arteries are then dilated by the blood pressure from within so as to let +through a larger quantity of blood. To diminish the supply, the muscle +contracts, making the caliber of the arteries less, so that less blood can +flow to this part of the body. Since the need of organs for blood varies +with their activity, the muscular coat serves in this way a very necessary +purpose. + + [Fig. 21] + + +Fig. 21--Diagram of network of capillaries between a very small artery and + a very small vein. Shading indicates the change of color of the blood as + it passes through the capillaries. _S._ Places between capillaries + occupied by the cells. + + +*Capillaries.*--The capillaries consist of a network of minute blood +vessels which connect the terminations of the smallest arteries with the +beginnings of the smallest veins (Fig. 21). They have an average diameter +of less than one two-thousandth of an inch (12 mu) and an average length of +less than one twenty-fifth of an inch (1 millimeter). Their walls consist +of a single coat which is continuous with the lining of the arteries and +veins. This coat is formed of a single layer of thin, flat cells placed +edge to edge (Fig. 22). With a few exceptions, the capillaries are found +in great abundance in all parts of the body. + + [Fig. 22] + + +Fig. 22--*Surface of capillary* highly magnified, showing its coat of thin + cells placed edge to edge. + + +*Functions of the Capillaries.*--On account of the thinness of their walls, +the capillaries are able to serve a twofold purpose in the body: + +1. They admit materials into the blood vessels. + +2. They allow materials to pass from the blood vessels to the surrounding +tissues. + +When it is remembered that the blood, as blood, does not escape from the +blood vessels under normal conditions, the importance of the work of the +capillaries is apparent. To serve its purpose as a carrier, there must be +places where the blood can load up with the materials which it is to +carry, and places also where these can be unloaded. Such places are +supplied by the capillaries. + +The capillaries also serve the purpose of spreading the blood out and of +bringing it very near the individual cells in all parts of the body (Fig. +21). + +*Functions of Arteries and Veins.*--While the capillaries provide the means +whereby materials may both enter and leave the blood, the arteries and +veins serve the general purpose of passing the blood from one set of +capillaries to another. Since pressure is necessary for moving the blood, +these tubes must connect with the source of the pressure, which is the +heart. In the arteries and veins the blood neither receives nor gives up +material, but having received or given up material at one set of +capillaries, it is then pushed through these tubes to where it can serve a +similar purpose in another set of capillaries (Fig. 23). + +*Divisions of the Circulation.*--Man, in common with all warm-blooded +animals, has a double circulation, a fact which explains the double +structure of his heart. The two divisions are known as the _pulmonary_ and +the _systemic_ circulations. By the former the blood passes from the right +ventricle through the lungs, and is then returned to the left auricle; by +the latter it passes from the left ventricle through all parts of the +body, returning to the right auricle. + +The general plan of the circulation is indicated in Fig. 23. All the blood +flows continuously through both circulations and passes the various parts +in the following order: right auricle, tricuspid valve, right ventricle, +right semilunar valve, pulmonary artery and its branches, capillaries of +the lungs, pulmonary veins, left auricle, mitral valve, left ventricle, +left semilunar valve, aorta and its branches, systemic capillaries, the +smaller veins, superior and inferior venae cavae, and then again into the +right auricle. + +In the pulmonary capillaries the blood gives up carbon dioxide and +receives oxygen, changing from a dark red to a bright red color. In the +systemic capillaries it gives up oxygen, receives carbon dioxide and other +impurities, and changes back to a dark red color. + +In addition to the two main divisions of the circulation, special circuits +are found in various places. Such a circuit in the liver is called the +_portal_ circulation, and another in the kidneys is termed the _renal_ +circulation. To some extent the blood supply to the walls of the heart is +also outside of the general movement; it is called the _coronary_ +circulation. + + [Fig. 23] + + + Fig. 23--*General scheme of the circulation*, showing places where the + blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3. + Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary + artery. 8. Aorta. + + +*Blood Pressure and Velocity.*--The blood, in obedience to physical laws, +passes continuously through the blood vessels, moving always from a place +of greater to one of less pressure. Through the contraction of the +ventricles, a relatively high pressure is maintained in the arteries +nearest the heart.(21) This pressure diminishes rapidly in the small +arteries, becomes comparatively slight in the capillaries, and falls +practically to nothing in the veins. Near the heart in the superior and +inferior venae cavae, the pressure at intervals is said to be _negative_. +This means that the blood from these veins is actually drawn into the +right auricle by the expansion of the chest walls in breathing.(22) + +The velocity of the blood is greatest in the arteries, less in the veins, +and _much_ less in the capillaries than in either the arteries or the +veins. The slower flow of the blood through the capillaries is accounted +for by the fact that their united area is many times greater than that of +the arteries which supply, or the veins which relieve, them. This allows +the same quantity of blood, flowing through them in a given time, a wider +channel and causes it to move more slowly. The time required for a +complete circulation is less than one minute. + +*Summary of Causes of Circulation.*--The chief factor in the circulation of +the blood is, of course, the heart. The ventricles keep a pressure on the +blood which is sufficient to force it through all the blood tubes and back +to the auricles. The heart is aided in its work by the elasticity of the +arteries, which keeps the blood under pressure while the ventricles are in +a state of relaxation. It is also aided by the muscles and elastic tissue +in all of the blood vessels. These, by keeping the blood vessels in a +state of "tone," or so contracted that their capacity just equals the +volume of the blood, enable pressure from the heart to be transmitted to +all parts of the blood stream. A further aid to the circulation is found +in the valves in the veins, which enable muscular contraction within the +body, and variable pressure upon its surface, to drive the blood toward +the heart. The heart is also aided to some extent by the movements of the +chest walls in breathing. The organs Of circulation are under the control +of the nervous system (Chapter XVIII). + + + +HYGIENE OF THE CIRCULATION + + +*Care of the Heart.*--The heart, consisting largely of muscle, is subject +to the laws of muscular exercise. It may be injured by over-exertion, but +is strengthened by a moderate increase in its usual work.(23) It may even +be subjected to great exertion without danger, if it be trained by +gradually increasing its work. Such training, by giving the heart time to +gain in size and strength, prepares it for tasks that could not at first +be accomplished. + +In taking up a new exercise requiring considerable exertion, precautions +should be observed to prevent an overstrain of the heart. The heart of the +amateur athlete, bicyclist, or mountain climber is frequently injured by +attempting more than the previous training warrants. The new work should +be taken up gradually, and feats requiring a large outlay of physical +energy should be attempted only after long periods of training. + +Since the heart is controlled by the nervous system, it frequently becomes +irregular in its action through conditions that exhaust the nervous +energy. Palpitations of the heart, the missing of beats, and pains in the +heart region frequently arise from this cause. It is through their effect +upon the nervous system that worry, overstudy, undue excitement, and +dissipation cause disturbances of the heart. In all such cases the remedy +lies in the removal of the cause. The nervous system should also be "toned +up" through rest, plenty of sleep, and moderate exercise in the open air. + +*Effect of Drugs.*--A number of substances classed as drugs, mainly by +their action on the nervous system, produce undesirable effects upon the +organs of circulation. Unfortunately some of these are extensively used, +alcohol being one of them. If taken in any but small quantities, alcohol +is a disturbing factor in the circulation. It increases the rate of the +heart beat and dilates the capillaries. Its effect upon the capillaries is +shown by the "bloodshot" eye and the "red nose" of the hard drinker. +Another bad effect from the use of much alcohol is the weakening of the +heart through the accumulation of fat around this organ and within the +heart muscle. The use of alcohol also leads in many cases to a hardening +of the walls of the arteries, such as occurs in old age. This effect makes +the use of alcohol especially dangerous for those in advanced years. + +Tobacco contains a drug, called nicotine, which has a bad effect upon the +heart in at least two ways: 1. When the use of tobacco is begun in early +life, it interferes with the growth of the heart, leading to its weakness +in the adult. 2. When used in considerable quantity, by young or old, it +causes a nervous condition both distressing and dangerous, known as +"tobacco heart." + +Tea and coffee contain a drug, called caffeine, which acts upon the +nervous system and which may, on this account, interfere with the proper +control of the heart. In some individuals the taking of a very small +amount of either tea or coffee is sufficient to cause irregularities in +the action of the heart. Tea is considered the milder of the two liquids +and the one less liable to injure. + +*Effect of Rheumatism.*--The disease which affects the heart more +frequently than any other is rheumatism. This attacks the lining membrane, +or endocardium, and causes, not infrequently, a shrinkage of the heart +valves. The heart is thus rendered defective and, to perform its function +in the body, must work harder than if it were in a normal condition. +Rheumatic attacks of the heart do most harm when they occur in early +life--the period when the valves are the most easily affected. Any tendency +toward rheumatism in children has, therefore, a serious significance and +should receive the attention of the physician. Any one having a defective +heart should avoid all forms of exercise that demand great exertion. + +*Strengthening of the Blood Vessels.*--Disturbances of the circulation, +causing too much blood to be sent to certain parts of the body and an +insufficient amount to others, when resulting from slight causes, are +usually due to weakness of the walls of the blood vessels, particularly of +the muscular coat. Such weakness is frequently indicated by extreme +sensitiveness to heat or cold and by a tendency to "catch cold." From a +health standpoint the preservation of the normal muscular "tone" of the +blood vessels is a problem of great importance. Though the muscles of the +blood vessels cannot be exercised in the same manner as the voluntary +muscles, they may be called actively into play through all the conditions +that induce changes in the blood supply to different parts of the body. +The usual forms of physical exercise necessitate such changes and +indirectly exercise the muscular coat. The exposure of the body to cold +for short intervals, because of the changes in the circulation which this +induces, also serves the same purpose. A cold bath taken with proper +precautions is beneficial to the circulation of many and so also is a +brisk walk on a frosty morning. Both indirectly exercise and strengthen +the muscular coat of the blood vessels. On the other hand, too much time +spent indoors, especially in overheated rooms, leads to a weakening of the +muscular coat and should be avoided. + +*Checking of Flow of Blood from Wounds.*--The loss of any considerable +quantity of blood is such a serious matter that every one should know the +simpler methods of checking its flow from wounds. In small wounds the flow +is easily checked by binding cotton or linen fiber over the place. The +absorbent cotton, sold in small packages at drug stores, is excellent for +this purpose and should be kept in every home. A simple method of checking +"nosebleed" is that of drawing air through the bleeding nostril, while the +other nostril is compressed with the finger.(24) Another method is to +"press with the finger (or insert a small roll of paper) under the lip +against the base of the nose." (25) Where the bleeding is persistent, the +nostril should be plugged with a small roll of clean cotton or paper. When +this is done, the plug should not be removed too soon because of the +likelihood of starting the flow afresh. + +In dealing with large wounds the services of a physician are +indispensable. But in waiting for the physician to arrive temporary aid +must be rendered. The one who gives such aid should first decide whether +an artery or a vein has been injured. This is easily determined by the +nature of the blood stream, which is in jets, or spurts, from an artery, +but flows steadily from a vein. If an artery is injured, the limb should +be tightly bandaged on the side of the wound nearest the heart; if a vein, +on the side farthest from the heart. In addition to this, the edges of the +wound should be closed and covered with cotton fiber and the limb should +be placed on a support above the level of the rest of the body. A large +handkerchief makes a convenient bandage if properly applied. This should +be folded diagonally and a knot tied in the middle. Opposite ends are then +tied, making a loose-fitting loop around the limb. The knot is placed +directly over the blood vessel to be compressed and a short stick inserted +in the loop. The necessary pressure is then applied by twisting the +handkerchief with the stick. Time must not be lost, however, in the +preparation of a suitable bandage. The blood vessel should be compressed +with the fingers while the bandage is being prepared. + +*Summary.*--The blood, to serve as a transporting agent, must be kept +continually moving through all parts of the body. The blood vessels hold +the blood, supply the channels and force necessary for its circulation, +and provide conditions which enable materials both to enter and to leave +the blood stream. The heart is the chief factor in propelling the blood, +although the muscles and the elastic tissue in the walls of the arteries +and the valves in the veins are necessary aids in the process. In the +capillaries the blood takes on and gives off materials, while the arteries +and veins serve chiefly as tubes for conveying the blood from one system +of capillaries to another. + +*Exercises.*--1. Of what special value in the study of the body was the +discovery of the circulation of the blood? + +2. State the necessity for a circulating liquid in the body. + +3. Show by a drawing the general plan of the heart, locating and naming +the essential parts. Show also the connection of the large blood vessels +with the cavities of the heart. + +4. Compare the purpose served by the chordae tendineae to that served by +doorstops (the strips against which the door strikes in closing). + +5. Explain how the heart propels the blood. To what class of pumps does it +belong? What special work is performed by each of its divisions? + +6. Define a valve. Of what use are the valves in the heart? In the veins? + +7. By what means is pressure from contracting muscles in different parts +of the body made to assist in the circulation? + +8. Of what advantage is the elasticity of the arteries? + +9. How is blood forced from the capillaries back to the heart? + +10. Why should there be a difference in structure between the two sides of +the heart? + +11. Following Fig. 23, trace the blood through a complete circulation, +naming all the divisions of the system in the order of the flow of the +blood. + +12. If the period of rest following the period of contraction of the heart +be as long as the period of contraction, how many hours is the heart able +to rest out of every twenty-four? + +13. State the functions of the capillaries. Show how their structure +adapts them to their work. + +14. What kind of physical exercise tends to strengthen the heart? What +forms of exercise tend to injure it? State the effects of alcohol and +tobacco on the heart. + +15. How may rheumatism injure the heart? + +16. Give directions for checking the flow of blood from small and from +large blood vessels. + + + +PRACTICAL WORK + + +In showing the relations of the different parts of the heart, a large +dissectible model is of great service (Fig. 24). Indeed, where the time of +the class is limited, the practical work may be confined to the study of +the heart model, diagrams of the heart and the circulation, and a few +simple experiments. However, where the course is more extended, the +dissection of the heart of some animal as described below is strongly +advised. + +*Observations on the Heart.*--Procure, by the assistance of a butcher, the +heart of a sheep, calf, or hog. To insure the specimen against mutilation, +the lungs and the diaphragm must be left attached to the heart. In +studying the different parts, good results will be obtained by observing +the following order: + +1. Observe the connection of the heart to the lungs, diaphragm, and large +blood vessels. Inflate the lungs and observe the position of the heart +with reference to them. + +2. Examine the sac surrounding the heart, called the _pericardium_. Pierce +its lower portion and collect the pericardial fluid. Increase the opening +thus made until it is large enough to slip the heart out through it. Then +slide back the pericardium until its connection with the large blood +vessels above the heart is found. Observe that a thin layer of it +continues down from this attachment, forming the outer covering of the +heart. + +3. Trace out for a short distance and study the veins and arteries +connected with the heart. The arteries are to be distinguished by their +thick walls. The heart may now be severed from the lungs by cutting the +large blood vessels, care being taken to leave a considerable length of +each one attached to the heart. + + [Fig. 24] + + + Fig. 24--Model for demonstrating the heart. + + +4. Observe the outside of the heart. The thick, lower portion contains the +cavities called _ventricles_; the thin, upper, ear-shaped portions are the +_auricles_. The thicker and denser side lies toward the left of the +animal's body and is called the _left_ side of the heart; the other is the +_right_ side. Locate the right auricle and the right ventricle; the left +auricle and the left ventricle. + +5. Lay the heart on the table with the front side up and the apex pointing +from the operator. This places the left side of the heart to his left and +the right side to his right. Notice the groove between the ventricles, +called the inter-ventricular groove. Make an incision half an inch to the +right of this groove and cut toward the base of the heart until the +pulmonary artery is laid open. Then, following within half an inch of the +groove, cut down and around the right side of the heart. The wall of the +right ventricle may now be raised and the cavity exposed. Observe the +extent of the cavity, its shape, its lining, its columns of muscles, its +half columns of muscles, its tendons (chordae tendineae), the tricuspid +valve from the under side, etc. Also notice the valve at the beginning of +the pulmonary artery (the right semilunar) and the sinuses, or +depressions, in the artery immediately behind its divisions. + +6. Now cut through the middle of the loosened ventricular wall from the +apex to the middle of the right auricle, laying it open for observation. +Observe the openings into the auricle, there being one each for the vena +cava superior, the vena cava inferior, and the coronary vein. Compare the +walls, lining, shape, size, etc., with the ventricle below. + +7. Cut off the end of the left ventricle about an inch above the apex. +This will show the extension of the cavity to the apex; it will also show +the thickness of the walls and the shape of the cavity. Split up the +ventricular wall far enough to examine the mitral valve and the chordae +tendineae from the lower side. + +8. Make an incision in the left auricle. Examine its inner surface and +find the places of entrance of the pulmonary veins. Examine the mitral +valve from above. Compare the two sides of the heart, part for part. + +9. Separate the aorta from the other blood vessels and cut it entirely +free from the heart, care being taken to leave enough of the heart +attached to the artery to insure the semilunar valve's being left in good +condition. After tying or plugging up the holes in the sides of the +artery, pour water into the small end and observe the closing of the +semilunar valve. Repeat the experiment until the action of the valve is +understood. Sketch the artery, showing the valve in a closed condition. + +*To illustrate the Action of a Ventricle.*--Procure a syringe bulb with an +opening at each end. Connect a rubber tube with each opening, letting the +tubes reach into two tumblers containing water. By alternately compressing +and releasing the bulb, water is pumped from one vessel into the other. +The bulb may be taken to represent one of the ventricles. What action of +the ventricle is represented by compressing the bulb? By releasing the +pressure? Show by a sectional drawing the arrangement of the valves in the +syringe bulb. + + [Fig. 25] + + + Fig. 25--Illustrating elasticity of arteries. + + +*To show the Advantage of the Elasticity of Arteries.*--Connect the syringe +bulb used in the last experiment with a rubber tube three or four feet in +length and having rather thin walls. In the opposite end of the rubber +tube insert a short glass tube which has been drawn (by heating) to a fine +point (Fig. 25). Pump water into the rubber tube, observing: + +1. The swelling of the tube (pulse) as the water is forced into it. (This +is best observed by placing the fingers on the tube.) + +2. The forcing of water from the pointed tubs during the interval when no +pressure is being applied from the bulb. Compare with the action of the +arteries when blood is forced into them from the ventricles. + +Repeat the experiment, using a long glass tube terminating in a point +instead of the rubber tube. (In fitting the glass tube to the bulb use a +very short rubber tube.) Observe and account for the differences in the +flow of water through the inelastic tube. + +*To show the Advantage of Valves in the Veins.*--Attach an open glass tube +one foot in length to each end of the rubber tube used in the preceding +experiment and fill with water (by sucking) to within about six inches of +the end. Lay on the table with the glass tubes secured in an upright +position (Fig. 26). Now compress the tube with the hand, noting that the +water rises in both tubes, being pushed in both directions. This effect is +similar to that produced on the blood when a vein having no valves is +compressed. + + [Fig. 26] + + + Fig. 26.--*Simple apparatus* for showing advantage of valves in veins. + + +Now imitate the action of a valve by clamping the tube at one point, or by +closing it by pressure from the finger, and then compressing with the hand +some portion of the tube on the table. Observe in this instance that the +water is *all* pushed in the same direction. The movement of the water is +now like the effect produced on the blood in veins having valves when the +veins are compressed. + +*To show the Position of the Valves in the Veins.*--Exercise the arm and +hand for a moment to increase the blood supply. Expose the forearm and +examine the veins on its surface. With a finger, stroke one of the veins +toward the heart, noting that, as the blood is pushed along on one side of +the finger the blood follows on the other side. Now stroke the vein toward +the hand. Places are found beyond which the blood does not follow the +finger. These mark the positions of valves. + +*To show Effect of Exercise upon the Circulation.*--1. With a finger on the +"pulse" at the wrist or temple, count the number of heart beats during a +period of one minute under the following conditions: (_a_) when sitting; +(_b_) when standing; (_c_) after active exercise, as running. What +relation, if any, do these observations indicate between the general +activity of the body and the work of the heart? + +2. Compare the size of the veins on the backs of the hands when they are +placed side by side on a table. Then exercise briskly the right hand and +arm, clenching and unclenching the fist and flexing the arm at the elbow. +Place the hands again side by side and, after waiting a minute, observe +the increase in the size of the veins in the hand exercised. How is this +accounted for? + +*To Show the Effect of Gravity on the Circulation.*--Hold one hand high +above the head, at the same time letting the other hand hang loosely by +the side. Observe the difference in the color of the hands and the degree +to which the large veins are filled. Repeat the experiment, reversing the +position of the hands. What results are observed? In what parts of the +body does gravity aid in the return of the blood to the heart? In what +parts does it hinder? Where fainting is caused by lack of blood in the +brain (the usual cause), is it better to let the patient lie down flat or +to force him into a sitting posture? + +*To study the Circulation in a Frog's Foot* (Optional).--A compound +microscope is needed in this study and for extended examination it is best +to destroy the frog's brain. This is done by inserting some blunt-pointed +instrument into the skull cavity from the neck and moving it about. A +small frog, on account of the thinness of its webs, gives the best +results. It should be attached to a thin board which has an opening in one +end over which the web of the foot may be stretched. Threads should extend +from two of the toes to pins driven into the board to secure the necessary +tension of the web, and the foot and lower leg should be kept moist. Using +a two-thirds-inch objective, observe the branching of the small arteries +into the capillaries and the union of the capillaries to form the small +veins. The appearance is truly wonderful, but allowance must be made for +the fact that the _motion_ of the blood is magnified, as well as the +different structures, and that it appears to move much faster than it +really does. With a still higher power, the movements of the corpuscles +through the capillaries may be studied. + +NOTE.--To perform this experiment without destroying the brain, the frog is +first carefully wrapped with strips of wet cloth and securely tied to the +board. The wrapping, while preventing movements of the frog, must not +interfere with the circulation. + + + + +CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY + + + [Fig. 27] + + + Fig. 27--*Diagram showing position of the lymph* with reference to the +blood and the cells. The central tube is a capillary. The arrows indicate + the direction of slight movements in the lymph. + + +The blood, it will be remembered, moves everywhere through the body in a +system of _closed_ tubes. These keep it from coming in contact with any of +the cells of the body except those lining the tubes themselves. The +capillaries, to be sure, bring the blood very near the cells of the +different tissues; still, there is need of a liquid to fill the space +between the capillaries and the cells and to transfer materials from one +to the other. The lymph occupies this position and does this work. The +position of the lymph with reference to the capillaries and the cells is +shown in Fig. 27. + +*Origin of the Lymph.*--The chief source of the lymph is the plasma of the +blood. As before described, the walls of the capillaries consist of a +single layer of flat cells placed edge to edge. Partly on account of the +pressure upon the blood and partly on account of the natural tendency of +liquids to pass through animal membranes, a considerable portion of the +plasma penetrates the thin walls and enters the spaces occupied by the +lymph. + +The cells themselves also help to form the lymph, since the water and +wastes leaving the cells add to its bulk. These mix with the plasma from +the blood, forming the resultant liquid which is the lymph. A considerable +amount of the material absorbed from the food canal also enters the lymph +tubes, but this passes into the blood before reaching the cells. + +*Composition and Physical Properties of the Lymph.*(26)--As would naturally +be expected, the composition of the lymph is similar to that of the blood. +In fact, nearly all the important constituents of the blood are found in +the lymph, but in different proportions. Food materials for the cells are +present in smaller amounts than in the blood, while impurities from the +cells are in larger amounts. As a rule the red corpuscles are absent from +the lymph, but the white corpuscles are present and in about the same +numbers as in the blood. + +The physical properties of the lymph are also similar to those of the +blood. Like the blood, the lymph is denser than water and also coagulates, +but it coagulates more slowly than does the blood. The most noticeable +difference between these liquids is that of color, the lymph being +colorless. This is due to the absence of red corpuscles. The quantity of +lymph is estimated to be considerably greater than that of the blood. + +*Lymph Vessels.*--Most of the lymph lies in minute cavities surrounding the +cells and in close relations with the capillaries (Figs. 27 and 30). These +are called _lymph spaces_. Connecting with the lymph spaces on the one +hand, and with certain blood vessels on the other, is a system of tubes +that return the lymph to the blood stream. The smallest of these, and the +ones in greatest abundance, are called _lymphatics_. They consist of +slender, thin-walled tubes, which resemble veins in structure, and, like +the veins, have valves. They differ from veins, however, in being more +uniform in size and in having thinner walls. + + [Fig. 28] + + + Fig. 28--*Diagram of drainage system for the lymph.* 1. Thoracic duct. 2. + Right lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein. + 5. Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes + connecting with the lymph spaces in all parts of the body are the + lymphatics. + + +The lymphatics in different places gradually converge toward, and empty +into, the two main lymph tubes of the body. The smaller of these tubes, +called the _right lymphatic duct_, receives the lymph from the lymphatics +in the right arm, the right side of the head, and the region of the right +shoulder. It connects with, and empties its contents into, the right +subclavian vein at the place where it is joined by the right jugular vein +(Fig. 28). + +The larger of the lymph tubes is called the _thoracic duct_. This receives +lymph from all parts of the body not drained by the right lymphatic duct, +and empties it into the left subclavian vein. Connection is made with the +subclavian vein on the upper side at the place where it is joined by the +left jugular vein. The thoracic duct has a length of from sixteen to +eighteen inches, and is about as large around as a goose quill. The lower +end terminates in an enlargement in the abdominal cavity, called the +_receptacle of the chyle_. It is provided with valves throughout its +course, in addition to one of considerable size which guards the opening +into the blood vessel. + +The lymphatics which join the thoracic duct from the small intestine are +called the _lacteals_ (Fig. 28). These do not differ in structure from the +lymphatics in other parts of the body, but they perform a special work in +absorbing the digested fat (Chapter XI). + +*Lymphatic Glands.*--The lymphatic glands, sometimes called lymph nodes, +are small and somewhat rounded bodies situated along the course of the +lymphatic tubes. They vary in size, some of them being an inch or more in +length. The lymph vessels generally open into them on one side and leave +them on the other (Figs. 28 and 30). They are not glands in function, but +are so called because of their having the general form of glands. They +provide favorable conditions for the development of white corpuscles (page +29). They also separate harmful germs and poisonous wastes from the lymph, +thereby preventing their entrance into the blood. + +*Relations of the Lymph, the Blood, and the Cells.*--While the blood is +necessary as a carrying, or transporting, agent in the body, the lymph is +necessary for transferring materials from the blood to the cells and _vice +versa_. Serving as a physiological "go between," or medium of exchange, +the lymph enables the blood to minister to the needs of the cells. But the +lymph and the blood, everything considered, can hardly be looked upon as +two separate and distinct liquids. Not only do they supplement each other +in their work and possess striking similarities, but each is made in its +movements to pass into the vessels occupied by the other, so that they are +constantly mixing and mingling. For these and other reasons, they are more +properly regarded as two divisions of a single liquid--one which, by +adapting itself to different purposes,(27) supplies all the conditions of +a nutrient fluid for the cells. + +*Movements of the Lymph.*--As compared with the blood, the lymph must be +classed as a quiet liquid. But, as already suggested, it has certain +movements which are necessary to the purposes which it serves. A careful +study shows it to have three well-defined movements as follows: + +1. A movement from the capillaries toward the cells. + +2. A movement from the cells toward the capillaries. + +3. A movement of the entire body of lymph from the lymph spaces into the +lymphatics and along these channels to the ducts through which it enters +the blood. + +By the first movement the cells receive their nourishment. By the second +and third movements the lymph, more or less laden with impurities, is +returned to the blood stream. (See Figs. 28 and 30.) + +*Causes of the Lymph Movements.*--Let us consider first the movement +through the lymph tubes. No pump, like the heart, is known to be connected +with these tubes and to supply the pressure necessary for moving the +lymph. There are, however, several forces that indirectly aid in its flow. +The most important of these are as follows: + +1. _Blood Pressure at the Capillaries._--The plasma which is forced through +the capillary walls by pressure from the heart makes room for itself by +pushing a portion of the lymph out of the lymph spaces. This in turn +presses upon the lymph in the tubes which it enters. In this way pressure +from the heart is transmitted to the lymph, forcing it to move. + +2. _Variable Pressure on the Walls of the Lymph Vessels._--Pressure exerted +on the sides of the lymph tubes by contracting muscles tends to close them +up and to push the lymph past the valves, which, by closing, prevent its +return (Fig. 29). Pressure at the surface of the body, provided that it is +variable, also forces the lymph along. The valves in the lymph vessels +serve the same purpose as those in the veins. + + [Fig. 29] + + +Fig. 29--*Diagram* to show how the muscles pump lymph. _A._ Relaxed muscle + beside which is a lymphatic tube. _B._ Same muscle in state of + contraction. + + +3. _The Inspiratory Force._--When the thoracic cavity is enlarged in +breathing, the unbalanced atmospheric pressure is exerted from all +directions towards the thoracic space. This not only causes the air to +flow into the lungs (Chapter VII), but also causes a movement of the blood +and lymph in such of their tubes as enter this cavity. It will be noted +that both of the large lymph ducts terminate where their contents may be +influenced by the respiratory movements. (See Practical Work.) + +*Where the Lymph enters the Blood.*--The fact that the lymph is poured into +the blood at but two places, and these very close to each other, requires +a word of explanation. As a matter of fact, it is impossible for the lymph +to flow into blood vessels at most places on account of the blood +pressure. This would force the blood into the lymph vessels, instead of +allowing the lymph to enter the blood. The lymph can enter only at some +place where the blood pressure is less than the pressure that moves the +lymph. Such a place is found in the thoracic cavity. As already pointed +out (page 54), the blood pressure in the veins entering this cavity +becomes, with each expansion of the chest, negative, i.e., less than the +pressure of the atmosphere on the outside of the body. This, as we have +seen, aids in the flow of the blood into the right auricle. It also aids +in the passage of lymph into the blood vessels. The lymph is said to be +"sucked in," which means that it is forced in by the unbalanced pressure +of the atmosphere.(28) Some advantage is also gained by the lymph duct's +entering the subclavian vein on the upper side and at its union with the +jugular vein. Everything considered, it is found that the lymph flows into +the blood vessels where it can be "drawn in" by the movements of breathing +and where it meets with no opposition from the blood stream itself (Fig. +30). + + [Fig. 30] + + + Fig. 30--*Diagram* showing general movement of lymph from the place of + relatively high pressure at the lymph spaces to the place of relatively + low pressure in the thoracic cavity. + + +*Lymph Movements at the Cells.*--The double movement of the lymph from the +capillaries toward the cells and from the cells toward the capillaries is +not entirely understood. Blood pressure in the capillaries undoubtedly has +much to do in forcing the plasma through the capillary walls, but this +tends to prevent the movement of the lymph in the opposite direction. +Movements between the blood and the lymph are known to take place in part +according to a general principle, known as _osmosis_, or dialysis. + + [Fig. 31] + + + Fig. 31--*Vessel* with an upright membranous partition for illustrating + osmosis. + + +*Osmosis.*--The term "osmosis" is used to designate the passage of liquids +through some partition which separates them. Thus, if a vessel with an +upright membranous partition be filled on the one side with pure water and +on the other with water containing salt, an exchange of materials will +take place through the membrane until the same proportion of salt exists +on the two sides (Fig. 31). The cause of osmosis is the motion of the +molecules, or minute particles, that make up the liquid substance. If the +partition were not present, this motion would simply cause a mixing of the +liquids. + +*Conditions under which Osmosis occurs.*--Osmosis may be shown by suitable +experiments (see Practical Work) to take place under the following +conditions: + +1. The liquids on the two sides of the partition must be _unlike_ either +in density or in composition. Since the effect of the movement is to +reduce the liquids to the same condition, _a difference in density causes +the flow to be greater from the less dense toward the denser liquid_, than +in the opposite direction; while _a difference in composition causes the +substances in solution to move from the place of greater abundance toward +places of less abundance_. + +2. The liquids must be capable of wetting, or penetrating, the partition. +If but one of the liquids penetrates the partition, the flow will be in +but one direction. + +3. The liquids on the two sides of the partition must readily mix with +each other. + +*Osmosis at the Cells.*--In the body osmosis takes place between the blood +and the lymph and between the lymph and the cells, the movements being +through the capillary walls and the membranes inclosing the cells (Fig. +27). Oxygen and food materials, which are found in great abundance in the +blood, are less abundant in the lymph and still less abundant in the +cells. According to the principle of osmosis, the main flow of oxygen and +food is from the capillaries toward the cells. On the other hand, the +wastes are most abundant in the cells where they are formed, less abundant +in the lymph, and least abundant in the blood. Hence the wastes flow from +the cells toward the capillaries. + +*Solutions.*--Neither the blood plasma nor the lymph, as already shown, are +simple liquids; but they consist of water and different substances +dissolved in the water. They belong to a class of substances called +_solutions_. The chief point of interest about substances in solution is +that they are very finely divided and that their little particles are free +to move about in the liquid that contains them. Both the motion and the +finely divided condition of the dissolved substances are necessary to the +process of osmosis. All substances, however, that appear to be in solution +are not able to penetrate membranes, or take part in osmosis. + +*Kinds of Solutions in the Body.*--The substances in solution in the body +liquids are of two general kinds known as _colloids_ and _crystalloids_. +The crystalloids are able to pass through membranous partitions, while the +colloids are not. An example of a colloid is found in the albumin of an +egg, which is unable to penetrate the membrane which surrounds it. +Examples of crystalloids are found in solutions of salt and sugar in +water. The inability of a colloid to penetrate a membrane is due to the +fact that it does not form a true solution. Its particles (molecules), +instead of being completely separated, still cling together, forming +little masses that are too large to penetrate the membrane. Since, +however, it has the appearance, on being mixed with water, of being +dissolved, it is called a _colloidal solution_. The crystalloid substance, +on the other hand, completely separates in the water and forms a _true +solution_--one which is able to penetrate the partition or membrane. + +*Osmosis not a Sufficient Cause.*--The passage of materials through animal +membranes, according to the principle of osmosis, is limited to +crystalloid substances. But colloid substances are also known to pass +through the various partitions of the body. An example of such is found in +the proteids of the blood which, as a colloidal solution, pass through the +capillary walls to become a part of the lymph. Perhaps the best +explanation offered as yet for this passage is that the colloidal +substances are changed by the cells lining the capillaries into substances +that form true solutions and that after the passage they are changed back +again to the colloidal condition. + +*Summary.*--Between the cells and the capillaries is a liquid, known as the +lymph, which is similar in composition and physical properties to the +blood. It consists chiefly of escaped plasma. The vessels that contain it +are connected with the system for the circulation of the blood. By adding +new material to the lymph and withdrawing waste material from it, the +blood keeps this liquid in a suitable condition for supplying the needs of +the cells. Supplementing each other in all respects, the blood and the +lymph together form the nutrient cell fluid of the body. The interchange +of material between the blood and the lymph, and the lymph and the cells, +takes place in part according to the principle of osmosis. + +*Exercises.*--1. Explain the necessity for the lymph in the body. + +2. Compare lymph and water with reference to density, color, and +complexity of composition. + +3. Compare lymph and blood with reference to color, composition, and +movement through the body. + +4. Show how blood pressure in the capillaries causes a flow of the lymph. + +5. Show how contracting muscles cause the lymph to move. Compare with the +effect of muscular contraction upon the blood in the veins. + +6. Trace the lymph in its flow from the right hand to where it enters the +blood; from the feet to where it enters the blood. + +7. What conditions prevail at the cells to cause a movement of food and +oxygen in one direction and of waste materials in the opposite direction? + +8. What part does water play in the exchanges at the cells? + +9. Show that the blood and the lymph together fulfill all the requirements +of a nutrient cell fluid in the body. + + + +PRACTICAL WORK + + +*To illustrate the Effect of Breathing upon the Flow of Lymph.*--Tightly +holding one end of a glass tube between the lips, let the other end extend +into water in a tumbler on a table. In this position quickly inhale air +through the nostrils, noting that with each inhalation there is a slight +movement of the water up the tube. (No sucking action should be exerted by +the mouth.) Apply to the movements in the large blood and lymph vessels +entering the thoracic cavity. + +*To illustrate Osmosis.*--1. Separate the shell from the lining membrane at +one end of an egg, over an area about one inch in diameter. To do this +without injuring the membrane, the shell must first be broken into small +pieces and then picked off with a pair of forceps, or a small knife blade. +Fit a small glass tube, eight or ten inches long, into the other end so +that it will penetrate the membrane and pass down into the yolk. Securely +fasten the tube to the shell by melting beeswax around it, and set the egg +in a small tumbler partly filled with water. Examine in the course of half +an hour. What evidence now exists that the water has passed through the +membrane? + +2. Tie over the large end of a "thistle tube" (used by chemists) a thin +animal membrane, such as a piece of the pericardium or a strip of the +membrane from around a sausage. Then fill the bulb and the lower end of +the tube with a concentrated solution of some solid, such as sugar, salt, +or copper sulphate. Suspend in a vessel of water so that the liquid which +it contains is just on a level with the water in the vessel. Examine from +time to time, looking for evidence of a movement in each direction through +the membrane. Why should the movement of the water into the tube be +greater than the movement in the opposite direction? (If the thistle tube +has a very slender stem, it is better to fill the bulb before tying on the +membrane. The opening in the stem may be plugged during the process of +filling.) + + [Fig. 32] + + + Fig. 32--An osmosometer. + + +NOTE.--With a special piece of apparatus, known as an _osmosometer_, the +principle of osmosis may be more easily illustrated than by the method in +either of the above experiments (Fig. 32). This apparatus may be obtained +from supply houses. + + + + +CHAPTER VII - RESPIRATION + + +Through the movements of the blood and the lymph, materials entering the +body are transported to the cells, and wastes formed at the cells are +carried to the organs which remove them from the body. We are now to +consider the passage of materials from outside the body to the cells and +_vice versa_. One substance which the body constantly needs is oxygen, and +one which it is constantly throwing off is carbon dioxide. Both of these +are constituents of + +*The Atmosphere.*--The atmosphere, or air, completely surrounds the earth +as a kind of envelope, and comes in contact with everything upon its +surface. It is composed chiefly of oxygen and nitrogen,(29) but it also +contains a small per cent of other substances, such as water-vapor, carbon +dioxide, and argon. All of the regular constituents of the atmosphere are +gases, and these, as compared with liquids and solids, are very light. +Nevertheless the atmosphere has weight and, on this account, exerts +pressure upon everything on the earth. At the sea level, its pressure is +nearly fifteen pounds to the square inch. The atmosphere forms an +essential part of one's physical environment and serves various purposes. +The process by which gaseous materials are made to pass between the body +and the atmosphere is known as + +*Respiration.*--As usually defined, respiration, or breathing, consists of +two simple processes--that of taking air into special contrivances in the +body, called the lungs, and that of expelling air from the lungs. The +first process is known as _inspiration_; the second as _expiration_. We +must, however, distinguish between respiration by the lungs, called +_external respiration_, and respiration by the cells, called _internal +respiration_. + +_The purpose of respiration_ is indicated by the changes that take place +in the air while it is in the lungs. Air entering the lungs in ordinary +breathing parts with about five per cent of itself in the form of oxygen +and receives about four and one half per cent of carbon dioxide, +considerable water-vapor, and a small amount of other impurities. These +changes suggest a twofold purpose for respiration: + +1. To obtain from the atmosphere the supply of oxygen needed by the body. + +2. To transfer to the atmosphere certain materials (wastes) which must be +removed from the body. + +The chief organs concerned in the work of respiration are + +*The Lungs.*--The lungs consist of two sac-like bodies suspended in the +thoracic cavity, and occupying all the space not taken up by the heart. +They are not simple sacs, however, but are separated into numerous +divisions, as follows: + +1. The lung on the right side of the thorax, called the right lung, is +made up of three divisions, or _lobes_, and the left lung is made up of +two lobes. + +2. The lobes on either side are separated into smaller divisions, called +_lobules_ (Fig. 33). Each lobule receives a distinct division of an air +tube and has in itself the structure of a miniature lung. + + [Fig. 33] + + +Fig. 33--*Lungs and air passages* seen from the front. The right lung shows + the lobes and their divisions, the lobules. The tissue of the left lung + has been dissected away to show the air tubes. + + +3. In the lobule the air tube divides into a number of smaller tubes, each +ending in a thin-walled sac, called an _infundibulum_. The interior of the +infundibulum is separated into many small spaces, known as the _alveoli_, +or air cells. + +The lungs are remarkable for their lightness and delicacy of +structure.(30) They consist chiefly of the tissues that form their sacs, +air tubes, and blood vessels; the membranes that line their inner and +outer surfaces; and the connective tissue that binds these parts together. +All these tissues are more or less elastic. The relation of the different +parts of the lungs to each other and to the outside atmosphere will be +seen through a study of the + +*Air Passages.*--The air passages consist of a system of tubes which form a +continuous passageway between the outside atmosphere and the different +divisions of the lungs. The air passes through them as it enters and +leaves the lungs, a fact which accounts for the name. + + [Fig. 34] + + + Fig. 34--*Model of section through the head*, showing upper air passages + and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of + mouth. 4. Larynx. 5. Trachea. 6. Esophagus. + + +The incoming air first enters the _nostrils_. These consist of two narrow +passages lying side by side in the nose, and connecting with the pharynx +behind. The lining of the nostrils, called _mucous membrane_ is quite +thick, and has its surface much extended by reason of being spread over +some thin, scroll-shaped bones that project into the passage. This +membrane is well supplied with blood vessels and secretes a considerable +quantity of liquid. Because of the nature and arrangement of the membrane, +the nostrils are able to _warm_ and _moisten_ the incoming air, and to +_free it from dust particles_, preparing it, in this way, for entrance +into the lungs (Fig. 34). + +The nostrils are separated from the mouth by a thin layer of bone, and +back of both the mouth and the nostrils is the pharynx. The _pharynx_ and +the _mouth_ serve as parts of the food canal, as well as air passages, and +are described in connection with the organs of digestion (Chapter X). Air +entering the pharynx, either by the nostrils or by the mouth, passes +through it into the _larynx_. The larynx, being the special organ for the +production of the voice, is described later (Chapter XXI). The entrance +into the larynx is guarded by a movable lid of cartilage, called the +_epiglottis_, which prevents food particles and liquids, on being +swallowed, from passing into the lower air tubes. The relations of the +nostrils, mouth, pharynx, and larynx are shown in Fig. 34. + +From the larynx the air enters the _trachea_, or windpipe. This is a +straight and nearly round tube, slightly less than an inch in diameter and +about four and one half inches in length. Its walls contain from sixteen +to twenty C-shaped, cartilaginous rings, one above the other and +encircling the tube. These incomplete rings, with their openings directed +backward, are held in place by thin layers of connective and muscular +tissue. At the lower end the trachea divides into two branches, called the +bronchi, each of which closely resembles it in structure. Each _bronchus_ +separates into a number of smaller divisions, called the _bronchial +tubes_, and these in turn divide into still smaller branches, known as the +_lesser bronchial tubes_ (Fig. 33). The lesser bronchial tubes, and the +branches into which they separate, are the smallest of the air tubes. One +of these joins, or expands into, each of the minute lung sacs, or +infundibula. Mucous membrane lines all of the air passages. + +*General Condition of the Air Passages.*--One necessary condition for the +movement of the air into and from the lungs is an unobstructed +passageway.(31) The air passages must be kept open and free from +obstructions. They are _kept open_ by special contrivances found in their +walls, which, by supplying a degree of stiffness, cause the tubes to keep +their form. In the trachea, bronchi, and larger bronchial tubes, the +stiffness is supplied by rings of cartilage, while in the smaller tubes +this is replaced by connective and muscular tissue. The walls of the +larynx contain strips and plates of cartilage; while the nostrils and the +pharynx are kept open by their bony surroundings. + + [Fig. 35] + + +Fig. 35--*Ciliated epithelial cells.* _A._ Two cells highly magnified. _c._ + Cilia, _n._ Nucleus. _B._ Diagram of a small air tube showing the lining + of cilia. + + +The air passages are _kept clean_ by cells especially adapted to this +purpose, known as the _ciliated epithelial cells_. These are slender, +wedge-shaped cells which have projecting from a free end many small, +hair-like bodies, called _cilia_ (Fig. 35). They line the mucous membrane +in most of the air passages, and are so placed that the cilia project into +the tubes. Here they keep up an inward and outward wave-like movement, +which is quicker and has greater force in the _outward_ direction. By this +means the cilia are able to move small pieces of foreign matter, such as +dust particles and bits of partly dried mucus, called phlegm, to places +where they can be easily expelled from the lungs.(32) + + [Fig. 36] + + +Fig. 36--*Terminal air sacs.* The two large sacs are infundibula; the small + divisions are alveoli. (Enlarged.) + + +*The Alveoli.*--The alveoli, or air cells, are the small divisions of the +infundibula (Fig. 36). They are each about one one-hundredth of an inch +(1/4 mm.) in diameter, being formed by the infolding of the infundibular +wall. This wall, which has for its framework a thin layer of elastic +connective tissue, supports a dense network of capillaries (Fig. 37), and +is lined by a single layer of cells placed edge to edge. By this +arrangement the air within the alveoli is brought very near a large +surface of blood, and the exchange of gases between the air and the blood +is made possible. It is at the alveoli that the oxygen passes from the air +into the blood, and the carbon dioxide passes from the blood into the air. +At no place in the lungs, however, do the air and the blood come in direct +contact. Their exchanges must in all cases take place through the +capillary walls and the layer of cells lining the alveoli. + + [Fig. 37] + + +Fig. 37--*Inner lung surface (magnified)*, the blood vessels injected with + coloring matter. The small pits are alveoli, and the vessels in their + walls are chiefly capillaries. + + + [Fig. 38] + + +Fig. 38.--*Diagram to show the double movement of air and blood through the +lungs.* The blood leaves the heart by the pulmonary artery and returns by + the pulmonary veins. The air enters and leaves the lungs by the same + system of tubes. + + + [Fig. 39] + + + Fig. 39--*Diagram to show air and blood movements in a terminal air sac.* + While the air moves into and from the space within the sac, the blood + circulates through the sac walls. + + +*Blood Supply to the Lungs.*--To accomplish the purposes of respiration, +not only the air, but the blood also, must be passed into and from the +lungs. The chief artery conveying blood to the lungs is the _pulmonary +artery_. This starts at the right ventricle and by its branches conveys +blood to the capillaries surrounding the alveoli in all parts of the +lungs. The branches of the pulmonary artery lie alongside of, and divide +similarly to, the bronchial tubes. At the places where the finest +divisions of the air tubes enter the infundibula, the little arteries +branch into the capillaries that penetrate the infundibular walls (Figs. +38 and 39). From these capillaries the blood is conveyed by the pulmonary +veins to the left auricle. + +The lungs also receive blood from two (in some individuals three) small +arteries branching from the aorta, known as the _bronchial arteries_. +These convey to the lungs blood that has already been supplied with +oxygen, passing it into the capillaries in the walls of the bronchi, +bronchial tubes, and large blood vessels, as well as the connective tissue +between the lobes of the lungs. This blood leaves the lungs partly by the +bronchial veins and partly by the pulmonary veins. No part of the body is +so well supplied with blood as the lungs. + + [Fig. 40] + + +Fig. 40--*The pleurae.* Diagram showing the general form of the pleural sacs +as they surround the lungs and line the inner surfaces of the chest (other + parts removed). _A, A'._ Places occupied by the lungs. _B, B'._ Slight + space within the pleural sacs containing the pleural secretion, _a, a'._ + Outer layer of pleura and lining of chest walls and upper surface of +diaphragm. _b, b'._ Inner layer of pleura and outer lining of lungs. _C._ + Space occupied by the heart. _D._ Diaphragm. + + +*The Pleura.*--The pleura is a thin, smooth, elastic, and tough membrane +which covers the outside of the lungs and lines the inside of the chest +walls. The covering of each lung is continuous with the lining of the +chest wall on its respective side and forms with it a closed sac by which +the lung is surrounded, the arrangement being similar to that of the +pericardium. Properly speaking, there are two pleurae, one for each lung, +and these, besides inclosing the lungs, partition off a middle space which +is occupied by the heart (Fig. 40). They also cover the upper surface of +the diaphragm, from which they deflect upward, blending with the +pericardium. A small amount of liquid is secreted by the pleura, which +prevents friction as the surfaces glide over each other in breathing. + +*The Thorax.*--The force required for breathing is supplied by the box-like +portion of the body in which the lungs are placed. This is known as the +thorax, or chest, and includes that part of the trunk between the neck and +the abdomen. The space which it incloses, known as the thoracic cavity, is +a _variable_ space and the walls surrounding this space are _air-tight._ A +framework for the thorax is supplied by the ribs which connect with the +spinal column behind and with the sternum, or breast-bone, in front. They +form joints with the spinal column, but connect with the sternum by strips +of cartilage. The ribs do not encircle the cavity in a horizontal +direction, but slope downward from the spinal column both toward the front +and toward the sides, this being necessary to the service which they +render in breathing. + +*How Air is Brought into and Expelled from the Lungs.*--The principle +involved in breathing is that air flows from a place of _greater_ to a +place of _less_ pressure. The construction of the thorax and the +arrangement of the lungs within it provide for the application of this +principle in a most practical manner. The lungs are suspended from the +upper portion of the thoracic cavity, and the trachea and the upper air +passages provide the only opening to the outside atmosphere. Air entering +the thorax must on this account pass into the lungs. As the thorax is +enlarged the air in the lungs expands, and there is produced within them a +place of _slightly less_ air pressure than that of the atmosphere on the +outside of the body. This difference causes the air to flow into the +lungs. + + [Fig. 41] + + + Fig. 41--*Diagram illustrating the bellows principle in breathing.* _A._ + The human bellows. _B._ The hand bellows. Compare part for part. + + +When the thorax is diminished in size, the air within the lungs is +slightly compressed. This causes it to become denser and to exert on this +account a pressure _slightly greater_ than that of the atmosphere on the +outside. The air now flows out until the equality of the pressure is again +restored. Thus the thorax, by making the pressure within the lungs first +slightly less and then slightly greater than the atmospheric pressure, +causes the air to move into and out of the lungs. + +Breathing is well illustrated by means of the common hand bellows, its +action being similar to that of the thorax. It will be observed that when +the sides are spread apart air flows into the bellows. When they are +pressed together the air flows out. If an air-tight sack were hung in the +bellows with its mouth attached to the projecting tube, the arrangement +would resemble closely the general plan of the breathing organs (Fig. 41). +One respect, however, in which the bellows differs from the thorax should +be noted. The thorax is never sufficiently compressed to drive out all the +air. Air is always present in the lungs. This keeps them more or less +distended and pressed against the thoracic walls. + +*How the Thoracic Space is Varied.*--One means of varying the size of the +thoracic cavity is through the movements of the ribs and their resultant +effect upon the walls of the thorax. In bringing about these movements the +following muscles are employed: + +1. The _scaleni_ muscles, three in number on each side, which connect at +one end with the vertebrae of the neck and at the other with the first and +second ribs. Their contraction slightly raises the upper portion of the +thorax. + +2. The _elevators of the ribs_, twelve in number on each side, which are +so distributed that each single muscle is attached, at one end, to the +back portion of a rib and, at the other, to a projection of the vertebra a +few inches above. The effect of their contraction is to' elevate the +middle portion of the ribs and to turn them outward or spread them apart. + +3. The _intercostal_ muscles, which form two thin layers between the ribs, +known as the _internal_ and the _external_ intercostal muscles. The +external intercostals are attached between the outer lower margin of the +rib above and the outer upper margin of the rib below, and extend +obliquely downward and forward. The internal intercostals are attached +between the inner margins of adjacent ribs, and they extend obliquely +downward and backward from the front. The contraction of the external +intercostal muscles raises the ribs, and the contraction of the internal +intercostals tends to lower them. + + [Fig. 42] + + + Fig. 42--*Simple apparatus* for illustrating effect of movements of the + ribs upon the thoracic space; strips of cardboard held together by pins, + the front part being raised or lowered by threads moving through + attachments at 1 and 2. As the front is raised the space between the + uprights is increased. The front upright corresponds to the breastbone, +the back one to the spinal column, the connecting strips to the ribs, and + the threads to the intercostal muscles. + + +By slightly raising and spreading apart the ribs the thoracic space is +increased in two directions--from front to back and from side to side. +Lowering and converging the ribs has, of course, the opposite effect (Fig. +42). Except in forced expirations the ribs are lowered and converged by +their own weight and by the elastic reaction of the surrounding parts. + +*The Diaphragm.*--Another means of varying the thoracic space is found in +an organ known as the diaphragm. This is the dome-shaped, _movable +partition_ which separates the thoracic cavity from the cavity of the +abdomen. The edges of the diaphragm are firmly attached to the walls of +the trunk, and the center is supported by the pericardium and the pleura. +The outer margin is muscular, but the central portion consists of a strong +sheet of connective tissue. By the contraction of its muscles the +diaphragm is pulled down, thereby increasing the thoracic cavity. By +raising the diaphragm the thoracic cavity is diminished. + +The diaphragm, however, is not raised by the contraction of its own +muscles, but _is pushed up_ by the organs beneath. By the elastic reaction +of the abdominal walls (after their having been pushed out by the lowering +of the diaphragm), pressure is exerted on the organs of the abdomen and +these in turn press against the diaphragm. This crowds it into the +thoracic space. In forced expirations the muscles in the abdominal walls +contract to push up the diaphragm. + +*Interchange of Gases in the Lungs.*--During each inspiration the air from +the outside fills the entire system of bronchial tubes, but the alveoli +are largely filled, at the same time, by the air which the last expiratory +effort has left in the passages. By the action of currents and eddies and +by the rapid diffusion of gas particles, the air from the outside mixes +with that in the alveoli and comes in contact with the membranous walls. +Here the oxygen, after being dissolved by the moisture in the membrane, +diffuses into the blood. The carbon dioxide, on the other hand, being in +excess in the blood, diffuses toward the air in the alveoli. The +interchange of gases at the lungs, however, is not fully understood, and +it is possible that other forces than osmosis play a part. + + [Fig. 43] + + + Fig. 43--*Diagram* illustrating lung capacity. + + +*Capacity of the Lungs.*--The air which passes into and from the lungs in +ordinary breathing, called the _tidal_ air, is but a small part of the +whole amount of air which the lungs contain. Even after a forced +expiration the lungs are almost half full; the air which remains is called +the _residual_ air. The air which is expelled from the lungs by a forced +expiration, less the tidal air, is called the _reserve_, or supplemental, +air. These several quantities are easily estimated. (See Practical Work.) +In the average individual the total capacity of the lungs (with the chest +in repose) is about one gallon. In forced inspirations this capacity may +be increased about one third, the excess being known as the _complemental_ +air (Fig. 43). + + [Fig. 44] + + +Fig. 44--*Diagram* illustrating internal respiration and its dependence on + external respiration. (Modified from Hall.) (See text.) + + +*Internal, or Cell, Respiration.*--The oxygen which enters the blood in the +lungs leaves it in the tissues, passing through the lymph into the cells +(Fig. 44). At the same time the carbon dioxide which is being formed at +the cells passes into the blood. An exchange of gases is thus taking place +between the cells and the blood, similar to that taking place between the +blood and the air. This exchange is known as _internal_, or cell, +respiration. By internal respiration the oxygen reaches the place where it +is to serve its purpose, and the carbon dioxide begins its movement toward +the exterior of the body. This "breathing by the cells" is, therefore, +_the final and essential act of respiration_. Breathing by the lungs is +simply the means by which the taking up of oxygen and *the* giving off of +carbon dioxide by the cells is made possible. + + + +HYGIENE OF RESPIRATORY ORGANS + + +The liability of the lungs to attacks from such dread diseases as +consumption and pneumonia makes questions touching their hygiene of first +importance. Consumption does not as a rule attack sound lung tissue, but +usually has its beginning in some weak or enfeebled spot in the lungs +which has lost its "power of resistance." Though consumption is not +inherited, as some suppose, lung weaknesses may be transmitted from +parents to children. This, together with the fact, now generally +recognized, that consumption is contagious, accounts for the frequent +appearance of this disease in the same family. Consumption as well as +other respiratory affections can in the majority of cases be _prevented_, +and in many cases cured, by an intelligent observation of well-known laws +of health. + +*Breathe through the Nostrils.*--Pure air and plenty of it is the main +condition in the hygiene of the lungs. One necessary provision for +obtaining _pure air_ is that of breathing through the nostrils. Air is the +carrier of dust particles and not infrequently of disease germs.(33) +Partly through the small hairs in the nose, but mainly through the moist +membrane that lines the passages, the nostrils serve as filters for +removing the minute solid particles (Fig. 45). While it is important that +nose breathing be observed at all times, it is especially important when +one is surrounded by a dusty or smoky atmosphere. Otherwise the small +particles that are breathed in through the mouth may find a lodging place +in the lungs. + + [Fig. 45] + + + Fig. 45--*Human air filter.* Diagram of a section through the nostrils; + shows projecting bones covered with moist membrane against which the air +is made to strike by the narrow passages. 1. Air passages. 2. Cavities in + the bones. 3. Front lower portion of the cranial cavity. + + +In addition to removing dust particles and germs, other purposes are +served by breathing through the nostrils. The warmth and moisture which +the air receives in this way, prepare it for entering the lungs. Mouth +breathing, on the other hand, looks bad and during sleep causes snoring. +The habit of nose breathing should be established early in life.(34) + +*Cultivate Full Breathing.*--Many people, while apparently taking in +sufficient air to supply their need for oxygen, do not breathe deeply +enough to "freely ventilate the lungs." "Shallow breathing," as this is +called, is objectionable because it fails to keep up a healthy condition +of the entire lung surface. Portions of the lungs to which air does not +easily penetrate fail to get the fresh air and exercise which they need. +As a consequence, they become weak and, by losing their "power of +resistance," become points of attack in diseases of the lungs.(35) The +breathing of each individual should receive attention, and where from some +cause it is not sufficiently full and deep, the means should be found for +remedying the defect. + +*Causes of Shallow Breathing.*--Anything that impedes the free movement of +air into the lungs tends to cause shallow breathing A drooping of the back +or shoulders and a curved condition of the spinal column, such as is +caused by an improper position in sitting, interfere with the free +movements of the ribs and are recognized causes. Clothing also may impede +the respiratory movements and lead to shallow breathing. If too tight +around the chest, clothing interferes with the elevation of the ribs; and +if too tight around the waist, it prevents the depression of the +diaphragm. Other causes of shallow breathing are found in the absence of +vigorous exercise, in the leading of an indoor and inactive life, in +obstructions in the nostrils and upper pharynx, and in the lack of +attention to proper methods of breathing. + +To prevent shallow breathing one should have the habit of sitting and +standing erect. The clothing must not be allowed to interfere with the +respiratory movements. The taking of exercise sufficiently vigorous to +cause deep and rapid breathing should be a common practice and one should +spend considerable time out of doors. If one has a flat chest or round +shoulders, he should strive by suitable exercises to overcome these +defects. Obstructions in the nostrils or pharynx should be removed. + +*Breathing Exercises.*--In overcoming the habit of shallow breathing and in +strengthening the lungs generally, the practicing of occasional deep +breathing has been found most valuable and is widely recommended. With the +hands on the hips, the shoulders drawn back and _down_, the chest pushed +upward and forward, and the chin slightly depressed, draw the air slowly +through the nostrils until the lungs are _completely_ full. After holding +this long enough to count three slowly, expel it quickly from the lungs. +Avoid straining. To get the benefit of pure air, it is generally better to +practice deep breathing out of doors or before an open window. + +By combining deep breathing with simple exercises of the arms, shoulders, +and trunk much may be done towards straightening the spine, squaring the +shoulders, and overcoming flatness of the chest. Though such movements are +best carried on by the aid of a physical director, one can do much to help +himself. One may safely proceed on the principle that slight deformities +of the chest, spine, and shoulders are corrected by gaining and keeping +the natural positions, and may employ any movements which will loosen up +the parts and bring them where they naturally belong.(36) + +*Serious Nature of Colds.*--That many cases of consumption have their +beginning in severe colds (on the lungs) is not only a matter of popular +belief, but the judgment also of physicians. Though the cold is a +different affection from that of consumption, it may so lower the vitality +of the body and weaken the lung surfaces that the germs of consumption +find it easy to get a start. On this account a cold on the chest which +does not disappear in a few days, but which persists, causing more or less +coughing and pain in the lungs, must be given serious consideration.(37) +The usual home remedies failing to give relief, a physician should be +consulted. It should also be noted that certain diseases of a serious +nature (pneumonia, diphtheria, measles, etc.) have in their beginning the +appearance of colds. On this account it is wise not only to call a +physician, but to call him early, in severe attacks of the lungs. +Especially if the attack be attended by difficult breathing, fever, and a +rapid pulse is the case serious and medical advice necessary. + +*Ventilation.*--The process by which the air in a room is kept fresh and +pure is known as ventilation. It is a double process--that of bringing +fresh air into the room and that of getting rid of air that has been +rendered impure by breathing (38) or by lamps. Outdoor air is usually of a +different temperature (colder in winter, warmer in summer) from that +indoors, and as a consequence differs from it slightly in weight. On +account of this difference, suitable openings in the walls of buildings +induce currents which pass between the rooms and the outside atmosphere +even when there is no wind. In winter care must be taken to prevent drafts +and to avoid too great a loss of heat from the room. A cold draft may even +cause more harm to one in delicate health than the breathing of air which +is impure. To ventilate a room successfully the problem of preventing +drafts must be considered along with that of admitting the fresh air. + + [Fig. 46] + + + Fig. 46--Window adjusted for ventilation without drafts. + + +The method of ventilation must also be adapted to the construction of the +building, the plan of heating, and the condition of the weather. Specific +directions cannot be given, but the following suggestions will be found +helpful in ventilating rooms where the air is not warmed before being +admitted: + +1. _Introduce, the air through many small openings_ rather than a few +large ones. If the windows are used for this purpose, raise the lower sash +and drop the upper one _slightly_ for _several_ windows, varying the width +to suit the conditions (Fig. 46). By this means sufficient air may be +introduced without causing drafts. + +2. _Introduce the air at the warmest portions of the room._ The air +should, if possible, be warmed before reaching the occupants. + +3. If the wind is blowing, _ventilate principally on the sheltered side of +the house_. + +Ample provision should be made for fresh air in sleeping rooms, and here +again drafts must be avoided. Especially should the bed be so placed that +strong air currents do not pass over the sleeper. In schoolhouses and +halls for public gatherings the means for efficient ventilation should, if +possible, be provided in the general plan of construction and method of +heating. + + [Fig. 47] + + + Fig. 47--*Artificial respiration* as a laboratory experiment. Expiration. + Prone-posture method of Schaffer. + + +*Artificial Respiration.*--When natural breathing is temporarily suspended, +as in partial drowning, or when one has been overcome by breathing some +poisonous gas, the saving of life often depends upon the prompt +application of artificial respiration. This is accomplished by alternately +compressing and enlarging the thorax by means of variable pressure on the +outside, imitating the natural process as nearly as possible. Following is +the method proposed by Professor E.A. Schaffer of England, and called by +him "the _prone-posture_ method of artificial respiration": + +The patient is laid face downward with an arm bent under the head, and +_intermittent_ pressure applied vertically over the shortest ribs. The +pressure drives the air from the lungs, both by compressing the lower +portions of the chest and by forcing the abdominal contents against the +diaphragm, while the elastic reaction of the parts causes fresh air to +enter (Figs. 47 and 48). "The operator kneels or squats by the side of, or +across the patient, places his hands over the lowest ribs and swings his +body backward and forward so as to allow his weight to fall vertically on +the wrists and then to be removed; in this way hardly any muscular +exertion is required.... The pressure is applied gradually and slowly, +occupying some three seconds; it is then withdrawn during two seconds and +again applied; and so on some twelve times per minute."(39) + + [Fig. 48] + + + Fig. 48--Artificial respiration. Inspiration. + + +The special advantages of the prone-posture method over others that have +been employed are: I. It may be applied by a single individual and fora +long period of time without exhaustion. 2. It allows the mucus and water +(in case of drowning) to run out of the mouth, and causes the tongue to +fall forward so as not to obstruct the passageway. 3. It brings a +sufficient amount of air into the lungs.(40) + +While applying artificial respiration, the heat of the body should not be +allowed to escape any more than can possibly be helped. In case of +drowning, the patient should be wrapped in dry blankets or clothing, while +bottles of hot water may be placed in contact with the body. The +circulation should be stimulated, as may be done by rubbing the hands, +feet, or limbs in the direction of the flow of the blood in the veins. + +*Tobacco Smoke and the Air* Passages.--Smoke consists of minute particles +of unburnt carbon, or soot, such as collect in the chimneys of fireplaces +and furnaces. If much smoke is taken into the lungs, it irritates the +delicate linings and tends to clog them up. Tobacco smoke also contains +the poison nicotine, which is absorbed into the blood. For these reasons +the cigarette user who inhales the smoke does himself great harm, injuring +his nervous system and laying the foundation for diseases of the air +passages. The practice of smoking indoors is likewise objectionable, since +every one in a room containing the smoke is compelled to breathe it. + +*Alcohol and Diseases of the Lungs.*--Pneumonia is a serious disease of the +lungs caused by germs. The attacks occur as a result of exposure, +especially when the body is in a weakened condition. A noted authority +states that "alcoholism is perhaps the most potent predisposing cause" of +pneumonia.(41) A person addicted to the use of alcohol is also less likely +to recover from the disease than one who has avoided its use, a result due +in part to the weakening effect of alcohol upon the heart. The congestion +of the lungs in pneumonia makes it very difficult for the heart to force +the blood through them. The weakened heart of the drunkard gives way under +the task. + +The statement sometimes made that alcohol is beneficial in pulmonary +tuberculosis is without foundation in fact. On the other hand, alcoholism +is a recognized cause of consumption. Some authorities claim that this +disease is more frequent in heavy drinkers than in those of temperate +habits, in the proportion of about three to one, and that possibly half of +the cases of tuberculosis are traceable to alcoholism.(42) + +*The Outdoor Cure for Lung Diseases*--Among the many remedies proposed for +consumption and kindred diseases, none have proved more beneficial, +according to reports, than the so-called "outdoor" cure. The person having +consumption is fed plentifully upon the most nourishing food, and is made +to spend practically his entire time, including the sleeping hours, _out +of doors_. Not only is this done during the pleasant months of summer, but +also during the winter when the temperature is below freezing. Severe +exposure is prevented by overhead protection at night and by sufficient +clothing to keep the body warm. The abundant supply of pure, cold air +toughens the lungs and invigorates the entire body, thereby enabling it to +throw off the disease. + +The success attending this method of treating consumptives suggests the +proper mode of strengthening lungs that are not diseased, but simply weak. +The person having weak lungs should spend as much time as he conveniently +can out of doors. He should provide the most ample ventilation at night +and have a sleeping room to himself. He should practice deep breathing +exercises and partake of a nourishing diet. While avoiding prolonged +chilling and other conditions liable to induce colds, he should take +advantage of every opportunity of exposing himself fully and freely to the +outside atmosphere. + +*Summary.*--The purpose of respiration is to bring about an exchange of +gases between the body and the atmosphere. The organs employed for this +purpose, called the respiratory organs, are adapted to handling materials +in the _gaseous_ state, and are operated in accordance with principles +governing the movements of the atmosphere. By alternately increasing and +diminishing the thoracic space, air is made to pass between the outside +atmosphere and the interior of the lungs. Finding its way into the +smallest divisions of the lungs, called the alveoli, the air comes very +near a large surface of blood. By this means the carbon dioxide diffuses +out of the blood, and the free oxygen enters. Through the combined action +of the organs of respiration and the organs that move the blood and the +lymph, the cells in all parts of the body are enabled to exchange certain +gaseous materials with the outside atmosphere. + + [Fig. 49] + + + Fig. 49--Model for demonstrating the lungs. + + +*Exercises.--*1. How does air entering the lungs differ in composition from +air leaving the lungs? What purposes of respiration are indicated by these +differences? + +2. Name the divisions of the lungs. + +3. Trace air from the outside atmosphere into the alveoli. Trace the blood +from the right ventricle to the alveoli and back again to the left +auricle. + +4. How does the movement of air into and from the lungs differ from that +of the blood through the lungs with respect to (_a_) the direction of the +motion. (_b_) the causes of the motion, and (_c_) the tubes through which +the motion takes place? + +5. How are the air passages kept clean and open? + +6. Describe the pleura. Into what divisions does it separate the thoracic +cavity? + +7. Describe and name uses of the diaphragm. + +8. If 30 cubic inches of air are passed into the lungs at each inspiration +and .05 of this is retained as oxygen, calculate the number of cubic feet +of oxygen consumed each day, if the number of inspirations be 18 per +minute. + +9. Find the _weight_ of a day's supply of oxygen, as found in the above +problem, allowing 1.3 ounces as the weight of a cubic foot. + +10. Make a study of the hygienic ventilation of the schoolroom. + +11. Give advantages of full breathing over shallow breathing. + +12. How may a flat chest and round shoulders be a cause of consumption? +How may these deformities be corrected? + +13. Give general directions for applying artificial respiration. + + + +PRACTICAL WORK + + +Examine a dissectible model of the chest and its contents (Fig. 49). Note +the relative size of the two lungs and their position with reference to +the heart and diaphragm. Compare the side to side and vertical diameters +of the cavity. Trace the air tubes from the trachea to their smallest +divisions. + +*Observation of Lungs* (Optional).--Secure from a butcher the lungs of a +sheep, calf, or hog. The windpipe and heart should be left attached and +the specimen kept in a moist condition until used. Demonstrate the +trachea, bronchi, and the bronchial tubes, and the general arrangement of +pulmonary arteries and veins. Examine the pleura and show lightness of +lung tissue by floating a piece on water. + +*To show the Changes that Air undergoes in the Lungs.*--1. Fill a quart jar +even full of water. Place a piece of cardboard over its mouth and invert, +without spilling, in a pan of water. Inserting a tube under the jar, blow +into it air that has been held as long as possible in the lungs. When +filled with air, remove the jar from the pan, keeping the top well +covered. Slipping the cover slightly to one side, insert a burning +splinter and observe that the flame is extinguished. This proves the +absence of sufficient oxygen to support combustion. Pour in a little +limewater(43) and shake to mix with the air. The change of the limewater +to a milky white color proves the presence of carbon dioxide. + + [Fig. 50] + + + Fig. 50--*Apparatus* for showing changes which air undergoes while in the + lungs. + + +2. The effects illustrated in experiment 1 may be shown in a somewhat more +striking manner as follows: Fill two bottles of the same size each one +fourth full of limewater and fit each with a two-holed rubber stopper +(Fig. 50). Fit into each stopper one short and one long glass tube, the +long tube extending below the limewater. Connect the short tube of one +bottle and the long tube of the other bottle with a Y-tube. Now breathe +slowly three or four times through the Y-tube. It will be found that the +inspired air passes through one bottle and the expired air through the +other. Compare the effect upon the limewater in the two bottles. Insert a +small burning splinter into the top of each bottle and note result. What +differences between inspired and expired air are thus shown? + +3. Blow the breath against a cold window pane. Note and account for the +collection of moisture. + +4. Note the temperature of the room as shown by a thermometer. Now breathe +several times upon the bulb, noting the rise in the mercury. What does +this experiment show the body to be losing through the breath? + +*To show Changes in the Thoracic Cavity.*--1. To a yard- or meter-stick, +attach two vertical strips, each about eight inches long, as shown in Fig. +51. The piece at the end should be secured firmly in place by screws or +nails. The other should be movable. With this contrivance measure the +sideward and forward expansion of a boy's thorax. Take the diameter first +during a complete inspiration and then during a complete expiration, +reading the difference. Compare the forward with the sideward expansion. + + [Fig. 51] + + + Fig. 51--*Apparatus* for measuring chest expansion. + + +2. With a tape-line take the circumference of the chest when all the air +possible has been expelled from the lungs. Take it again when the lungs +have been fully inflated. The difference is now read as the chest +expansion. + + [Fig. 52] + + + Fig. 52--*Simple apparatus* for illustrating the action of the diaphragm. + + +*To illustrate the Action of the Diaphragm.*--Remove the bottom from a +large bottle having a small neck. (Scratch a deep mark with a file and +hold on the end of this mark a hot poker. When the glass cracks, lead the +crack around the bottle by heating about one half inch in advance of it.) +Place the bottle in a large glass jar filled two thirds full of water +(Fig. 52). Let the space above the water represent the chest cavity and +the water surface represent the diaphragm. Raise the bottle, noting that +the water falls, thereby increasing the space and causing air to enter. +Then lower the bottle, noting the opposite effect. To show the movement of +the air in and out of the bottle, hold with the hand (or arrange a support +for) a burning splinter over the mouth of the bottle. + +*To estimate the Capacity of the Lungs.*--Breathing as naturally as +possible, expel the air into a spirometer (lung tester) during a period, +say of ten respirations (Fig. 53). Note the total amount of air exhaled +and the number of "breaths" and calculate the amount of air exhaled at +each breath. This is called the _tidal_ air. + + [Fig. 53] + + +Fig. 53--*Apparatus* (spirometer) for measuring the capacity of the lungs. + + +2. After an ordinary inspiration empty the lungs as completely as possible +into the spirometer, noting the quantity exhaled. This amount, less the +tidal air, is known as the _reserve_ air. The air which is now left in the +lungs is called the _residual_ air. On the theory that this is equal in +amount to the reserve air, calculate the capacity of the lungs in an +ordinary inspiration. + +3. Now fill the lungs to the full expansion of the chest and empty them as +completely as possible into the spirometer, noting the amount expelled. +This, less the tidal air and the reserve air, is called the _complemental_ +air. Now calculate the total capacity of the lungs. + + + + +CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY + + +What is the nature of oxygen? What is its purpose in the body and how does +it serve this purpose? How is the blood able to take it up at the lungs +and give it off at the cells? What becomes of it after being used? These +are questions touching the maintenance of life and they deserve careful +consideration. + +*Nature of Oxygen.*--To understand the relation which oxygen sustains to +the body we must acquaint ourselves with certain of its chemical +properties. It is an element(44) of intense affinity, or combining power, +and is one of the most active of all chemical agents. It is able to +combine with most of the other elements to form chemical compounds. A +familiar example of its combining action is found in ordinary combustion, +or burning. On account of the part it plays in this process, oxygen is +called the _supporter of combustion_; but it supports combustion by the +simple method of uniting. The ashes that are left and the invisible gases +that escape into the atmosphere are the compounds formed by the uniting +process. It thus appears that oxygen, in common with the other elements, +may exist in either of two forms: + +1. That in which it is in a _free_, or uncombined, condition--the form in +which it exists in the atmosphere. + +2. That in which it is a part of compounds, such as the compounds formed +in combustion. + +Oxygen manifests its activity to the best advantage when it is in a free +state, or, more accurately speaking, when it is passing from the free +state into one of combination. It is separated from its compounds and +brought again into a free state by overcoming with heat, or some other +force, the affinity which causes it to unite. + +*How Oxygen unites.*--The chemist believes oxygen, as well as all other +substances, to be made up of exceedingly small particles, called _atoms_. +The atoms do not exist singly in either elements or compounds, but are +united with each other to form groups of atoms that are called +_molecules_. In an element the molecules are made up of one kind of atoms, +but in a compound the molecules are made up of as many kinds of atoms as +there are elements in the compound. Changes in the composition of +substances (called chemical changes) are due to rearrangements of the +atoms and the formation of new molecules. The atoms, therefore, are the +units of chemical combination. In the formation of new compounds they +unite, and in the breaking up of existing compounds they separate. + +The uniting of oxygen is no exception to this general law. All of its +combinations are brought about by the uniting of its atoms. In the burning +of carbon, for example, the atoms of oxygen and the atoms of carbon unite, +forming molecules of the compound known as carbon dioxide. The chemical +formula of this compound, which is CO_2, shows the proportion in which the +atoms unite--one atom of carbon uniting with two atoms of oxygen in each of +the molecules. The affinity of oxygen for other elements, and the affinity +of other elements for oxygen, and for each other, resides in their atoms. + +*Oxidation.*--The uniting of oxygen with other elements is termed +_oxidation_. This may take place slowly or rapidly, the two rates being +designated as _slow_ oxidation and _rapid_ oxidation. Examples of slow +oxidation are found in certain kinds of decay and in the rusting of iron. +Combustion is an example of rapid oxidation. Slow and rapid oxidation, +while differing widely in their effects upon surrounding objects, are +alike in that both produce heat and form compounds of oxygen. In slow +oxidation, however, the heat may come off so gradually that it is not +observed. + +*Movement of Oxygen through the Body.*--Oxygen has been shown in the +preceding chapters to pass from the lungs into the blood and later to +leave the blood and, passing through the lymph, to enter the cells. That +oxygen does not become a permanent constituent of the cells is shown by +the constancy of the body weight. Nearly two pounds of oxygen per day are +known to enter the cells of the average-sized person. If this became a +permanent part of the cells, the body would increase in weight from day to +day. Since the body weight remains constant, or nearly so, we must +conclude that oxygen leaves the body about as fast as it enters. Oxygen +enters the body as a _free_ element. The form in which it leaves the body +will be understood when we realize the purpose which it serves and the +method by which it serves this purpose. + +*Purpose of Oxygen in the Body.*--The question may be raised: Is it +possible for oxygen to serve a purpose in the body without remaining in +it? This, of course, depends upon what the purpose is. That it is possible +for oxygen to serve a purpose and at the same time pass on through the +place where it serves that purpose, is seen by studying the combustion in +an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most +part passes out at the flue, but in passing through the stove it unites +with, or oxidizes, the fuel, causing the combustion which produces the +heat. + + [Fig. 54] + + + Fig. 54--*Coal stove* illustrating rapid oxidation. + + +Now it is found that certain chemical processes, mainly oxidations, are +taking place in the body. These produce the heat for keeping it warm and +also supply other forms of energy,(45) including motion. It is the purpose +of oxygen to keep up these oxidations and, by so doing, to aid in +supplying the body with energy. It serves this purpose in much the same +way that it supports combustion, _i.e._, by uniting with, or oxidizing, +materials derived from foods that are present in the cells. + +*Does Oxygen serve Other Purposes?*--It has been suggested that oxygen may +serve the purpose of oxidizing, or destroying, substances that are +injurious and of acting, in this way, as a purifying agent in the body. In +support of this view is the natural tendency of oxygen to unite with +substances and the well-known fact that oxygen is an important natural +agent in purifying water. It seems probable, therefore, that it may to a +slight extent serve this purpose in the body. It is probable also that +oxygen aids through its chemical activity in the formation of compounds +which are to become a part of the cells. Both of these uses, however, are +of minor importance when compared with _the main use of oxygen_, which _is +that of an aid in supplying energy to the body_. + +*Oxygen and the Maintenance of Life.*--In the supplying of energy to the +body, one of the conditions necessary to the maintenance of life is +provided. Because oxygen is necessary to this process, and because death +quickly results when the supply of it is cut off, oxygen is frequently +called the supporter of life. This idea is misleading, for oxygen has no +more to do with the maintenance of life than have the food materials with +which it unites. Life appears to be more dependent upon oxygen than upon +food, simply because the supply of it in the body at any time is +exceedingly small. Being continually surrounded by an atmosphere +containing free oxygen, the body depends upon this as a constant source of +supply, and does not store it up. Food, on the other hand, is taken in +excess of the body's needs and stored in the various tissues, the supply +being sufficient to last for several days. When the supply of either +oxygen or food is exhausted in the body, life must cease. + +*The Oxygen Movement a Necessity.*--Since _free_ oxygen is required for +keeping up the chemical changes in the cells, and since it ceases to be +free as soon as it goes into combination, its continuous movement through +the body is a necessity. The oxygen compounds must be removed as fast as +formed in order to make room for more free oxygen. This movement has +already been studied in connection with the blood and the organs of +respiration, but the consideration of certain details has been deferred +till now. By what means and in what form is the oxygen passed _to_ and +_from_ the cells? + +*Passage of Oxygen through the Blood.*--In serving its purpose at the +cells, the oxygen passes twice through the blood--once as it goes toward +the cells and again as it passes from the cells to the exterior of the +body: + +_Passage toward the Cells._--This is effected mainly through the hemoglobin +of the red corpuscles. At the lungs the oxygen and the hemoglobin form a +weak chemical compound that breaks up and liberates the oxygen when it +reaches the capillaries in the tissues. The separation of the oxygen from +the hemoglobin at the tissues appears to be due to two causes: first, to +the weakness of the chemical attraction between the atoms of oxygen and +the atoms that make up the hemoglobin molecule; and second, to a +difference in the so-called _oxygen pressure_ at the lungs and at the +tissues.(46) + +The attraction of the oxygen and the hemoglobin is sufficient to cause +them to unite where the oxygen pressure is more than one half pound to the +square inch, but it is not sufficiently strong to cause them to unite or +to prevent their separation, if already united, where the oxygen pressure +is less than one half pound to the square inch. The oxygen pressure at the +lungs, which amounts to nearly three pounds to the square inch, easily +causes the oxygen and the hemoglobin to unite, while the almost complete +absence of any oxygen pressure at the tissues, permits their separation. +The blood in its circulation constantly flows from the place of high +oxygen pressure at the lungs to the place of low oxygen pressure at the +tissues and, in so doing, loads up with oxygen at one place and unloads it +at the other (Fig. 55). + +_Passage from the Cells._--Since oxygen leaves the free state at the cells +and becomes a part of compounds, we are able to trace it from the body +only by following the course of these compounds. Three waste compounds of +importance are formed at the cells--carbon dioxide (CO2), water (H2O), and +urea (N2H4CO). The first is formed by the union of oxygen with carbon, the +second by its union with hydrogen, and the third by its union with +nitrogen, hydrogen, and carbon. These compounds are carried by the blood +to the organs of excretion, where they are removed from the body. The +water leaves the body chiefly as a liquid, the urea as a solid dissolved +in water, and the carbon dioxide as a gas. The passage of carbon dioxide +through the blood requires special consideration. + + [Fig. 55] + + + Fig. 55--*Diagram illustrating movement, of oxygen and carbon dioxide + through the body* (S.D. Magers). Each moves from a place of relatively + high to a place of relatively low pressure. (See text.) + + +*Passage of Carbon Dioxide through the Blood.*--Part of the carbon dioxide +is dissolved in the plasma of the blood, and part of it is in weak +chemical combination with substances found in the plasma and in the +corpuscles. Its passage through the blood is accounted for in the same way +as the passage of the oxygen. Its ability to dissolve in liquids and to +enter into chemical combination varies as the _carbon dioxide +pressure_(47) This in turn varies with the amount of the carbon dioxide, +which is greatest at the cells (where it is formed), less in the blood, +and still less in the lungs. Because of these differences, the blood is +able to take it up at the cells and release it at the lungs (Fig. 55). + + [Fig. 56] + + +Fig. 56--*Soap bubble* floating in a vessel of carbon dioxide, illustrating + the difference in weight between air and carbon dioxide gas. + + +*Properties of Carbon Dioxide.*--Carbon dioxide is a colorless gas with +little or no odor. It is classed as a heavy gas, being about one third +heavier than air(48) (Fig. 56). It does not support combustion, but on the +contrary is used to some extent to extinguish fires. It is formed by the +oxidation of carbon in the body, and by the combustion of carbon outside +of the body. It is also formed by the decay of animal and vegetable +matter. From these sources it is continually finding its way into the +atmosphere. Although not a poisonous gas, carbon dioxide may, if it +surround the body, shut out the supply of oxygen and cause death.(49) + +*Final Disposition of Carbon Dioxide.*--It is readily seen that the union +of carbon and oxygen, which is continually removing oxygen from the air +and replacing it with carbon dioxide, tends to make the whole atmosphere +deficient in the one and to have an excess of the other. This tendency is +counteracted through the agency of vegetation. Green plants absorb the +carbon dioxide from the air, decompose it, build the carbon into compounds +(starch, etc.) that become a part of the plant, and return the free oxygen +to the air (Fig. 57). In doing this, they not only preserve the necessary +proportion of oxygen and carbon dioxide in the atmosphere, but also put +the carbon and oxygen in such a condition that they can again unite. The +force which enables the plant cells to decompose the carbon dioxide is +supplied by the sunlight (Chapter XII). + + [Fig. 57] + + +Fig. 57--*Under surface* of a geranium leaf showing breathing pores, highly + magnified (O.H.). + + +*Summary.*--Oxygen, by uniting with materials at the cells, keeps up a +condition of chemical activity (oxidation) in the body. This supplies heat +and the other forms of bodily energy. Entering as a free element, oxygen +leaves the body as a part of the waste compounds which it helps to form. +The free oxygen is transported from the lungs to the cells by means of the +hemoglobin of the red corpuscles, while the combined oxygen in carbon +dioxide and other compounds from the cells is carried mainly by the +plasma. The limited supply of free oxygen in the body at any time makes +necessary its continuous introduction into the body. + +*Exercises.*--1. Describe the properties of oxygen. How does it unite with +other elements? How does it support combustion? + +2. State the purpose of oxygen in the body. What properties enable it to +fulfill this purpose? + +3. What is the proof that oxygen does not remain permanently in the body? +How does the oxygen entering the body differ from the same oxygen as it +leaves the body? + +4. What is the necessity for the _continuous_ introduction of oxygen into +the body, while food is introduced only at intervals? + +5. How are the red corpuscles able to take up and give off oxygen? How is +the plasma able to take up and give off carbon dioxide? + +6. If thirty cubic inches of air pass from the lungs at each expiration +and 4.5 per cent of this is carbon dioxide, calculate the number of cubic +feet of the gas expelled in twenty-four hours, estimating the number of +respirations at eighteen per minute. + +7. What is the weight of this volume of carbon dioxide, if one cubic foot +weigh 1.79 ounces? + +8. What portion of this weight is oxygen and what carbon, the ratio by +weight of carbon to oxygen in carbon dioxide being twelve to thirty-two? + +9. What is the final disposition of carbon dioxide in the atmosphere? + + + +PRACTICAL WORK + + +*To show the Difference between Free Oxygen and Oxygen in +Combination.*--Examine some crystals of potassium chlorate (KClO3). They +contain oxygen _in combination_ with potassium and chlorine. Place a few +of these in a small test tube and heat strongly in a gas or alcohol flame. +The crystals first melt, and the liquid which they form soon appears to +boil. If a splinter, having a spark on the end, is now inserted in the +tube, it is kindled into a flame. This shows the presence of _free_ +oxygen, the heat having caused the potassium chlorate to decompose. The +difference between free and combined oxygen may also be shown by +decomposing other compounds of oxygen, such as water and mercuric oxide. + +*Preparation and Properties of Oxygen.*--Intimately mix 3 grams (1/2 +teaspoonful) of potassium chlorate with half its bulk of manganese +dioxide, and place the mixture in a large test tube. Close the test tube +with a tight-fitting stopper which bears a glass tube of sufficient length +and of the right shape to convey the escaping gas to a small trough or pan +partly filled with water, on the table. Fill four large-mouthed bottles +with water and, by covering with cardboard, invert each in the trough of +water. Arrange the test tube conveniently for heating, letting the end of +the glass tube terminate under the mouth of one of the bottles (Fig. 58). +Using an alcohol lamp or a Bunsen burner, heat over the greater portion of +the tube at first, but gradually concentrate the flame upon the mixture. +Do not heat too strongly, and when the gas is coming off rapidly, remove +the flame entirely, putting it back as the action slows down. After all +the bottles have been filled, remove the end of the glass tube from the +water, but leave the bottles of oxygen inverted in the trough until they +are to be used. On removing the bottles from the trough, keep the tops +covered with wet cardboard. + + [Fig. 58] + + + Fig. 58--*Apparatus* for generating oxygen. + + +1. Examine a bottle of oxygen, noting its lack of color. Insert a small +burning splinter in the upper part of the bottle and observe the change in +the rate of burning. The air contains free oxygen, but it is diluted with +nitrogen. Compare this with the undiluted oxygen in the bottle as to +effect in causing the splinter to burn. + +2. In a second bottle of oxygen insert a splinter without the flame, but +having a small spark on the end. As soon as the oxygen kindles the spark +into a flame, withdraw from the bottle and blow out the flame, but again +insert the spark. Repeat the experiment as long as the spark is kindled by +the oxygen into a flame. This experiment is usually performed as a test +for undiluted oxygen. + +3. Make a hollow cavity in the end of a short piece of crayon. Fasten a +wire to the crayon, and fill the cavity with powdered sulphur. Ignite the +sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it +into a bottle of oxygen. Observe the change in the rate of burning, the +color of the flame, and the material formed in the bottle by the burning. +The gas remaining in the bottle is sulphur dioxide (SO2), formed by the +_uniting_ of the sulphur and the oxygen. + +4. Bend a small loop on the end of a piece of picture wire. Heat the loop +in a flame and insert it in some powdered sulphur. Ignite the melted +sulphur which adheres, and insert it quickly in a bottle of oxygen. +Observe the dark, brittle material which is formed by the burning of the +iron. It is a compound of the iron with oxygen, similar to iron rust, and +formed by their uniting. + +*Preparation and Properties of Carbon Dioxide.*--1. (_a_) Attach a piece of +carbon (charcoal) no larger than the end of the thumb to a piece of wire. +Ignite the charcoal in a hot flame and lower it into a vessel of oxygen. +Observe its combustion, letting it remain in the bottle until it ceases to +burn. Note that the burning has consumed a part of the carbon and has used +up the free oxygen. Has anything been formed in their stead? + +(_b_) Remove the charcoal and add a little limewater. Cover the bottle +with a piece of cardboard, and bring the gas and the limewater in contact +by shaking. Note any change in the color of the limewater. If it turns +white, the presence of carbon dioxide is proved. + +2. Burn a splinter in a large vessel of air, keeping the top covered. Add +limewater and shake. Note and account for the result. + +3. Place several pieces of marble (limestone) in a jar holding at least +half a gallon. Barely cover the marble with water, and then add +hydrochloric acid until a gas is rapidly evolved. This gas is carbon +dioxide. + +(_a_) Does it possess color? + +(_b_) Insert a burning splinter to see if it supports combustion. + +(_c_) Place a bottle of oxygen by the side of the vessel of carbon +dioxide. Light a splinter and extinguish the flame by lowering it into the +vessel of carbon dioxide. Withdraw immediately, and if a spark remains on +the splinter, thrust it into the bottle of oxygen. Then insert the +relighted splinter into the carbon dioxide. Repeat several times, kindling +the flame in one gas and extinguishing it in the other. Finally show that +the spark also may be extinguished by holding the splinter a little longer +in the carbon dioxide. + +(_d_) Tip the jar containing the carbon dioxide over the mouth of a +tumbler, as in pouring water, though not far enough to spill the acid, and +then insert a burning splinter in the tumbler. Account for the result. +Inference as to the weight of carbon dioxide. + + [Fig. 59] + + +Fig. 59--*Simple apparatus* for illustrating passage of oxygen through the + body. + + +(_e_) Review experiments (page 101) showing the presence of carbon dioxide +in the breath. + +*To illustrate the General Movement of Oxygen through the Body.*--Into a +glass tube, six inches in length and open at both ends, place several +small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means +of a stopper, a smaller glass tube which is bent at right angles and which +is made to pass through a close-fitting stopper to the bottom of a small +bottle. Another small tube is fitted into a second hole in this stopper, +but terminating near the top of the bottle, and to this is connected a +rubber tube about eighteen inches in length. The arrangement is now such +that by sucking air from the top of the bottle, it is made to enter at the +distant end of the tube containing the charcoal. After filling the bottle +one third full of limewater, heat the tube containing the charcoal until +it begins to glow. Then suck the air through the apparatus (as in smoking, +without drawing it into the lungs), observing what happens both in the +tube and in the bottle. What are the proofs that the oxygen, in passing +through the tube, unites with the carbon, forms carbon dioxide, and +liberates energy? Compare the changes which the oxygen undergoes while +passing through the tube with the changes which it undergoes in passing +through the body. + + + + +CHAPTER IX - FOODS AND THE THEORY OF DIGESTION + + +The body is constantly in need of new material. Oxidation, as shown in the +preceding chapter, rapidly destroys substances at the cells, and these +have to be replaced. Upon this renewal depends the supply of energy. +Moreover, there is found to be an actual breaking down of the living +material, or protoplasm, in the body. While this does not destroy the +cells, as is sometimes erroneously stated, it reduces the quantity of the +protoplasm and makes necessary a process of repair, or rebuilding, of the +tissues. This also requires new material. Finally, substances, such as +water and common salt, are required for the aid which they render in the +general work of the body. Since these are constantly being lost in one way +or another, they also must be replaced. These different needs of the body +for new materials are supplied through + +*The Foods.*--Foods are substances that, on being taken into the healthy +body, are of assistance in carrying on its work. This definition properly +includes oxygen, but the term is usually limited to substances introduced +through the digestive organs. As suggested above, foods serve at least +three purposes: + +1. They, with oxygen, supply the body with energy. + +2. They provide materials for rebuilding the tissues. + +3. They supply materials that aid directly or indirectly in the general +work of the body. + +*The Simple Foods, or Nutrients.*--From the great variety of things that +are eaten, it might appear that many different kinds of substances are +suitable for food. When our various animal and vegetable foods are +analyzed, however, they are found to be similar in composition and to +contain only some five or six kinds of materials that are essentially +different. While certain foods may contain only a single one of these, +most of the foods are mixtures of two or more. These few common materials +which, in different proportions, form the different things that are eaten, +are variously referred to as simple foods, food-stuffs, and _nutrients_, +the last name being the one generally preferred. The different classes of +nutrients are as follows: + + Nutrients: + Proteids + (Albuminoids) + Carbohydrates + Fats + Mineral salts + Water + +It is now necessary to become somewhat familiar with the different +nutrients and the purposes which they serve in the body. + +*Proteids.*--The proteids are obtained in part from the animal and in part +from the plant kingdom, there being several varieties. A well-known +variety, called _albumin_, is found in the white of eggs and in the plasma +of the blood, while the muscles contain an abundance of another variety, +known as _myosin_. Cheese consists largely of a kind of proteid, called +_casein_, which is also present in milk, but in a more diluted form. If a +mouthful of wheat is chewed for some time, most of it is dissolved and +swallowed, but there remains in the mouth a sticky, gum-like substance. +This is _gluten_, a form of proteid which occurs in different grains. +Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind +of proteid which is called _legumen_. + +Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small +per cent of sulphur. Certain ones (the nucleo-proteids from grains) also +contain phosphorus. All of the proteids are highly complex compounds and +form a most important class of nutrients. + +*Purposes of Proteids.*--The chief purpose of proteids in the body is to +rebuild the tissues. Not only do they supply all of the main elements in +the tissues, but they are of such a nature chemically that they are +readily built into the protoplasm. They are absolutely essential to life, +no other nutrients being able to take their place. An animal deprived of +them exhausts the proteids in its body and then dies. In addition to +rebuilding the tissues, proteids may also be oxidized to supply the body +with energy. + +*Albuminoids* form a small class of foods, of minor importance, which are +similar to proteids in composition, but differ from them in being unable +to rebuild the tissues. Gelatin, a constituent of soup and obtained from +bones and connective tissue by boiling, is the best known of the +albuminoid foods. On account of the nitrogen which they contain, proteids +and albuminoids are often classed together as _nitrogenous foods_. + +*Carbohydrates.*--While the carbohydrates are not so essential to life as +are the proteids, they are of very great value in the body. They are +composed of carbon, hydrogen, and oxygen, and are obtained mainly from +plants. There are several varieties of carbohydrates, but they are similar +in composition. All of those used as food to any great extent are starch +and certain kinds of sugar. + +*Starch* is the carbohydrate of greatest importance as a food, and it is +also the one found in the greatest abundance. All green plants form more +or less starch, and many of them store it in their leaves, seeds, or roots +(Fig. 60). From these sources it is obtained as food. _Glycogen_, a +substance closely resembling starch, is found in the body of the oyster. +It is also formed in the liver and muscles of the higher animals, being +prepared from the sugar of the blood, and is stored by them as reserve +food (Chapter XI). Glycogen is, on this account, called _animal starch_. +Starch on being eaten is first changed to sugar, after which it may be +converted into glycogen in the liver and in the muscles. + + [Fig. 60] + + + Fig. 60--*Starch grains* in cells of potato as they appear under the + microscope. (See practical work.) + + +*Sugars.*--There are several varieties of sugar, but the important ones +used as foods fall into one or the other of two classes, known as _double +sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the +first class belong _cane sugar_, found in sugar cane and beets, _milk +sugar_, found in sweet milk, and _maltose_, a kind of sugar which is made +from starch by the action of malt. The important members of the second +class are _grape sugar_, or dextrose, and _fruit sugar_, or levulose, both +of which are found in fruits and in honey. + +The most important of all sugars, so far as its use in the body is +concerned, is _dextrose_. To this form all the other sugars, and starch +also, are converted before they are finally used in the body. The close +chemical relation between the different carbohydrates makes such a +conversion easily possible. + +*Fats.*--The fats used as foods belong to one or the other of two classes, +known as solid fats and oils. The solid fats are derived chiefly from +animals, and the oils are obtained mostly from plants. Butter, the fat of +meats, olive oil, and the oil of nuts are the fats of greatest importance +as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen, +and oxygen. They are rather complex chemical compounds, though not so +complex as proteids. Since neither fats nor carbohydrates contain +nitrogen, they are frequently classed together as _non-nitrogenous_ foods. + +*Purpose Served by Carbohydrates, Fats, and Albuminoids.*--These classes of +nutrients all serve the common purpose of supplying energy. By uniting +with oxygen at the cells, they supply heat and the other forms of bodily +force. This is perhaps their only purpose.(50) Proteids also serve this +purpose, but they are not so well adapted to supplying energy as are the +carbohydrates and the fats. In the first place they do not completely +oxidize and therefore do not supply so much energy; and, in the second +place, they form waste products that are removed with difficulty from the +body. + +*Mineral Salts and their Uses.*--Mineral salts are found in small +quantities in all of the more common food materials, and, as a rule, find +their way into the body unnoticed. They supply the elements which are +found in the body in small quantities and serve a variety of purposes.(51) +Calcium phosphate and calcium carbonate are important constituents of the +bones and teeth; and the salts containing iron renew the hemoglobin of the +blood. Others perform important functions in the vital processes. The +mineral compound of greatest importance perhaps is sodium chloride, or +common salt.(52) This is a natural constituent of most of our foods, and +is also added to food in its preparation for the table. When it is +withheld from animals for a considerable length of time, they suffer +intensely and finally die. It is necessary in the blood and lymph to keep +their constituents in solution, and is thought to play an important role +in the chemical changes of the cells. It is constantly leaving the body as +a waste product and must be constantly supplied in small quantities in the +foods. + +*Importance of Water.*--Water finds its way into the body as a pure liquid, +as a part of such mixtures as coffee, chocolate, and milk, and as a +constituent of all our solid foods. (See table of foods, page 126.) It is +also formed in the body by the oxidation of hydrogen. It passes through +the body unchanged, and is constantly being removed by all the organs of +excretion. Though water does not liberate energy in the body nor build up +the tissues in the sense that other foods do, it is as necessary to the +maintenance of life as oxygen or proteids. It occurs in all the tissues, +and forms about 70 per cent of the entire weight of the body. Its presence +is necessary for the interchange of materials at the cells and for keeping +the tissues soft and pliable. As it enters the body, it carries digested +food substances with it, and as it leaves it is loaded with wastes. Its +chief physiological work, which is that of a _transporter of material_, +depends upon its ability to dissolve substances and to flow readily from +place to place. + +*Relative Quantity of Nutrients Needed.*--Proteids, carbohydrates, and fats +are the nutrients that supply most of the body's nourishment. The most +hygienic diet is the one which supplies the proteids in sufficient +quantity to rebuild the tissues and the carbohydrates and fats in the +right amounts to supply the body with energy. Much experimenting has been +done with a view to determining these proportions, but the results so far +are not entirely satisfactory. According to some of the older estimates, a +person of average size requires for his daily use five ounces of proteid, +two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent +investigations of this problem seem to show that the body is as well, if +not better, nourished by a much smaller amount of proteid--not more than +two and one half ounces (60 grams) daily.(53) + +While there is probably no necessity for the healthy individual's taking +his proteid, fat, and carbohydrate in _exact_ proportions (if the +proportions best suited to his body were known), the fact needs to be +emphasized that proteids, although absolutely necessary, should form but a +small part (not over one fifth) of the daily bill of fare. In recognition +of this fact is involved a principle of health and also one of economy. +The proteids, especially those in meats, are the most expensive of the +nutrients, whereas the carbohydrates, which should form the greater bulk +of one's food, are the least expensive. + +*Effects of a One-sided Diet.*--The plan of the body is such as to require +a _mixed diet_, and all of the great classes of nutrients are necessary. +If one could subsist on any single class, it would be proteids, for +proteids are able both to rebuild tissue and to supply energy. But if +proteids are eaten much in excess of the body's need for rebuilding the +tissues, and this excess is oxidized for supplying energy, a strain is +thrown upon the organs of excretion, because of the increase in the +wastes. Not only is there danger of overworking certain of these organs +(the liver and kidneys), but the wastes may linger too long in the body, +causing disorder and laying the foundation for disease. On the other hand, +if an insufficient amount of proteid is taken, the tissues are improperly +nourished, and one is unable to exert his usual strength. What is true of +the proteids is true, though in a different way, of the other great +classes of foods. A diet which is lacking in proteid, carbohydrate, or +fat, or which has any one of them in excess, is not adapted to the +requirements of the body. + +*Composition of the Food Materials.*--One who intelligently provides the +daily bill of fare must have some knowledge of the nature and quantity of +the nutrients present in the different materials used as food. This +information is supplied by the chemist, who has made extensive analyses +for this purpose. Results of such analyses are shown in Table 1 (page +126), which gives the percentage of proteids, fats, carbohydrates, water, +and mineral salts in the edible portions of the more common of our foods. + + [Fig. 61] + + + Fig. 61--Relative proportions of different nutrients in well-known foods. + + +*Food Supply to the Table.*--The main problem in supplying the daily bill +of fare is that of securing through the different food materials the +requisite amounts of proteids, carbohydrates, and fats. In this matter a +table showing the composition of foods can be used to great advantage. +Consulting the table on page 126, it is seen that large per cents of +proteids are supplied by lean meat, eggs, cheese, beans, peas, peanuts, +and oatmeal, while fat is in excess in fat meat, butter, and nuts (Fig. +61). Carbohydrates are supplied in abundance by potatoes, rice, corn, +sugar, and molasses. The different cereals also contain a large percentage +of carbohydrates in the form of starch. + + TABLE I. THE COMPOSITION OF + FOOD MATERIALS(54) +Food Water Solids Proteid Fat Carbohydrates Mineral Heat +Materials Matter Value of + One + Pound +Animal Per cent Per cent Per cent Per cent Per cent Per cent Calories(55) +foods, +edible +portion +Beef: 63.9 36.1 19.5 15.6 ... 1 1020 +Shoulder + Rib 48.1 51.9 15.4 35.6 ... .9 1790 + Sirloin 60 40 18.5 20.5 ... 1 1210 + Round 68.2 31.8 20.5 10.1 ... 1.2 805 +Veal: 68.8 31.2 20.2 9.8 ... ... 790 +Shoulder +Mutton: 61.8 38.2 18.3 19 ... .9 1140 +Leg + Loin 49.3 50.7 15 35 ... .7 1755 +Pork: 50.3 49.7 16 32.8 ... .9 1680 +Shoulder + Ham, 41.5 58.5 16.7 39.1 ... 2.7 1960 + salted, + smoked + Fat, 12.1 87.9 .9 82.8 ... 4.2 3510 + salted +Sausage: 41.5 58.8 13.8 42.8 ... 2.2 2065 +Pork +Bologna 62.4 37.6 18.8 42.8 ... 3 1015 +Chicken 72.2 27.8 24.4 1 ... 1.4 540 +Eggs 73.8 26.2 14.9 10.5 ... .8 721 +Milk 87 13 3.6 4 4.7 .7 325 +Butter 10.5 89 .6 85 .5 .3 3515 +Cheese: 30.2 69.8 28.3 35.5 1.8 4.2 2070 +Full +cream + Skim milk 41.3 58.7 38.4 6.8 6.9 4.6 1165 +Fish: 82.6 17.4 15.8 .5 ... 1.2 310 +Codfish + Salmon 63.6 36.4 21.6 13.4 ... 1.4 965 + Oysters 87.1 12.9 6 1.2 3.7 2 230 +Vegetable +foods +Wheat 12.5 87.5 11 1.1 74.9 .5 1645 +flour +Graham 13.1 86.9 11.7 1.7 71.7 1.8 1635 +flour +(wheat) +Rye flour 13.1 86.9 6.7 .8 78.7 .7 1625 +Buckwheat 14.6 85.4 6.9 1.4 76.1 1 1605 +flour +Oatmeal 7.6 92.4 15.1 7.1 68.2 2 1850 +Cornmeal 15 85 9.2 3.8 70.6 1.4 1645 +Rice 12.4 87.6 7.4 .4 79.4 .4 1630 +Peas 12.3 87.7 26.7 1.7 56.4 2.9 1565 +Beans 12.6 87.4 23.1 2 59.2 3.1 1615 +Potatoes 78.9 21.1 2.1 .1 17.9 1 375 +Tomatoes 95.3 4.7 .8 .4 3.2 .3 80 +Apples 83.2 16.8 .2 .4 15.9 .3 315 +Sugar, 2 98 ... ... 97.8 .3 1820 +granulated +White 32.3 67.7 8.2 1.7 56.3 .0 1280 +bread +(wheat) +Peanuts 9.2 90.8 25.8 24.4 38.6 2 2560 +Almonds 4.8 95.2 21 17.3 54.9 2 3030 +Walnuts 2.5 97.5 16.6 16.1 63.4 1.4 3285 +(English) + +_Variety_ in the selection of foods for the table is an essential feature, +but this should not increase either the work or the expense of supplying +the meals. Each single meal can, and should, be simple in itself and, at +the same time, differ sufficiently from the meal preceding and the one +following to give the necessary variety in the course of the day. The bill +of fare should, of course, include fruits (for their tonic effects) and +very small amounts perhaps of substances which stimulate the appetite, +such as pepper, mustard, etc., known as condiments. + +*Purity of Food.*--The fact that many of the food substances are perishable +makes it possible for them to be eaten in a slightly decayed condition. +Such substances are decidedly unwholesome (some containing poisons) and +should be promptly rejected. Not only do fresh meats, fruits, and +vegetables need careful inspection, but canned and preserved goods as +well. If canned foods are imperfectly sealed or if not thoroughly cooked +in the canning process, they decay and the acids which they generate act +on the metals lining the cans, forming poisonous compounds. The contents +of "tin" cans should for this reason be transferred to other vessels as +soon as opened. + +Foods are also rendered impure or weakened through adulteration, the +watering of milk being a familiar example. The manufacture of jellies, +preserves, sirups, and various kinds of pickles and condiments has perhaps +afforded the largest field for adulterations, although it is possible to +adulterate nearly all of the leading articles of food. A long step in the +prevention of food and drug adulteration was taken in this country by the +passage of the _Pure Food Law_. By forcing manufacturers of foods and +medicines to state on printed labels the composition of their products, +this law has made it possible for the consumer to know what he is +purchasing and putting into his body. + +*Alcohol not a Food.*--Many people in this and other countries drink in +different beverages, such as whisky, beer, wine, etc., a varying amount of +alcohol. This substance has a temporary stimulating or exciting effect, +and the claim has been made that it serves as a food. Recently it has been +shown that alcohol when introduced into the body in small quantities and +in a greatly diluted form, is nearly all oxidized, yielding energy as does +fat or sugar. If no harmful effects attended the use of alcohol, it might +on this account be classed as a food. But alcohol is known to be harmful +to the body. When used in large quantities, it injures nearly all of the +tissues, and when taken habitually, even in small doses, it leads to the +formation of the alcohol habit which is now recognized and treated as a +disease. This and other facts show that alcohol is not adapted to the body +plan of taking on and using new material (Chapter XI), and no substance +lacking in this respect can properly be classed as a food.(56) Instead of +classing alcohol as a food, it should be placed in that long list of +substances which are introduced into the body for special purposes and +which are known by the general name of + +*Drugs.*--Drugs act strongly upon the body and tend to bring about unusual +and unnatural results. Their use should in no way be confused with that of +foods. If taken in health, they tend to disturb the physiological balance +of the body by unduly increasing or diminishing the action of the +different organs. In disease where this balance is already disturbed, they +may be administered for their counteractive effects, but always under the +advice and direction of a physician. Knowing the nature of the disturbance +which the drug produces, the physician can administer it to advantage, +should the body be out of physiological balance, or diseased. Not only are +drugs of no value in health, but their use is liable to do much harm. + + + +NATURE OF DIGESTION + + +Before the nutrients can be oxidized at the cells, or built into the +protoplasm, they undergo a number of changes. These are necessary for +their entrance into the body, for their distribution by the blood and the +lymph, and for the purposes which they finally serve. The first of these +changes is preparatory to the entrance of the nutrients and is known as +_digestion_. The organs which bring about this change, called digestive +organs, have a special construction which adapts them to their work. It +will assist materially in understanding these organs if we first learn +something of the nature of the work which they have to perform. + +*How the Nutrients get into the Body.*--The nature of digestion is +determined by the conditions affecting the entrance of nutrients into the +body. Food in the stomach and air in the lungs, although surrounded by the +body, are still outside of what is called the _body proper_. To gain +entrance into the body proper, a substance must pass through the body +wall. This consists of the skin on the outside and of the mucous linings +of the air passages and other tubes and cavities which are connected with +the external surface. + +To get from the digestive organs into the blood, the nutrients must pass +through the mucous membrane lining these organs and also the walls of +blood or lymph vessels. Only _liquid materials_ can make this passage. It +is necessary, therefore, to reduce to the liquid state all nutrients not +already in that condition. _This reduction to the liquid state constitutes +the digestive process_. + +*How Substances are Liquefied.*--While the reduction of solids to the +liquid state is accomplished in some instances by heating them until they +melt, they are more frequently reduced to this state by subjecting them to +the action of certain liquids, called _solvents_. Through the action of +the solvent the minute particles of the solid separate from each other and +disappear from view. (Shown in dropping salt in water.) At the same time +they mix with the solvent, forming a _solution_, from which they separate +only with great difficulty. For this reason solids in solution can diffuse +through porous partitions along with the solvents in which they are +dissolved (page 73). + +By digestion the nutrients are reduced to the form of a solution. _The +process is_, simply speaking, _one of dissolving_. The liquid employed as +_the digestive solvent is water_. The different nutrients dissolve in +water, mixing with it to form a solution which is then passed into the +body proper. + +*Digestion not a Simple Process.*--Digestion is by no means a simple +process, such, for instance, as the dissolving of salt or sugar in water. +These, being soluble in water, dissolve at once on being mixed with a +sufficient amount of this liquid. The majority of the nutrients, however, +are insoluble in water and are unaffected by it when acting alone. Fats, +starch, and most of the proteids do not dissolve in water. Before these +can be dissolved they have to be changed chemically and converted into +substances that are _soluble in water_. This complicates the process and +_prevents the use of water alone_ as the digestive solvent. + +*A Similar Case.*--If a piece of limestone be placed in water, it does not +dissolve, because it is insoluble in water. If hydrochloric acid is now +added to the water, the limestone is soon dissolved (Fig. 62). (See +Practical Work.) It seems at first thought that the acid dissolves the +limestone, but this is not the case. The acid produces a chemical change +in the limestone (calcium carbonate) and converts it into a compound +(calcium chloride) that is soluble in water. As fast as this is formed it +is dissolved by the water, which is the real solvent in the case. The acid +simply plays the part of a chemical converter. + + [Fig. 62] + + + Fig. 62--The dissolving of limestone in water containing acid, suggesting + the double action in the digestion of most foods. + + +*The Digestive Fluids.*--Several fluids--saliva, gastric juice, pancreatic +juice, bile, and intestinal juice--are employed in the digestion of the +food. The composition of these fluids is in keeping with the nature of the +digestive process. While all of them have water for their most abundant +constituent, there are dissolved in the water small amounts of active +chemical agents. It is the work of these agents to convert the insoluble +nutrients into substances that are soluble in water. The digestive fluids +are thus able to act in a _double_ manner on the nutrients--to change them +chemically and to dissolve them. The chemical agents which bring about the +changes in the nutrients are called _enzymes_, or digestive ferments. + +*Foods Classed with Reference to Digestive Changes.*--With reference to the +changes which they undergo during digestion, foods may be divided into +three classes as follows: + +1. Substances already in the liquid state and requiring no digestive +action. Water and solutions of simple foods in water belong to this class. +Milk and liquid fats, or oils, do not belong to this class. + +2. Solid foods soluble in water. This class includes common salt and +sugar. These require no digestive action other than dissolving in water. + +3. Foods that are insoluble in water. These have first to be changed into +soluble substances, after which they are dissolved. + +*Summary.*--Materials called foods are introduced into the body for +rebuilding the tissues, supplying energy, and aiding in its general work. +Only a few classes of substances, viz., proteids, carbohydrates, fats, +water, and some mineral compounds have all the qualities of foods and are +suitable for introduction into the body. Substances known as drugs, which +may be used as medicines in disease, should be avoided in health. Before +foods can be passed into the body proper, they must be converted into the +liquid form, or dissolved. In this process, known as digestion, water is +the solvent; and certain chemical agents, called enzymes, convert the +insoluble nutrients into substances that are soluble in water. + +*Exercises.*--1. How does oxidation at the cells make necessary the +introduction of new materials into the body? + +2. What different purposes are served by the foods? + +3. What is a nutrient? Name the important classes. + +4. What are food materials? From what sources are they obtained? + +5. Name the different kinds of proteids; the different kinds of +carbohydrates. Why are proteids called nitrogenous foods and fats and +carbohydrates non-nitrogenous foods? + +6. Show why life cannot be carried on without proteids; without water. + +7. What per cents of proteid, fat, and carbohydrate are found in wheat +flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts? + +8. State the objection to a meal consisting of beef, eggs, beans, bread, +and butter; to one consisting of potatoes, rice, bread, and butter. Which +is the more objectionable of these meals and why? + +9. State the general plan of digestion. + +10. Show that digestion is not a simple process like that of dissolving +salt in water. + + + +PRACTICAL WORK + + +*Elements supplied by the Foods.*--The following brief study will enable +the pupil to identify most of the elements present in the body and which +have, therefore, to be supplied by the foods. + +_Carbon._--Examine pieces of charred wood, coke, or coal, and also the +"lead" in lead pencils. Show that the charred wood and the coal will burn. +Recall experiment (page 114) showing that carbon in burning forms carbon +dioxide. + +_Hydrogen._--Fill a test tube one third full of strong hydrochloric acid +and drop into it several small scraps of zinc. The gas which is evolved is +hydrogen. When the hydrogen is coming off rapidly, bring a lighted +splinter to the mouth of the tube. The gas should burn. Hold a cold piece +of glass over the flame and observe the deposit of moisture. Hydrogen in +burning forms water. Extinguish the flame by covering the top of the tube +with a piece of cardboard. Now let the escaping gas collect in a tumbler +inverted over the tube. After holding the tumbler in this position for two +or three minutes, remove and, keeping inverted, thrust a lighted splinter +into it. (The gas should either burn or explode.) What does this +experiment show relative to the weight of hydrogen as compared with that +of air? + +_Nitrogen._--Nitrogen forms about four fifths of the atmosphere, where, +like oxygen, it exists in a free state. It may be separated from the +oxygen of an inclosed portion of air by causing that gas to unite with +phosphorus. Place a piece of phosphorus the size of a pea in a depression +in a flat piece of cork. (Handle phosphorus with wet fingers or with +forceps.) Place the cork on water and have ready a glass fruit jar holding +not more than a quart. Ignite the phosphorus with a hot wire and invert +the jar over it, pushing the mouth below the surface of the water. The +phosphorus uniting with the oxygen fills the jar with white fumes of +phosphoric oxide. These soon dissolve in the water, leaving a clear gas +above. This is nitrogen. Place a cardboard under the mouth of the jar and +turn it right side up, leaving in the water and keeping the top covered. +Light a splinter and, slipping the cover to one side, thrust the flame +into the jar of nitrogen, noting the effect. (Flame is extinguished.) +Compare nitrogen with oxygen in its relation to combustion. What purpose +is served by each in the atmosphere? + +_Oxygen._--Review experiments (page 114) showing the properties of oxygen. + +_Phosphorus._--Examine a small piece of phosphorus, noting that it has to +be kept under water. Lay a small piece on the table and observe the tiny +stream of white smoke rising from it, formed by slow oxidation. Dissolve a +piece as large as a pea in a teaspoonful of carbon disulphide in a test +tube, pour this on a piece of porous paper, and lay the paper on an iron +support. When the carbon disulphide evaporates the phosphorus takes fire +spontaneously. (The heat from the slow oxidation is sufficient to ignite +the phosphorus in the finely divided condition.) What is the most striking +property of phosphorus? What purpose does it serve in the match? + +_Sulphur._--Examine some sulphur, noting its color and the absence of odor +or taste. (Impure sulphur may have an odor and a taste.) Burn a little +sulphur in an iron spoon, noting that the compound which it forms with +oxygen by burning has a decided odor. + +_Other Elements._--_Magnesium._ Examine and burn a piece of magnesium +ribbon, noting the white compound of magnesium oxide which is formed. +_Iron._ Examine pieces of the metal and also some of its compounds, as +ferrous sulphate, ferric chloride, and ferric oxide or iron rust. +_Sodium._ Drop a piece of the metal on water and observe results. Sodium +decomposes water. It has to be kept under some liquid, such as kerosene, +which contains no oxygen. (It should not be touched except with the +fingers wet with kerosene.) _Chlorine._ Pour strong hydrochloric acid on a +little manganese dioxide in a test tube, and warm gently over a low flame. +The escaping gas is chlorine. Avoid breathing much of it. + +*Composition of the Nutrients.*--The simplest way of determining what +elements make up the different nutrients is by heating them and studying +the products of decomposition, as follows: + +_To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen._--Place +one half teaspoonful of powdered starch in a test tube and heat strongly. +Observe that _water_ condenses on the sides of the tube and that a black, +charred mass remains behind. The black mass consists mainly of _carbon_. +The water is composed of hydrogen and oxygen. These three elements are +thus shown to be present in the starch. The experiment may be repeated, +using sugar instead of starch. + +_To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and +Sulphur._--Place in a test tube some finely divided proteid which has been +thoroughly dried (dried beef or the lean of hard cured bacon). Heat +strongly in the hood of a chemical laboratory or some other place where +the odors do not get into the room. First hold in the escaping gases a wet +strip of red litmus paper. This will be turned blue, showing _ammonia_ +(NH3) to be escaping. Next hold in the mouth of the tube a strip of a +paper wet with a solution of lead nitrate. This is turned black or brown +on account of _hydrogen sulphide_(H2S) which is being driven off. Observe +also that _water_ condenses in the upper part of the tube and that a +black, charred mass remains behind. Since the products of decomposition +(H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen, +sulphur, and carbon, these elements are of course present in the proteid +tested. + +_To show the Presence of Mineral Matter._--Burn a piece of dry bread by +holding it in a clear, hot flame, and observe the ash that is left behind. +This is the mineral matter present in the bread. + +*Tests for Nutrients.* _Proteids._--Cover the substance to be tested with +strong nitric acid and heat gradually to boiling. If proteid is present it +turns yellow and partly dissolves in the acid, forming a yellow solution. +Let cool and then add ammonia. The yellow solid and the solution are +turned a deep orange color. Apply this test to foods containing proteid +such as white of egg, cheese, lean meat, etc. + +_Starch._--_(a)_ Place a small lump of starch in one fourth of a pint of +water and heat gradually to boiling, stirring well. Then add enough water +to form a thin liquid and fill a test tube half full. Add to this a few +drops of a solution of iodine. (Prepare by dissolving a crystal of iodine +in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in +water and add water to this until it is a light amber color.) The starch +solution is turned blue, _(b)_ Cut with a razor a thin slice from a +potato. Place this in a weak solution of iodine for a few minutes and then +examine with the microscope, using first a low and then a high power. +Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60). + +_Dextrose, or Grape Sugar._--Place a solution of the substance supposed to +contain grape sugar in a test tube and add a few drops of a dilute +solution of copper sulphate. Then add sodium hydroxide solution until the +precipitate which first forms is redissolved and a clear blue liquid +obtained. Heat the upper portion of the liquid slowly to near the boiling +point. A little below the boiling point the blue color disappears and a +yellow-red precipitate is formed. If the upper layer of the liquid is now +boiled, the color deepens and this may be contrasted with the blue color +below. Apply this test to the sugar in raisins and in honey. + +_Fat._--Fat is recognized by its effect on paper, making a greasy stain +which does not disappear on heating and which renders the paper +translucent. Try butter, lard, or olive oil. Also show the presence of fat +in peanuts by crushing them in a mortar and rubbing the powder on thin +paper. If the substance to be tested contains but little fat, this may be +dissolved out with ether. If a drop of ether containing the fat is placed +on paper, it evaporates, leaving the fat, which then forms the stain. + +*To show the Effect of Alcohol upon Proteid.*--Place some of the white of a +raw egg in a glass vessel and cover it with a small amount of alcohol. As +the albumin (proteid) hardens, or coagulates, observe that the quantity of +clear liquid increases. This is due to the _withdrawal_ of water from the +albumin by the alcohol. Since the tissues are made up chiefly of proteids, +a piece of muscle or of liver may be used in the experiment, instead of +the egg, with similar results. + +*To illustrate the Digestive Process.*--To a tumbler two thirds full of +water add a little salt. Stir and observe that the salt is dissolved. +Taste the solution to see that the salt has not been changed chemically. +Now add a little powdered limestone to the water and stir as before. +Observe that the limestone does not dissolve. Then add some hydrochloric +acid and observe the result. State the part played by the acid and by the +water in dissolving the limestone. Apply to the digestion of the different +classes of foods. + + + + +CHAPTER X - ORGANS AND PROCESSES OF DIGESTION + + +The organs of digestion are adapted to the work of dissolving the foods by +both their structure and arrangement. Most of them consist either of tubes +or cavities and these are so connected, one with the other, as to form a +continuous passageway entirely through the body. This passageway is known +as + +*The Alimentary Canal. *--The alimentary canal has a length of about thirty +feet and, while it begins at the mouth, all but about eighteen inches of +it is found in the abdominal cavity. On account of its length it lies for +the most part in coils, the two largest ones being known as the small +intestine and the large intestine. Connected with the alimentary canal are +the glands that supply the liquids for acting on the food. The divisions +of the canal and most of the glands that empty liquids into it are shown +in Fig. 63 and named in the table below: + + [Table] + +*Coats of the Alimentary Canal.*--The walls of the alimentary canal, except +at the mouth, are distinct from the surrounding tissues and consist in +most places of at least three layers, or coats, as follows: + + [Fig. 63] + + + Fig. 63--*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3. +Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7. +Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12. + Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid + flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and + pancreas. 19. Liver. + + Diagram does not show comparative length of the small intestine. + + +1. An _inner coat_, or lining, known as the mucous membrane. This membrane +is not confined to the alimentary canal, but lines, as we have seen, the +different air passages. It covers, in fact, all those internal surfaces of +the body that connect with the external surface. It derives its name from +the substance which it secretes, called _mucus_. In structure it resembles +the skin, being continuous with the skin where cavities open to the +surface. It is made up of two layers--a thick underlayer which contains +blood vessels, nerves, and glands, and a thin surface layer, called the +_epithelium._ The epithelium, like the cuticle, is without blood vessels, +nerves, or glands. + +2. A _middle coat_, which is muscular and which forms a continuous layer +throughout the canal, except at the mouth. (Here its place is taken by the +strong muscles of mastication which are separate and distinct from each +other.) As a rule the muscles of this coat are involuntary. They surround +the canal as thin sheets and at most places form two distinct layers. In +the inner layer the fibers encircle the canal, but in the outer layer they +run longitudinally, or lengthwise, along the canal.(57) + +3. An _outer_ or _serous coat_, which is limited to those portions of the +canal that occupy the abdominal cavity. This coat is not found above the +diaphragm. It is a part of the lining membrane of the cavity of the +abdomen, called + + [Fig. 64] + + +Fig. 64--*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3. + Small intestine. 4. Pancreas. + + +*The Peritoneum.*--The peritoneum is to the abdominal cavity what the +pleura is to the thoracic cavity. It forms the outer covering for the +alimentary canal and other abdominal organs and supplies the inner lining +of the cavity itself. It is also the means of holding these organs in +place, some of them being suspended by it from the abdominal walls (Fig. +64). By the secretion of a small amount of liquid, it prevents friction of +the parts upon one another. + +*Digestive Glands.*--The glands which provide the different fluids for +acting on the foods derive their constituents from the blood. They are +situated either in the mucous membrane or at convenient places outside of +the canal and pass their liquids into it by means of small tubes, called +ducts. In the canal the food and the digestive fluids come in direct +contact--a condition which the dissolving processes require. Each kind of +fluid is secreted by a special kind of gland and is emptied into the canal +at the place where it is needed. + +*The Digestive Processes.*--Digestion is accomplished by acting upon the +food in different ways, as it is passed along the canal, with the final +result of reducing it to the form of a solution. Several distinct +processes are necessary and they occur in such an order that those +preceding are preparatory to those that follow. These processes are known +as _mastication, insalivation, deglutition, stomach digestion_, and +_intestinal_ digestion. As the different materials become liquefied they +are transferred to the blood, and substances not reduced to the liquid +state are passed on through the canal as waste. The first two of the +digestive processes occur in + +*The Mouth.*--This is an oval-shaped cavity situated at the very beginning +of the canal. It is surrounded by the lips in front, by the cheeks on the +sides, by the hard palate above and the soft palate behind, and by the +tissues of the lower jaw below. The mucous membrane lining the mouth is, +soft and smooth, being covered with flat epithelial cells. The external +opening of the mouth is guarded by the lips, and the soft palate forms a +_movable_ partition between the mouth and the pharynx. In a condition of +repose the mouth space is practically filled by the teeth and the tongue, +but the cavity may be enlarged and room provided for food by depressing +the lower jaw. + +The mouth by its construction is well adapted to carrying on the processes +of mastication and insalivation. By the first process the solid food is +reduced, by the cutting and grinding action of the teeth, to a finely +divided condition. By the second, the saliva becomes mixed with the food +and is made to act upon it. + + [Fig. 65] + + +Fig. 65--*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine. + 3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower + jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and + lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4. +Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact. + + +*Accessory Organs of the Mouth.*--The work of mastication and insalivation +is accomplished through organs situated in and around the mouth cavity. +These comprise: + +1. _The Teeth._--The teeth are set in the upper and lower jaws, one row +directly over the other, with their hardened surfaces facing. In reducing +the food, the teeth of the lower jaw move against those of the upper, +while the food is held by the tongue and cheeks between the grinding +surfaces. The front teeth are thin and chisel-shaped. They do not meet so +squarely as do the back ones, but their edges glide over each other, like +the blades of scissors--a condition that adapts them to cutting off and +separating the food (_D_, Fig. 65). The back teeth are broad and +irregular, having surfaces that are adapted to crushing and grinding. + +Each tooth is composed mainly of a bone-like substance, called _dentine_, +which surrounds a central space, containing blood vessels and nerves, +known as the _pulp cavity_. It is set in a depression in the jaw where it +is held firmly in place by a bony substance, known as _cement_. The part +of the tooth exposed above the gum is the _crown_, the part surrounded by +the gum is the _neck_, and the part which penetrates into the jaw is the +_root_ (_A_, Fig. 65). A hard, protective material, called _enamel_, +covers the exposed surface of the tooth. + +The teeth which first appear are known as the _temporary_, or milk, teeth +and are twenty in number, ten in each jaw. They usually begin to appear +about the sixth month, and they disappear from the mouth at intervals from +the sixth to the thirteenth year. As they leave, teeth of the second, or +_permanent_, set take their place. This set has thirty-two teeth of four +different kinds arranged in the two jaws as follows: + +In front, above and below, are four chisel-shaped teeth, known as the +_incisors_. Next to these on either side is a tooth longer and thicker +than the incisors, called the _canine_. Back of these are two short, +rounded and double pointed teeth, the _bicuspids_, and back of the +bicuspids are three heavy teeth with irregular grinding surfaces, called +the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each +jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The +molars are known as the superadded permanent teeth because they do not +take the place of milk teeth, but form farther back as the jaw grows in +length. + + [Fig. 66] + + + Fig. 66--*Diagram* showing directions of muscular fibers in tongue. + + +2. _The Tongue._--The tongue is a muscular organ whose fibers extend +through it in several directions (Fig. 66). Its structure adapts it to a +variety of movements. During mastication the tongue transfers the food +from one part of the mouth to another, and, with the aid of the cheeks, +holds the food between the rows of teeth. (By an outward pressure from the +tongue and an inward pressure from the cheek the food is kept between the +grinding surfaces.) The tongue has functions in addition to these and is a +most useful organ. + +3. _The Muscles of Mastication._--These are attached to the lower jaw and +bring about its different movements. The _masseter_ muscles, which are the +heavy muscles in the cheeks, and the _temporal_ muscles, located in the +region of the temples, raise the lower jaw and supply the force for +grinding the food. Small muscles situated below the chin depress the jaw +and open the mouth. + + [Fig. 67] + + + Fig. 67--*Salivary glands* and the ducts connecting them with the mouth. + + +4. _The Salivary Glands._--These glands are situated in the tissues +surrounding the mouth, and communicate with it by means of ducts (Fig. +67). They secrete the saliva. The salivary glands are six in number and +are arranged in three pairs. The largest, called the _parotid_ glands, +lie, one on either side, in front of and below the ears. A duct from each +gland passes forward along the cheek until it opens in the interior of the +mouth, opposite the second molar tooth in the upper jaw. Next in size to +the parotids are the _submaxillary_ glands. These are located, one on +either side, just below and in front of the triangular bend in the lower +jaw. The smallest of the salivary glands are the _sublingual_. They are +situated in the floor of the mouth, on either side, at the front and base +of the tongue. Ducts from the submaxillary and sublingual glands open into +the mouth below the tip of the tongue. + +*The Saliva and its Uses.*--The saliva is a transparent and somewhat slimy +liquid which is slightly alkaline. It consists chiefly of water (about 99 +per cent), but in this are dissolved certain salts and an active chemical +agent, or enzyme, called _ptyalin_, which acts on the starch. The ptyalin +changes starch into a form of sugar (maltose), while the water in the +saliva dissolves the soluble portions of the food. In addition to this the +saliva moistens and lubricates the food which it does not dissolve, and +prepares it in this way for its passage to the stomach. The last is +considered the most important use of the saliva, and dry substances, such +as crackers, which require a considerable amount of this liquid, cannot be +eaten rapidly without choking. Slow mastication favors the secretion and +action of the saliva. + +*Deglutition.*--Deglutition, or swallowing, is the process by which food is +transferred from the mouth to the stomach. Though this is not, strictly +speaking, a digestive process, it is, nevertheless, necessary for the +further digestion of the food. Mastication and insalivation, which are +largely mechanical, prepare the food for certain chemical processes by +which it is dissolved. The first of these occurs in the stomach and to +this organ the food is transferred from the mouth. The chief organs +concerned in deglutition are the tongue, the pharynx, and the esophagus. + +*The Pharynx* is a round and somewhat cone-shaped cavity, about four and +one half inches in length, which lies just back of the nostrils, mouth, +and larynx. It is remarkable for its openings, seven in number, by means +of which it communicates with other cavities and tubes of the body. One of +these openings is into the mouth, one into the esophagus, one into the +larynx, and one into each of the nostrils, while two small tubes (the +eustachian) pass from the upper part of the pharynx to the middle ears. + +The pharynx is the part of the food canal that is crossed by the +passageway for the air. To keep the food from passing out of its natural +channel, the openings into the air passages have to be carefully guarded. +This is accomplished through the soft palate and epiglottis, which are +operated somewhat as valves. The muscular coat of the pharynx is made up +of a series of overlapping muscles which, by their contractions, draw the +sides together and diminish the cavity. The mucous membrane lining the +pharynx is smooth, like that of the mouth, being covered with a layer of +flat epithelial cells. + +*The Esophagus*, or gullet, is a tube eight or nine inches long, +connecting the pharynx with the stomach. It lies for the most part in the +thoracic cavity and consists chiefly of a thick mucous lining surrounded +by a heavy coat of muscle. The muscular coat is composed of two layers--an +inner layer whose fibers encircle the tube and an outer layer whose fibers +run lengthwise. + +*Steps in Deglutition.*--The process of deglutition varies with the kind of +food. With bulky food it consists of three steps, or stages, as follows: +1. By the contraction of the muscles of the cheeks, the food ball, or +bolus, is pressed into the center of the mouth and upon the upper surface +of the tongue. Then the tongue, by an upward and backward movement, pushes +the food under the soft palate and into the pharynx. + +2. As the food passes from the mouth, the pharynx is drawn up to receive +it. At the same time the soft palate is pushed upward and backward, +closing the opening into the upper pharynx, while the epiglottis is made +to close the opening into the larynx. By this means all communication +between the food canal and the air passages is temporarily closed. The +upper muscles of the pharynx now contract upon the food, forcing it +downward and into the esophagus. + +3. In the esophagus the food is forced along by the successive +contractions of muscles, starting at the upper end of the tube, until the +stomach is reached. + +Swallowing is doubtless aided to some extent by the force of gravity. That +it is independent of this force, however, is shown by the fact that one +may swallow with the esophagus in a horizontal position, as in lying down. + + [Fig. 68] + + + Fig. 68--*Gastric Glands.* _A._ Single gland showing the two kinds of +secreting cells and the duct where the gland opens on to the surface. _B._ + Inner surface of stomach magnified. The small pits are the openings from + the glands. + + +*The Stomach.*--The stomach is the largest dilatation of the alimentary +canal. It is situated in the abdominal cavity, immediately below the +diaphragm, with the larger portion toward the left side. Its connection +with the esophagus is known as the _cardiac orifice_ and its opening into +the small intestine is called the _pyloric orifice_. It varies greatly in +size in different individuals, being on the average from ten to twelve +inches at its greatest length, from four to five inches at its greatest +width, and holding from three to five pints. It has the coats common to +the canal, but these are modified somewhat to adapt them to its work. + +_The mucous membrane_ of the stomach is thick and highly developed. It +contains great numbers of minute tube-shaped bodies, known as the _gastric +glands_ (Fig. 68). These are of two general kinds and secrete large +quantities of a liquid called the gastric juice. When the stomach is +empty, the mucous membrane is thrown into folds which run lengthwise over +the inner surface. These disappear, however, when the walls of the stomach +are distended with food. + +_The muscular coat_ consists of _three_ separate layers which are named, +from the direction of the fibers, the circular layer, the longitudinal +layer, and the oblique layer (Fig. 69). The circular layer becomes quite +thick at the pyloric orifice, forming a distinct band which serves as a +valve. + + [Fig. 69] + + +Fig. 69--*Muscles of the stomach* (from Morris' _Human Anatomy_). The layer + of Longitudinal fibers removed. + + +The outer coat of the stomach, called the _serous coat_, is a continuation +of the peritoneum, the membrane lining the abdominal cavity. + +*Stomach Digestion.*--In the stomach begins the definite work of dissolving +those foods which are insoluble in water. This, as already stated, is a +double process. There is first a chemical action in which the insoluble +are changed into soluble substances, and this is followed immediately by +the dissolving action of water. The chief substances digested in the +stomach are the proteids. These, in dissolving, are changed into two +soluble substances, known as _peptones_ and _proteoses_. The digestion of +the proteids is, of course, due to the + +*Gastric Juice.*--The gastric juice is a thin, colorless liquid composed of +about 99 per cent of water and about 1 per cent of other substances. The +latter are dissolved in the water and include, besides several salts, +three active chemical agents--hydrochloric acid, pepsin, and rennin. +_Pepsin_ is the enzyme which acts upon proteids, but it is able to act +only in an acid medium--a condition which is supplied by the _hydrochloric +acid_. Mixed with the hydrochloric acid it converts the proteids into +peptones and proteoses. + +*Other Effects of the Gastric Juice.*--In addition to digesting proteids, +the gastric juice brings about several minor effects, as follows: + +1. It checks, after a time, the digestion of the starch which was begun in +the mouth by the saliva.(58) This is due to the presence of the +hydrochloric acid, the ptyalin being unable to act in an acid medium. + +2. While there is no appreciable action on the fat itself, the proteid +layers that inclose the fat particles are dissolved away (Fig. 79), and +the fat is set free. By this means the fat is broken up and prepared for a +special digestive action in the small intestine. + +3. Dissolved albumin, like that in milk, is curded, or coagulated, in the +stomach. This action is due to the _rennin_. The curded mass is then acted +upon by the pepsin and hydrochloric acid in the same manner as the other +proteids. + +4. The hydrochloric acid acts on certain of the insoluble mineral salts +found in the foods and reduces them to a soluble condition. + +5. It is also the opinion of certain physiologists that cane sugar and +maltose (double sugars) are converted by the hydrochloric acid into +dextrose and levulose (single sugars). + +After a variable length of time, the contents of the stomach is reduced to +a rather uniform and pulpy mass which is called _chyme_. Portions of this +are now passed at intervals into the small intestine. + +*Muscular Action of the Stomach.*--The muscles in the walls of the stomach +have for one of their functions the mixing of the food with the gastric +juice. By _alternately_ contracting and relaxing, the different layers of +muscle keep the form of the stomach changing--a result which agitates and +mixes its contents. This action varies in different parts of the organ, +being slight or entirely absent at the cardiac end, but quite marked at +the pyloric end. + +Another purpose of the muscular coat is to empty the stomach into the +small intestine. During the greater part of the digestive period the +muscular band at the pyloric orifice is contracted. At intervals, however, +this band relaxes, permitting a part of the contents of the stomach to be +forced into the small intestine. After the discharge the pyloric muscle +again contracts, and so remains until the time arrives for another +discharge. + +In addition to emptying the stomach into the small intestine, these +muscles also aid in emptying the organ upward and through the esophagus +and mouth, should occasion require. Vomiting in case of poisoning, or if +the food for some reason fails to digest, is a necessary though unpleasant +operation. It is accomplished by the contraction of all the muscles of the +stomach, together with the contraction of the walls of the abdomen. During +these contractions the pyloric valve is closed, and the muscles of the +esophagus and pharynx are in a relaxed condition.(59) + + [Fig. 70] + + + Fig. 70--*Passage from stomach* into small intestine. Illustration also + shows arrangement of mucous membrane in the two organs. _D._ Bile duct. + + +*The Small Intestine.*--This division of the alimentary canal consists of a +coiled tube, about twenty-two feet in length, which occupies the central, +lower portion of the abdominal cavity (Fig. 71). At its upper extremity it +connects with the pyloric end of the stomach (Fig. 70), and at its lower +end it joins the large intestine. It averages a little over an inch in +diameter, and gradually diminishes in size from the stomach to the large +intestine. The first eight or ten inches form a short curve, known as the +_duodenum_. The upper two fifths of the remainder is called the _jejunum_, +and the lower three fifths is known as the _ileum_. The ileum joins that +part of the large intestine known as the caecum, and at their place of +union is a marked constriction which prevents material from passing from +the large into the small intestine (Fig. 73). This is known as the +_ileo-caecal valve_. + +_The mucous membrane_ of the small intestine is richly supplied with blood +vessels and contains glands that secrete a digestive fluid known as the +_intestinal juice_. The membrane is thrown into many transverse, or +circular, folds which increase its surface and also prevent materials from +passing too rapidly through the intestine. One important respect in which +the small intestine differs from all other portions of the food canal is +that its surface is covered with great numbers of minute elevations known +as the villi. The purpose of these is to aid in the absorption of the +nutrients as they become dissolved (Chapter XI). + +_The muscular coat_ of the small intestine is made up of two distinct +layers--the inner layer consisting of circular fibers and the outer of +longitudinal fibers. These muscles keep the food materials mixed with the +juices of the small intestine, but their main purpose is to force the +materials undergoing digestion through this long and much-coiled tube. + +The outer, or _serous_, coat of the small intestine, like that of the +stomach, is an extension from the general lining of the abdominal cavity, +or peritoneum. In fact, the intestine lies in a fold of the peritoneum, +somewhat as an arm in a sling, while the peritoneum, by connecting with +the back wall of the abdominal cavity, holds this great coil of digestive +tubing in place (Fig. 64). The portion of the peritoneum which attaches +the intestine to the wall of the abdomen is called the _mesentery_. + +Most of the liquid acting on the food in the small intestine is supplied +by two large glands, the liver and the pancreas, that connect with it by +ducts. + + [Fig. 71] + + + Fig. 71--*Abdominal cavity* with organs of digestion in position. + + +*The Liver* is situated immediately below the diaphragm, on the right side +(Figs. 71 and 72), and is the largest gland in the body. It weighs about +four pounds and is separated into two main divisions, or lobes. It is +complex in structure and differs from the other glands in several +particulars. It receives blood from two distinct sources--the portal vein +and the hepatic artery. _The portal vein_ collects the blood from the +stomach, intestines, and spleen, and passes it to the liver. This blood is +loaded with food materials, but contains little or no oxygen. The _hepatic +artery_, which branches from the aorta, carries to the liver blood rich in +oxygen. In the liver the portal vein and the hepatic artery divide and +subdivide, and finally empty their blood into a single system of +capillaries surrounding the liver cells. These capillaries in turn empty +into a single system of veins which, uniting to form the _hepatic veins_ +(two or three in number), pass the blood into the inferior vena cava (Fig. +72). + + [Fig. 72] + + + Fig. 72--*Relations of the liver.* Diagram showing the connection of the + liver with the large blood vessels and the food canal. + + +The liver secretes daily from one to two pounds of a liquid called _bile_. +A reservoir for the bile is provided by a small, membranous sack, called +the _gall bladder_, located on the underside of the liver. The bile passes +from the gall bladder, and from the right and left lobes of the liver, by +three separate ducts. These unite to form a common tube which, uniting +with the duct from the pancreas, empties into the duodenum. Though usually +described as a digestive gland, the liver has other functions of equal or +greater importance (Chapter XIII). + +*The Bile* is a golden yellow liquid, having a slightly alkaline reaction +and a very bitter taste. It consists, on the average, of about 97 per cent +of water and 3 per cent of solids.(60) The solids include bile pigments, +bile salts, a substance called cholesterine, and mineral salts. The +pigments (coloring matter) of the bile are derived from the hemoglobin of +broken-down red corpuscles (page 27). + +Much about the composition of the bile is not understood. It is known, +however, to be necessary to digestion, its chief use being to aid in the +digestion and absorption of fats. It is claimed also that the bile aids +the digestive processes in some general ways--counteracting the acid of the +gastric juice, preventing the decomposition of food in the intestines, and +stimulating muscular action in the intestinal walls. No enzymes have been +discovered in the bile. + +*The Pancreas* is a tapering and somewhat wedge-shaped gland, and is so +situated that its larger extremity, or head, is encircled by the duodenum. +From here the more slender portion extends across the abdominal cavity +nearly parallel to and behind the lower part of the stomach. It has a +length of six or eight inches and weighs from two to three and one half +ounces. Its secretion, the pancreatic juice, is emptied into the duodenum +by a duct which, as a rule, unites with the duct from the liver. + +*The Pancreatic Juice* is a colorless and rather viscid liquid, having an +alkaline reaction. It consists of about 97.6 per cent of water and 2.4 per +cent of solids. The solids include mineral salts (the chief of which is +sodium carbonate) and four different chemical agents, or enzymes,--trypsin, +amylopsin, steapsin, and a milk-curding enzyme. These active constituents +make of the pancreatic juice the most important of the digestive fluids. +It acts with vigor on all of the nutrients insoluble in water, producing +the following changes: + +1. It converts the starch into maltose, completing the work begun by the +saliva. This action is due to the _amylopsin_,(61) which is similar to +ptyalin but is more vigorous. + +2. It changes proteids into peptones and proteoses, completing the work +begun by the gastric juice. This is accomplished by the _trypsin_, which +is similar to, but more active than, the pepsin. + +3. It digests fat. In this work the active agent is the _steapsin_. + +The necessity of a milk-curding enzyme, somewhat similar to the rennin of +the gastric juice, is not understood. + +*Digestion of Fat.*--Several theories have been proposed at different times +regarding the digestion and absorption of fat. Among these, what is known +as the "solution theory" seems to have the greatest amount of evidence in +its favor. According to this theory, the fat, under the influence of the +steapsin, absorbs water and splits into two substances, recognized as +glycerine and fatty acid. This finishes the process so far as the +glycerine is concerned, as this is soluble in water; but the fatty acid, +which (from certain fats) is insoluble in water,(62) requires further +treatment. The fatty acid is now supposed to be acted on in one, or both, +of the following ways: 1. To be dissolved as fatty acid by the action of +the bile (since bile is capable of dissolving it under certain +conditions). 2. To be converted by the sodium carbonate into a form of +soap which is soluble in water. + +The emulsification of fat is known to occur in the small intestine. By +this process the fat is separated into minute particles which are +suspended in water, but not changed chemically, the mixture being known as +an _emulsion_. While this is believed by some to be an actual process of +digestion, the advocates of the solution theory claim that it is a process +accompanying and aiding the conversion of fat into fatty acid and +glycerine.(63) + +*The Intestinal Juice* is a clear liquid with an alkaline reaction, +containing water, mineral salts, and certain proteid substances that may +act as enzymes. It assists in bringing about an alkaline condition in the +small intestine and aids in the reduction of cane sugar and maltose to the +simple sugars, dextrose and levulose. Since it is difficult to obtain this +liquid in sufficient quantities for experimenting, its uses have not been +fully determined. Recent investigators, however, assign to it an important +place in the work of digestion. + +*Work of the Small Intestine.*--The small intestine is the most important +division of the alimentary canal. It serves as a receptacle for holding +the food while it is being acted upon; it secretes the intestinal juice +and mixes the food with the digestive fluids; it propels the food toward +the large intestine; and, in addition to all this, serves as an organ of +absorption. + +Digestion is practically finished in the small intestine, and a large +portion of the reduced food is here absorbed. There is always present, +however, a variable amount of material that is not digested. This, +together with a considerable volume of liquid, is passed into + +*The Large Intestine.*--The large intestine is a tube from five to six feet +in length and averaging about one and one half inches in diameter. It +begins at the lower right side of the abdominal cavity, forms a coil which +almost completely surrounds the coil of small intestine, and finally +terminates at the surface of the body (Figs. 2, 71 and 73). It has three +divisions, known as the caecum, the colon, and the rectum. + + [Fig. 73] + + +Fig. 73--*Passage from small into large intestine.* At the ileo-caecal valve + is the narrowest constriction of the food canal. + + +_The caecum_ is the pouch-like dilatation of the large intestine which +receives the lower end of the small intestine. It measures about two and +one half inches in diameter and has extending from one side a short, +slender, and blind tube, called the _vermiform appendix_. This structure +serves no purpose in digestion, but appears to be the rudiment of an organ +which may have served a purpose at some remote period in the history of +the human race. The caecum gradually blends into the second division of the +large intestine, called the colon. + +_The colon_ consists of four parts, described as the ascending colon, the +transverse colon, the descending colon, and the sigmoid flexure, or +sigmoid colon. The first three divisions are named from the direction of +the movement of materials through them and the last from its shape, which +is similar to that of the Greek letter sigma ({~GREEK CAPITAL LETTER SIGMA~}). + +_The rectum_ is the last division of the large intestine It is a nearly +straight tube, from six to eight inches in length, and connects with the +external surface of the body. + +The general structure of the large intestine is similar to that of the +small intestine, and, like the small intestine, it is held in place by the +peritoneum. It differs from the small intestine, however, in its lining of +mucous membrane and in the arrangement of the muscular coat. The mucous +membrane presents a smooth appearance and has no villi, while the +longitudinal layer of the muscular coat is limited to three narrow bands +that extend along the greater length of the tube (Fig. 74). These bands +are shorter than the coats, and draw the large intestine into a number of +shallow pouches, by which it is readily distinguished from the small +intestine (Fig. 71). + + [Fig. 74] + + + Fig. 74--*Section of large intestine*, showing the coats. 1. Serous coat. + 2. Circular layer of muscle. 3. Submucous coat. 4. Mucous membrane. 5. + Muscular bands extending lengthwise over the intestine. + + +*Work of the Large Intestine.*--The large intestine serves as a receptacle +for the materials from the small intestine. The digestive fluids from the +small intestine continue their action here, and the dissolved materials +also continue to be absorbed. In these respects the work of the large +intestine is similar to that of the small intestine. It does, however, a +work peculiar to itself in that it collects and retains undigested food +particles, together with other wastes, and ejects them periodically from +the canal. + +*Work of the Alimentary Muscles.*--The mechanical part of digestion is +performed by the muscles that encircle the food canal. Their uses, which +have already been mentioned in connection with the different organs of +digestion, may be here summarized: They supply the necessary force for +masticating the food. They propel the food through the canal. They mix the +food with the different juices. At certain places they partly or +completely close the passage until a digestive process is completed. They +may even cause a reverse movement of the food, as in vomiting. All of the +alimentary muscles, except those around the mouth, are involuntary. Their +work is of the greatest importance. + +*Other Purposes of the Digestive Organs.*--The digestive organs serve other +important purposes besides that of dissolving the foods. They provide +favorable conditions for passing the dissolved material into the blood. +They dispose of such portions of the foods as fail, in the digestive +processes, to be reduced to a liquid state. A considerable amount of waste +material is also separated from the blood by the glands of digestion +(especially the liver), and this is passed from the body with the +undigested portions of food. Then the food canal (stomach in particular) +is a means of holding, or storing, food which is awaiting the processes of +digestion. Considering the number of these purposes, the digestive organs +are remarkably simple, both in structure and in method of operation. + + + +HYGIENE OF DIGESTION + + +Many of the ills to which flesh is heir are due to improper methods of +taking food and are cured by observing the simple rules of eating. Habit +plays a large part in the process and children should, for this reason, be +taught early to eat properly. Since the majority of the digestive +processes are involuntary and the food, after being swallowed, is +practically beyond control, careful attention must be given to the proper +mastication of the food and to such other phases of digestion as are under +control. + +*Necessity for Thorough Mastication.*--Mastication prepares the food for +the digestive processes which follow. Unless the food has been properly +masticated, the digestive fluids in the stomach and intestines cannot act +upon it to the best advantage. When the food is carefully chewed, a larger +per cent of it is actually digested--a point of importance where economy in +the use of food needs to be practiced. + +A fact not to be overlooked is that one cannot eat hurriedly and practice +thorough mastication. The food must not be swallowed in lumps, but reduced +to a finely divided and pulpy mass. This requires time. The one who +hurries through the meal is necessarily compelled to bolt his food. Thirty +minutes is not too long to give to a meal, and a longer period is even +better. + +Perhaps the most important result of giving plenty of time to the taking +of food is that of _stimulating the digestive glands to a proper degree of +activity_. That both the salivary and gastric glands are excited by the +sight, smell, and thought of food and, through taste, by the presence of +food in the mouth, has been fully demonstrated. Food that is thoroughly +masticated and relished will receive more saliva and gastric juice, and +probably more of other juices, than if hastily chewed and swallowed. This +has a most important bearing upon the efficiency of the digestive +processes. + +*Order of Taking Food.*--There has been evolved through experience a rather +definite order of taking food, which our knowledge of the process of +digestion seems to justify. The heavy foods (proteids for the most part) +are eaten first; after which are taken starchy foods and fats; and the +meal is finished off with sweetmeats and pastry.(64) The scientific +arguments for this order are the following: + +1. By receiving the first of the gastric flow the proteids can begin +digesting without delay. Since these are the main substances acted on in +the stomach, the time required for their digestion is shortened by eating +them first. + +2. Sugar, being of the nature of predigested starch, quickly gets into the +blood and _satisfies the relish_ for food. The result of taking sugar +first may be to cause one to eat less than he needs and to diminish the +activity of the glands. + +3. Fat or grease, if taken first, tends to form a coating over the walls +of the stomach and around the material to be digested. This prevents the +juices from getting to and mixing with the foods upon which they are to +act. + +4. Starch following the proteids, for the most part, does not so quickly +come in contact with the gastric juice. This enables the ptyalin of the +saliva to continue its action for a longer time than if the starch were +eaten first. + +*Liquids during the Meal.*--Liquids as ordinarily taken during the meal are +objectionable. They tend to diminish the secretion of the saliva and to +cause rapid eating. Instead of eating slowly and swallowing the food only +so fast as the glands can supply the necessary saliva, the liquid is used +to wash the food down. Water or other drinks should be taken after the +completion of the meal or when the mouth is completely free from food. +Even then it should be taken in small sips. While the taking of a small +amount of water in this way does no harm, a large volume has the effect of +weakening the gastric juice. Most of the water needed by the body should +be taken between meals. + +*The State of Mind* has much to do with the proper digestion of the food. +Worry, anger, fear, and other disturbed mental states are known to check +the secretion of fluids and to interfere with the digestive processes. +While the cultivation of cheerfulness is important for its general +hygienic effects, it is of especial value in relation to digestion. +Intense emotions, either during or following the meal, should if possible +be avoided. The table is no place for settling difficulties or +administering rebuke. The conversation, on the other hand, should be +elevating and joy giving, thereby inducing a desirable reactionary +influence upon the digestive processes. + +*Care of the Teeth.*--The natural teeth are indispensable for the proper +mastication of the food. Of especial value are the molars--the teeth that +grind the food. The development of the profession of dentistry has made +possible the preservation of the teeth, even when naturally poor, as long +as one has need of them. To preserve the teeth they must be kept clean. +They should be washed at least once a day with a soft-bristled brush, and +small particles of food, lodged between them, should be removed with a +wooden pick. The biting of hard substances, such as nuts, should be +avoided, on account of the danger of breaking the enamel, although the +chewing of tough substances is considered beneficial. + +Decayed places in the teeth should be promptly filled by the dentist. It +is well, even when decayed places are not known to exist, to have the +teeth examined occasionally in order to detect such places before they +become large. On account of the expense, pain, and inconvenience there is +a tendency to put off dental work which one knows ought to be done. +Perhaps in no other instance is procrastination so surely punished. The +decayed places become larger and new points of decay are started; and the +pain, inconvenience, and expense are increased proportionately. + +*The Natural Appetite* should be followed with reference to both the kind +and the amount of food eaten. No system of knowledge will ever be devised +which can replace the appetite as an aid in the taking of food. _It is_ +_nature's means of indicating the needs of the body_. The natural appetite +may be spoiled, however, by overeating and by the use of highly seasoned +foods, or by indulging in stimulants during the meal. It is spoiled in +children by too free indulgence in sweetmeats. By cultivating the natural +appetite and heeding its suggestions, one has at his command an almost +infallible guide in the taking of food. + +*Preparation of Meals.*--The cooking of food serves three important +purposes. It renders the food more digestible, relieving the organs of +unnecessary work; it destroys bacteria that may be present in the food, +diminishing the likelihood of introducing disease germs into the body; and +it makes the food more palatable, thereby supplying a necessary stimulus +to the digestive glands. While the methods employed in the preparation of +the different foods have much to do with the ease with which they are +digested and with their nourishing qualities, the scope of our subject +does not permit of a consideration of these methods. + +*Quantity of Food.*--Overeating and undereating are both objectionable from +a hygienic standpoint. Overeating, by introducing an unnecessary amount of +food into the body, overworks the organs of digestion and also the organs +of excretion. It may also lead to the accumulation of burdensome fat and +of harmful wastes. On the other hand, the taking of too little food +impoverishes the blood and weakens the entire body. As a rule, however, +more people eat too much than too little, and to quit eating before the +appetite is fully satisfied is with many persons a necessary precaution. +The power of self-control, valuable in all phases of life, is +indispensable in the avoidance of overeating. + +*Frequency of Taking Food.*--Eating between meals is manifestly an +unhealthful practice. The question has also been raised as to whether the +common habit of eating three times a day is best suited to all classes of +people. Many people of weak digestive organs have been benefited by the +plan of two meals a day, while others adopt the plan of eating one heavy +meal and two light ones. Either plan gives the organs of digestion more +time to rest and diminishes the liability of overeating. On the other +hand, those doing heavy muscular work can hardly derive the energy which +they need from less than three good meals a day. Though no definite rule +can be laid down, there is involved a hygienic principle which all should +follow: _Meals should not overlap_. The stomach should be free from food +taken at a previous meal before more is introduced into it. When this +principle is not observed, material ferments in the stomach, causing +indigestion and other disorders. It should be noted, however, that the +overlapping may be due to overeating as well as to eating too frequently. + +*Dangers from Impure Food.*--Food is frequently the carrier of disease +germs and for this reason requires close inspection (page 128). Typhoid +fever, a most dangerous disease, is usually contracted through either +impure food or impure water (Chapter XXIII). One safeguard against disease +germs, as stated above, is thorough cooking. Too much care cannot be +exercised with reference to the water for drinking purposes. Water which +is not perfectly clear, which smells of decaying material, or which forms +a sediment on standing is usually not fit to drink. It can, however, be +rendered comparatively harmless by boiling. The objections which many +people have to drinking boiled water are removed when it is boiled the day +before it is used, so as to give it time to cool, settle, and replace the +air driven off by the boiling. + +*Care of the Bowels.*--In considering the hygiene of the alimentary canal, +the fact that it is used as a means of separating the impurities from the +body must not be overlooked. Frequently, through lack of exercise, +negligence in evacuating the bowels, or other causes, a weakened condition +of the canal is induced which results in the retention of impurities +beyond the time when they should be discharged. This is a great annoyance +and at the same time a menace to the health. + +In most cases this condition can be relieved, and prevented from +recurring, by observing the following habits: 1. Have a regular time each +day for evacuating the bowels. This is a most important factor in securing +the necessary movements. 2. Drink a cup of cold water on rising in the +morning and on retiring at night. 3. Eat generously of fruits and other +coarse foods, such as corn bread, oatmeal, hominy, cabbage, etc. 4. +Practice persistently such exercises as bring the abdominal muscles into +play. These exercises strengthen indirectly the muscles of the canal. 5. +Avoid overwork, especially of the nervous system. + +*Alcohol and Digestion.*--Though exciting temporarily a greater flow of the +digestive fluids, alcoholic drinks taken in any but very small quantities +are considered detrimental to the work of digestion. Large doses retard +the action of enzymes, inflame the mucous lining of the stomach,(65) and +bring about a diseased condition of the liver. It may be noted, however, +that the bad effects of alcoholic beverages upon the stomach, the liver, +and the body in general are less pronounced when these are taken as a part +of the regular meals. + +*Effects of Tea and Coffee.*--In addition to the stimulating agent +caffeine, tea and coffee contain a bitter, astringent substance, known as +tannin. On account of the tannin these beverages tend to retard digestion +and to irritate the lining of the stomach--effects that may be largely +obviated by methods of preparing tea and coffee which dissolve little of +the tannin. (They should be made without continued boiling or steeping.) +The caffeine may do harm through its stimulating effect upon the nervous +system (page 56) and through the introduction of a special waste into the +body. In chemical composition caffeine closely resembles a waste, called +uric acid, and in the body is converted into this substance. If one is in +a weakened condition, the uric acid may fail to be oxidized to urea, as +occurs normally, or to be thrown off as uric acid. In this case it +accumulates in the body, causing rheumatism and related diseases. It thus +happens that while some people may use tea and coffee without detriment, +others are injured by them. + +*Summary.*--The main structure in the digestive system is the alimentary +canal. This provides cavities where important dissolving processes take +place, and tubes for joining these cavities, while glands connecting with +the canal supply the necessary liquids for changing and dissolving the +foods. The general plan of digestion is that of passing the food through +the canal, beginning with the mouth, and of acting on it at various +places, with the final result of reducing most of it to the liquid state. +The digestive fluids supply water which acts as a solvent and carries the +active chemical agents, or enzymes, that convert the insoluble foods into +substances that are soluble. The muscles in the walls of the canal perform +the mechanical work of digestion, while the nervous system controls and +regulates the activity of the various organs concerned in this work. + +Exercises.--1. State the general purpose of digestion. How does digested +food differ from that not digested? + +2. Name all the divisions of the alimentary canal in the order in which +the food passes through them. + +3. What other work besides digestion is carried on by the alimentary +canal? + +4. What is gained by the mastication of the food? Why should mastication +precede the other processes of digestion? + +5. What is the work of the tongue in digestion? + +6. State the purposes served by the gastric juice. + +7. Give reasons for regarding the small intestine as the most important +division of the food canal. + +8. At what places, and by the action of what liquids, are fats, proteids, +and starch digested? + +9. What enzymes are found in the pancreatic juice? What is the digestive +action of each? + +10. Describe the work performed by the muscles of the stomach, the mouth, +the esophagus, and the small intestine. + +11. What advantages are derived from the use of cooked food? + +12. State the advantages of drinking pure water. + +13. If all the food that one needs to take at a single meal can be +thoroughly masticated in fifteen minutes, why is it better to spend a +longer time at the table? + +14. What is meant by the overlapping of meals? What bad results follow? +How avoided? + + + +PRACTICAL WORK + + +Examine a dissectible model of the human abdomen (Fig. 75), noting the +form, location, and connection of the different organs. Find the +connection of the esophagus with the stomach, of the stomach with the +small intestine, and of the small intestine with the large intestine. +Sketch a general outline of the cavity, and locate in this outline its +chief organs. + +Where it is desirable to learn something of the actual structure of the +digestive organs, the dissection of the abdomen of some small animal is +necessary. On account of unpleasant features likely to be associated with +such a dissection, however, this work is not recommended for immature +pupils. + + [Fig. 75] + + + Fig. 75--Model for demonstrating the abdomen and its contents. + + +*Dissection of the Abdomen.* (Optional)--For individual study, or for a +small class, a half-grown cat is perhaps the best available material. It +should be killed with chloroform, and then stretched, back downward, on a +board, the feet being secured to hold it in place. + +The teacher should make a preliminary examination of the abdomen to see +that it is in a fit condition for class study. If the bladder is +unnaturally distended, its contents may be forced out by slight pressure. +The following materials will be needed during the dissection, and should +be kept near at hand: a sharp knife with a good point, a pair of heavy +scissors, a vessel of water, some cotton or a damp sponge, and some fine +cord. During the dissection the specimen should be kept as clean as +possible, and any escaping blood should be mopped up with the cotton or +the sponge. The dissection is best carried out by observing the following +order: + +1. Cut through the abdominal wall in the center of the triangular space +where the ribs converge. From here cut a slit downward to the lower +portion of the abdomen, and sideward as far as convenient. Tack the +loosened abdominal walls to the board, and proceed to study the exposed +parts. Observe the muscles in the abdominal walls, and the fold of the +_peritoneum_ which forms an apron-like covering over the intestines. + +2. Observe the position of the stomach, liver, spleen, and intestines, and +then, by pushing the intestines to one side, find the kidneys and the +bladder. + +3. Study the liver with reference to its location, size, shape, and color. +On the under side, find the gall bladder, from which a small tube leads to +the small intestine. Observe the portal vein as it passes into the liver. +As the liver is filled with blood, neither it nor its connecting blood +vessels should be cut at this time. + +4. Trace out the continuity of the canal. Find the esophagus where it +penetrates the diaphragm and joins the stomach. Find next the union of the +stomach with the small intestine. Then, by carefully following the coils +of the small intestine, discover its union with the large intestine. + +5. Within the first coil of the small intestine, as it leaves the stomach, +find the _pancreas_. Note its color, size, and branches. Find its +connection with the small intestine. + +6. Beginning at the cut portion of the abdominal wall, lift the thin +lining of the peritoneum and carefully follow it toward the back and +central portion of the abdomen. Observe whether it extends back of or in +front of the kidneys, the aorta, and the inferior vena cava. Find where it +leaves the wall as a _double_ membrane, the _mesentery_, which surrounds +and holds in place the large and small intestines. Sketch a coil of the +intestine, showing the mesentery. + +7. Find in the center of the coils of small intestine a long, slender body +having the appearance of a gland. This is the beginning of the _thoracic +duct_ and is called the _receptacle of the chyle_. From this the thoracic +duct rapidly narrows until it forms a tiny tube difficult to trace in a +small animal. + +8. Cut away about two inches of the small intestine from the remainder, +having first tied the tube on the two sides of the section removed. Split +it open for a part of its length, and wash out its contents. Observe its +coats. Place it in a shallow vessel containing water, and examine the +mucous membrane with a lens to find the _villi_. Make a drawing of this +section, showing the coats. + +9. Study the connection of the small intestine with the large. Split them +open at the place of union, wash out the contents, and examine the +ileo-caecal valve. + +10. Observe the size, shape, and position of the kidneys. Do they lie in +front of or back of the peritoneum? Do they lie exactly opposite each +other? Note the connection of each kidney with the aorta and the inferior +vena cava by the renal artery and the renal vein. Find a slender tube, the +_ureter_, running from each kidney to the bladder. Do the ureters connect +with the top or with the base of the bladder? Show by a sketch the +connection of the kidneys with the large blood vessels and the bladder. + +*To demonstrate the Teeth.*--Procure from the dentist a collection of +different kinds of teeth, both sound and decayed. + +(_a_) Examine external surfaces of different kinds of teeth, noting +general shape, cutting or grinding surfaces, etc. Make a drawing of an +incisor and also of a molar. + +(_b_) After soaking some of the teeth for a couple of days in warm water +saw one of them in two lengthwise, and another in two crosswise, and +smooth the cut surfaces with fine emery or sand paper. Examine both kinds +of sections, noting arrangement and extent of dentine, enamel, and pulp. +Make drawings. + +(_c_) Examine a decayed tooth. Which substance of the tooth appears to +decay most readily? Why is it necessary to cut away a part of the tooth +before filling? + +(_d_) Test the effect of acids upon the teeth by leaving a tooth over +night in a mixture of one part hydrochloric acid to four parts water, and +by leaving a second tooth for a couple of days in strong vinegar. Examine +the teeth exposed to the action of acids, noting results. + +*To show the Importance of Mastication.*--Fill two tumblers each half full +of water. Into one put a lump of rock salt. Into the other place an equal +amount of salt that has been finely pulverized. Which dissolves first and +why? + +*To illustrate Acid and Alkaline Reactions.*--To a tumbler half full of +water add a teaspoonful of hydrochloric or other acid, as vinegar. To a +second tumbler half full of water add an equal amount of cooking soda. +Taste each liquid, noting the sour taste of the acid, and the alkaline +taste of the soda. Hold a piece of red litmus paper in the soda solution, +noting that it is turned blue. Then hold a piece of blue litmus paper in +the acid solution, noting that it is turned red. Add acid to the soda +solution, and soda to the acid solution, until the conditions are +reversed, testing with the red and blue litmus papers. + +Hold, for a minute or longer, a narrow strip of red litmus paper in the +mouth, noting any change in the color of the paper. Repeat, using blue +litmus paper. What effect, if any, has the saliva upon the color of the +papers? Has the mouth an acid or an alkaline reaction? + +*To show the Action of Saliva on Starch.*--1 (Optional). Prepare starch +paste by mixing half a teaspoonful of starch in half a pint of water and +heating the mixture to boiling. Place some of this in a test tube and thin +it by adding more water. Then add a small drop of iodine solution (page +136) to the solution of starch. It should turn a deep blue color. This is +the test for starch. + +Now collect from the mouth, in a clean test tube, two or three +teaspoonfuls of saliva. Add portions of this to small amounts of fresh +starch solution in two test tubes. Let the tubes stand for five or ten +minutes surrounded by water having about the temperature of the body. Test +for changes that have occurred as follows: + +(_a_) To one tube add a little of the iodine solution. If it does not turn +blue, it shows that the starch has been converted into some other +substance by the saliva, (_b_) To the other tube add a few drops of a very +dilute solution of copper sulphate. Then add sodium (or potassium) +hydroxide, a few drops at a time, until the precipitate which first forms +dissolves and turns a deep blue. Then gradually heat the upper portion of +the liquid to boiling. If it turns an orange or yellowish red color, the +presence of a form of sugar (maltose or dextrose) is proved. See page 136. + +2. Hold some powdered starch in the mouth until it completely dissolves +and observe that it gradually acquires a sweetish taste. This shows the +change of starch into sugar. + +*To illustrate the Action of the Gastric Juice.*--Add to a tumbler two +thirds full of water as much scale pepsin (obtained from a drug store) as +will stay on the end of the large blade of a penknife. Then add enough +hydrochloric acid to give a slightly sour taste. Place in the artificial +gastric juice thus prepared some boiled white of egg which has been finely +divided by pressing it through a piece of wire gauze. Also drop in a +single large lump. Keep in a warm place (about the temperature of the +body) for several hours or a day, examining from time to time. What is the +general effect of the artificial gastric juice upon the egg? + +*To illustrate Effect of Alcohol upon Gastric Digestion.*--Prepare a +tumbler half full of artificial gastric juice as in the above experiment, +and add 10 cubic centimeters of this to each of six clean test tubes +bearing labels. To five of the tubes add alcohol from a burette as +follows: (1) .5 c.c., (2) 1 c.c., (3) 1.5 c.c., (4) 2 c.c., and (5) 3 +c.c., leaving one tube without alcohol. Now add to each tube about 1/4 +gram of finely divided white of egg from the experiment above, and place +all of the tubes in a beaker half full of water. Keep the water a little +above the temperature of the body for several hours, examining the tubes +at intervals to note the progress of digestion. Inferences. + + + + +CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION + + +The dissolved nutrients, to reach the cells, must be transferred from the +alimentary canal to the blood stream. This process is known as +_absorption_. In general, absorption means the penetration of a liquid +into the pores of a solid, and takes place according to the simple laws of +molecular movements. The absorption of food is, however, not a simple +process, and the passage takes place through an _active_ (living) +membrane. Another difference is that certain foods undergo chemical change +while being absorbed. + +*Small Intestine as an Organ of Absorption.*--While absorption may occur to +a greater or less extent along the entire length of the alimentary canal, +most of it takes place at the small intestine. Its great length, its small +diameter, and its numerous blood vessels all adapt the small intestine to +the work of absorption. The transverse folds in the mucous membrane, by +retarding the food in its passage and by increasing the absorbing surface, +also aid in the process. But of greatest importance are the minute +elevations that cover the surface of the mucous membrane, known as + +*The Villi.*--Each single elevation, or villus, has a length of about one +fiftieth of an inch and a diameter about half as great (_A_, Fig. 76), and +contains the following essential parts: + +1. An outer layer of epithelial cells, resting upon a connective tissue +support. + +2. A small lymph tube, called a _lacteal_, which occupies the center of +the villus and connects at the base with other lymph tubes, also called +lacteals (_B_, Fig. _76_). + +3. A network of capillaries. + +The villi are structures especially adapted to the work of absorption, and +they are found only in the small intestine. The mucous membrane in all +parts of the canal, however, is capable of taking up some of the digested +materials. + + [Fig. 76] + + +Fig. 76--*The villi.* _A._ Diagram of a small section of mucous membrane of + small intestine. 1. Villi. 2. Small glands, called _crypts_. + + _B._ Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3. +Villus showing termination of the lacteal. 4. Villus showing capillaries. +5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7. + Layer of epithelial cells. + + +*Work of Capillaries and Lacteals.*--The capillaries and lacteals act as +receivers of material as it passes through the layer of epithelial cells +covering the mucous membrane. The lacteals take up the digested fats,(66) +and the capillaries receive all the other kinds of nutrients. These +vessels do not, of course, retain the absorbed materials, but pass them +on. Their final destination is the general circulation, which they reach +by two well-defined channels, or routes. + +*Routes to the Circulation.*--The two routes from the place of absorption +to the general circulation are as follows: + +1. _Route taken by the Fat._--The fat is conveyed by the lacteals from the +villi to the receptacle of the chyle. At this place it mingles with the +lymph from the lower parts of the body, and with it passes through the +thoracic duct to the left subclavian vein. Here it enters the general +circulation. Thus, to reach the general circulation, the fat has to pass +through the villi, the lacteals, the receptacle of the chyle, and the +thoracic duct (Fig. 77). Its passage through these places, like the +movements in all lymph vessels, is slow, and it is only gradually admitted +to the blood stream. + + [Fig. 77] + + + Fig. 77--*Diagram of routes* from food canal to general circulation. See + text. + + +2. _Route of All the Nutrients except Fat._--Water and salts and the +digested proteids and carbohydrates, in passing into the capillaries, mix +there with the blood. But this blood, instead of flowing directly to the +heart, is passed through the portal vein to the liver, where it enters a +_second set of capillaries_ and is brought very near the liver cells. From +the liver it is passed through the hepatic veins into the inferior vena +cava, and by these it is emptied into the right auricle. This route then +includes the capillaries in the mucous membrane of the stomach and +intestines, the branches of the portal vein, the portal vein proper, the +liver, and the hepatic veins (Fig. 77). In passing through the liver, a +large portion of the food material is temporarily retained for a purpose +and in a manner to be described later (page 177). + +*Absorption Changes.*--During digestion the insoluble foods are converted +into certain soluble materials, such as peptones, maltose, and +glycerine,--the conversion being necessary to their solution. A natural +supposition is that these materials enter and become a part of the blood, +but examination shows them to be absent from this liquid. (See Composition +of the Blood, page 30.) There are present in the blood, however, +substances closely related to the peptones, maltose, glycerine, etc.; +substances which have in fact been formed from them. During their transfer +from the food canal, the dissolved nutrients undergo changes, giving rise +to the materials in the blood. Thus are the serum albumin and serum +globulin of the blood derived from the peptones and proteoses; the +dextrose, from the maltose and other forms of sugar; and the fat droplets, +from the glycerine, fatty acid, and soluble soap. + +While considerable doubt exists as to the cause of these changes and as to +the places also where some of them occur, their purpose is quite apparent. +The materials forming the dissolved foods, although adapted to absorption, +are not suited to the needs of the body, and if introduced in this form +are likely to interfere with its work.(67) They are changed, therefore, +into the forms which the body can use. + +*A Second Purpose of Digestion.*--Comparing the digestive changes with +those of absorption, it is found that they are of a directly opposite +nature; that while digestion is a process of tearing down, or +separating,--one which reduces the food to a more finely divided +condition--there is in absorption a process of building up. From the +comparatively simple compounds formed by digestion, there are formed +during absorption the more complex compounds of the blood. The one +exception is dextrose, which is a simple sugar; but even this is combined +in the liver and the muscles to form the more complex compound known as +glycogen. (See Methods of Storage, below.) These facts have suggested a +second purpose of digestion--that of reducing foods to forms sufficiently +simple to enable the body to construct out of them the more complex +materials that it needs. Evidence that digestion serves such a purpose is +found in the fact that both proteids and carbohydrates are reduced to a +simpler form than is necessary for dissolving them.(68) + +*The Storage of Nutriment.*--For some time after the taking of a meal, food +materials are being absorbed more rapidly than they can be used by the +cells. Following this is an interval when the body is taking no food, but +during which the cells must be supplied with nourishment. It also happens +that the total amount of food absorbed during a long interval may be in +excess of the needs of the cells during that time; and it is always +possible, as in disease, that the quantity absorbed is not equal to that +consumed. To provide against emergencies, and to keep up a uniform supply +of food to the cells, it is necessary that the body store up nutrients in +excess of its needs. + +*Methods of Storage.*--The general plan of storage varies with the +different nutrients as follows: + +1. _The carbohydrates_ are stored in the form of _glycogen_. This, as +already stated (page 120), is a substance closely resembling starch. It is +stored in the cells of both the liver and the muscles, but mainly in the +liver (Fig. 78). It is a chief function of the liver to collect the excess +of dextrose from the blood passing through it, and to convert it into +glycogen, which it then stores within its cells. It does not, however, +separate all of the dextrose from the blood, a small amount being left for +supplying the immediate needs of the tissues. As this is used, the +glycogen in the liver is changed back to dextrose and, dissolving, again +finds its way into the blood. In this way, the amount of dextrose in the +blood is kept practically constant. The carbohydrates are stored also by +converting them into fat. + + [Fig. 78] + + + Fig. 78--*Liver cells* where is stored the glycogen. _C._ Capillaries. + + + [Fig. 79] + + + Fig. 79--*Stored-up fat.* The figure shows four connective tissue cells + containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4. + Connective tissue fibers. + + +2. _The fat_ is stored for the most part in the connective tissue. Certain +of the connective tissue cells have the property of taking fat from the +blood and of depositing it within their inclosing membranes (Fig. 79). +When this is done to excess, and the cells become filled with fat, they +form the so-called _adipose tissue_. Most of this tissue is found under +the skin, between the muscles, and among the organs occupying the +abdominal cavity. If one readily takes on fat, it may also collect in the +connective tissue around the heart. The stored-up fat is redissolved as +needed, and enters the blood, where it again becomes available to the +active cells. + +3. _The proteids_ form a part of all the tissues, and for this reason are +stored in larger quantities than any of the other food substances. The +large amount of proteid found in the blood may also be looked upon as +storage material. The proteids in the various tissues are spoken of as +_tissue proteids_, and those in the blood as _circulating proteids_. The +proteids of the tissues serve the double purpose of forming a working part +of the cell protoplasm, and of supplying reserve food material. That they +are available for supplying energy, and are properly regarded as _storage +material_, is shown by the rapid loss of proteid in starving animals. When +the proteids are eaten in excess of the body's need for rebuilding the +tissues, they are supposed to be broken up in such a manner as to form +glycogen and fat, which may then be stored in ways already described. + +*General Facts Relating to Storage.*--The form into which the food is +converted for storage in the body is that of _solids_--the form that takes +up the least amount of space. These solids are of such a nature that they +can be changed back into their former condition and, by dissolving, +reenter the blood. + +Only energy-yielding foods are stored. Water and salts, though they may be +absorbed in excess of the needs of the body, are not converted into other +substances and stored away. Oxygen, as already stated (page 108), is not +stored. The interval of storage may be long or short, depending upon the +needs of the body. In the consumption of stored material the glycogen is +used first, then as a rule the fat, and last of all the proteids. + +*Storage in the Food Canal.*--Not until three or four hours have elapsed +are all the nutrients, eaten at a single meal, digested and passed into +the body proper. The undigested food is held in reserve, awaiting +digestion, and is only gradually absorbed as this process takes place. It +may properly, on this account, be regarded as _stored material_. That such +storage is of advantage is shown by the observed fact that substances +which digest quickly (sugar, dextrin, "predigested foods," etc.) do not +supply the needs of the body so well as do substances which, like starch +and proteids, digest slowly. Even substances digesting quite slowly +(greasy foods and pastry), since they can be stored longer in the food +canal, may be of real advantage where, from hard work or exposure, the +body requires a large supply of energy for some time. These "stay by" the +laborer, giving him strength after the more easily digested foods have +been used up. Storage by the food canal is limited chiefly to the stomach. + +*Regulation of the Food Supply to the Cells.*--The storage of food +materials is made to serve a second purpose in the plan of the body which +is even more important than that of supplying nourishment to the cells +during the intervals when no food is being taken. It is largely the means +whereby the rate of supply of materials to the cells is regulated. The +cells obtain their materials from the lymph, and the lymph is supplied +from the blood. Should food substances, such as sugar, increase in the +blood beyond a low per cent, they are converted into a form, like +glycogen, in which they are held in reserve, or, for the time being, +placed beyond the reach of the cells. When, however, the supply is +reduced, the stored-up materials reenter the blood and again become +available to the cells. By this means their rate of supply to the cells is +practically constant. + +We are now in a position to understand why carbohydrates, fats, and +proteids are so well adapted to the needs of the body, while other +substances, like alcohol, which may also liberate energy, prove injurious. +It is because foods are of such a chemical nature that they are adapted in +all respects to the body plan of taking up and using materials, while the +other substances are lacking in some particular. + + [Fig. 80] + + + Fig. 80--*Diagrams illustrating the relation of nutrients* and the + non-relation of these to alcohol. _A._ Inter-relation and convertibility + of proteids, fats, and carbohydrates (after Hall). + +_B._ Diagram showing disposition of alcohol if this substance is taken in + quantity corresponding to that of the nutrients (F.M.W.). The alcohol + thrown off as waste is unoxidized and yields no energy. + + +*Why Alcohol is not a Food.*--If the passage of alcohol through the body is +followed, it is seen, in the first place, that it is a simple liquid and +undergoes no digestive change; and in the second place, that it is rapidly +absorbed from the stomach in both weak and concentrated solutions. This +introduces it quickly into the blood, and once there, it diffuses rapidly +into the lymph and then into the cells. Since the body cannot store +alcohol or convert it into some nutrient that can be stored (Fig. 80), +_there is no way of_ _regulating the amount that shall be present in the +blood, or of supplying it to the cells as their needs require_. They must +take it in excess of their needs, regardless of the effect, at least until +the organs of excretion can throw off the surplus as waste. Compared with +proteid, carbohydrates, or fats, alcohol is an _unmanageable_ substance in +the body. Attempting to use it as a food is as foolish as trying to burn +gasolene or kerosene in an ordinary wood stove. It may be done to a +limited extent, but is an exceedingly hazardous experiment. Not being +adapted to the body method of using materials, alcohol cannot be classed +as a food. + +*Assimilation.*--Digestion, absorption, circulation, and storage of foods +are the processes that finally make them available to the cells in the +different parts of the body. There still remains another process for these +materials to undergo before they serve their final purposes. This last +process, known as _assimilation_, is the appropriation of the food +material by the cell protoplasm. In a sense the storage of fat by +connective tissue cells and of glycogen by the liver cells is +assimilation. The term is limited, however, to the disposition of material +with reference to its final use. Whether all the materials used by the +cells actually become a part of the protoplasm is not known. It is known, +however, that the cells are the places where most of the oxidations of the +body occur and that materials taking part in these oxidations must, at +least, come in close contact with the protoplasm. Assimilation, then, is +the last event in a series of processes by which oxygen, food materials, +and cell protoplasm are brought into close and _active_ relations. The +steps leading up to assimilation are shown in Table II. + + TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS +MATERIALS DIGESTION ABSORPTION ROUTE TO STORAGE CONDITION + THE GENERAL IN THE + CIRCULATION BLOOD +Proteids Changed In passing Through the Become a As proteids + into into the portal vein part of the in + proteoses capillaries, to the protoplasm colloidal + and the liver and of all the solution. + peptones by proteoses from there cells. + the action and through the + of the peptones hepatic + gastric and change into veins into + pancreatic the the + juices. proteids of inferior + the blood. vena cava. +Fat Changed In passing Through the As fat in Chiefly as + into fatty into the lacteals to the cells minute oil + acid, lacteals, the of droplets. + glycerine, the thoracic collective + and glycerine duct, by tissue. + soluable unites with which it is + soap by the the soluable emptied + bile and soap and into the + pancreatic fatty acid left + juice. to form the subclavian + oil droplets vein. + of the + blood. +Starch Reduced to Enters the Through the As glycogen As dextrose + some of the capillaries portal chiefly by in + different as dextrose. vein, the liver, solution. + forms of liver, but to some + sugar, as hepatic extent by + maltose, veins, into muscle + dextrose, inferior cells. + etc. vena cava. +Water Undergoes Taken up by Both Is not As the + no change. both the routes, but stored in water which + lacteals and mostly by the sense serves as a + capillaries, way of the that energy carrier of + but to the liver. foods are. all the + greater other + extent by constituents + the of the + capilaries. blood. +Common salt Undergoes Taken up by By way of Not stored. In solution. + no change. the portal + capillaries vein, + without liver, and + undergoing hepatic + apparent veins into + change. inferior + vena cava. +Oxygen Taken up by Already in Is not United with + the the general stored. the + capillaries circulation. hemoglobin + at the and to a + lungs. small extent + in solution + in the + plasma. + +*Tissue Enzymes.*--The important part played by enzymes in the digestion of +the food has suggested other uses for them in the body. It has been +recently shown that many of the chemical changes in the tissues are in all +probability due to the presence of enzymes. An illustration of what a +tissue enzyme may do is seen in the changes which fat undergoes. In order +for the body to use up its reserve fat, it must be transferred from the +connective tissue cells, where it is stored, to the cells of the active +tissues where it is to be used. This requires that it be reduced to the +form of a solution and that it reenter the blood. In other words, it must +be _redigested_. For bringing about these changes a substance identical in +function with the steapsin of the pancreatic juice has been shown to exist +in several of the tissues. + +Although this subject is still under investigation, it may be stated with +certainty that there are present in the tissues, enzymes that change +dextrose to glycogen and _vice versa_, that break down and build up the +proteids, and that aid in the oxidations at the cells. The necessity for +such enzymes is quite apparent. + +*Summary.*--The digested nutrients are taken up by the capillaries and the +lymph vessels and transferred by two routes to the circulation. In passing +from the alimentary canal into the circulation the more important of the +foods undergo changes which adapt them to the needs of the body. Since +materials are absorbed more rapidly than they are used, means are provided +for storing them and for supplying them to the cells as their needs +require. _Capability of storage is an essential quality of energy-yielding +foods_; and substances, such as alcohol, which lack this quality are not +adapted to the needs of the body. For causing the chemical changes that +occur in the storage of foods, as well as the oxidations at the cells, the +presence of active agents, or enzymes, is necessary. + +*Exercises.*--1. In what respects does the absorption of food materials +from the alimentary canal differ from the absorption of a simple liquid by +a solid? + +2. In what different ways is the small intestine especially adapted to the +work of absorption? + +3. What are the parts of a villus? What are the lacteals? Account for the +name. + +4. What part is played by the capillaries and the lacteals in the work of +absorption? How does their work differ? + +5. What changes, if any, take place in water, common salt, fat, proteids, +and carbohydrates during absorption? + +6. What double purpose is served by the processes of digestion? + +7. Trace the passage of proteids, fats, and carbohydrates from the small +intestine into the general circulation. + +8. What is the necessity for storing nutrients in the body? Why is it not +also necessary to store up oxygen? + +9. In what form and at what places is each of the principal nutrients +stored? + +10. How is the rate of supply of food to the cells regulated? Why is the +body unable to regulate the supply of alcohol to the cells when this +substance is taken? + +11. Explain Fig. 80, page 181. What becomes of the alcohol if this is +taken in any but very small quantities? + +12. State the general purpose of enzymes in the body. Name the enzymes +found in each of the digestive fluids. What ones are found in the tissues? + + + +PRACTICAL WORK + + +Illustrate the ordinary meaning of the term "absorption" by bringing the +end of a piece of crayon in contact with water, or a piece of blotting +paper in contact with ink, noting the passage of the liquid into the +crayon or the paper. Show how absorption from the food canal differs from +this kind of absorption. + +Show by a diagram similar to Fig. 77 the two routes by which the foods +pass from the alimentary canal into the blood stream. + + + + +CHAPTER XII - ENERGY SUPPLY OF THE BODY + + +If one stops taking food, it becomes difficult after a time for him to +move about and to keep warm. These results show that food has some +relation to the energy of the body, for motion and heat are forms of +energy. The relation of oxygen to the supply of energy has already been +discussed (Chapter VIII). We are now to inquire more fully into the energy +supply of the body, and to consider those conditions which make necessary +the introduction of both food and oxygen for this purpose. + +*Kinds of Bodily Energy.*--The healthy body has at any time a considerable +amount of _potential_, or reserve, energy,--energy which it is not using at +the time, but which it is able to use as its needs require. When put to +use, this energy is converted into such forms of _kinetic_ energy(69) as +are indicated by the different kinds of bodily power. These are as +follows: + +1. _Power of Motion._--The body can move itself from place to place and it +can give motion to things about it. + +2. _Heat Power._--The body keeps itself warm and is able to communicate +warmth to its surroundings. + +3. _Nervous Power._--Through the nervous system the body exercises the +power of control over its different parts. + +As motion, heat, and nervous power the body uses most of its energy. + +*The Source of Bodily Energy.*--As already indicated, the energy of the +body is supplied through the food and the oxygen. These contain energy in +the potential form, which becomes kinetic (active) through their uniting +with each other in the body. Somewhat as the power of the steam engine is +derived from the combustion of fuel in the furnaces, the energy of the +body is supplied through the oxidations at the cells. How the food and +oxygen come to possess energy is seen by a study of the general methods by +which energy is stored up and used. + + [Fig. 81] + + + Fig. 81--*Simple device* for storing energy through gravity. + + +*Simple Methods of Storing Energy.*--Energy is stored by converting the +kinetic into the potential form. Two of the simplest ways of doing this +are the following: + +1. _Storing of Energy through Gravity._--On account of the attraction +between the earth and all bodies upon the earth, the mere lifting of a +weight puts it in a position where gravity can cause it to move (Fig. 81). +As a consequence _the raising of bodies above the earth's surface is a +means of storing energy_--the energy remaining stored until the bodies +fall. As they fall, the stored-up (potential) energy becomes kinetic and +can be made to do work. + +2. _Storing of Energy through Elasticity._--Energy is stored also by doing +work in opposition to elasticity, as in bending a bow or in winding a +clock spring. The bending, twisting, stretching, or compressing of elastic +substances puts them in a condition of _strain_ which causes them to exert +a pressure (called elastic force) that tends to restore them to their +former condition. Energy stored by this means becomes active as the +distorted or compressed substance returns to its former shape or volume. + +These simple methods of storing energy will serve to illustrate the +general principles upon which such storage depends: + +1. To store energy, energy must be expended, or work done. + +2. The work must be against some force, such as gravity or elasticity, +which can undo the work, i.e., bring about an effect opposite to that of +the work. + +3. The stored energy becomes active (kinetic) as the force through which +the energy was stored undoes the work, or puts the substance upon which +the work was done into its former condition (gravity causing bodies to +fall, etc.). + +These principles are further illustrated by the + +*Storing of Energy through Chemical Means.*--A good example of storing +energy by chemical means is that of decomposing water with electricity. If +a current of electricity is passed through acidulated water in a suitable +apparatus (Fig. 82), the water separates into its component gases, oxygen +and hydrogen. These gases now have power (energy) which they did not +possess before they were separated. The hydrogen will burn in the oxygen, +giving heat; and if the two gases are mixed in the right proportions and +then ignited, they explode with violence. This energy was derived from the +electricity. It was stored by _decomposing_ the water. + + [Fig. 82] + + + Fig. 82--*Storing energy by chemical means.* Apparatus for decomposing + water with electricity. + + +Energy is stored by chemical means by causing it to do work in opposition +to the force of chemism, or chemical affinity. Instead of changing the +form of bodies or moving them against gravity, it overcomes the force that +causes atoms to unite and to hold together after they have united. Since +in most cases the atoms on separating from any given combination unite at +once to form other combinations, we may say that _energy is stored when +strong chemical combinations are broken up and weak ones formed_. Energy +stored by this means becomes active when the atoms of weak combinations +unite to form combinations that are strong.(70) + +*How Plants store the Sun's Energy.*--The earth's supply of energy comes +from the sun. While much of this, after warming and lighting the earth's +surface, is lost by radiation, a portion of it is stored up and retained. +The sun's energy is stored both through the force of gravity(71) and by +chemical means, the latter being the more important of the two methods. +Plants supply the means for storing it chemically (Fig. 83). Attention has +already been called to the fact (page 112) that growing plants are +continually taking carbon dioxide into their leaves from the air. This +they decompose, adding the carbon to compounds in their tissues and +returning the oxygen to the air. It is found, however, that this process +does not occur unless the plants are exposed to sunlight. The sunlight +supplies the energy for overcoming the attraction between the atoms of +oxygen and the atoms of carbon, while the plant itself serves as the +instrument through which the sunlight acts. The energy for decomposing the +carbon dioxide then comes from the sun, and through the decomposition of +the carbon dioxide the sun's energy is stored--becomes potential. It +remains stored until the carbon of the plant again unites with the oxygen +of the air, as in combustion. + + [Fig. 83] + + + Fig. 83--*Nature's device* for storing energy from the sun. See text. + + +*The Sun's Energy in Food and Oxygen.*--Food is derived directly or +indirectly from plants and sustains the same relation to the oxygen of the +air as do the plants themselves. (The elements in the food have an +attraction for the oxygen, but are separated chemically from it.) On +account of this relation they have potential energy--the energy derived +through the plant from the sun. When a person eats the food and breathes +the oxygen, this energy becomes the possession of the body. It is then +converted into kinetic energy as the needs of the body require. + + [Fig. 84] + + + Fig. 84--*Simple apparatus* for illustrating transformation of energy. + Potential energy is converted into heat and heat into motion. + + +*From the Sun to the Cells.*--It thus appears that the body comes into +possession of energy, and is able to use it, through a series of +transferences and transformations that can be traced back to the sun.(72) +Coming to the earth as kinetic energy, it is transformed into potential +energy and stored in the compounds of plants and in the oxygen of the air. +Through the food and the oxygen the potential energy is transferred to the +cells of the body. Then by the uniting of the food and the oxygen at the +cells (oxidation), the potential becomes kinetic energy and is used by the +body in doing its work. The phrase "Child of the Sun" has sometimes been +applied to man to express his dependence upon the sun for his supply of +energy. + +*Why Oxygen and Food are Both Necessary.*--The necessity for introducing +both oxygen and food into the body for the purpose of supplying energy is +now apparent. The energy which is used in the body is not the energy of +food alone. Nor is it the energy of oxygen alone. It belongs to both. It +is due to their attraction for each other and their condition of +separation. It cannot, therefore, become kinetic except through their +union. To introduce one of these substances into the body without the +other, would neither introduce the energy nor set it free. They must both +be introduced into the body and there caused to unite. + +*Bodily Control of Energy.*--A fact of importance in the supply of energy +to the body is that the rate of transformation (changing of potential to +kinetic) is just sufficient for its needs. It is easily seen that too +rapid or too slow a rate would prove injurious. The oxidations at the +cells are, therefore, under such control that the quantity of kinetic +energy supplied to the body as a whole, and to the different organs, is +proportional to the work that is done. This is attained, in part at least, +through the ability of the body to store up the food materials and hold +them in reserve until they are to be oxidized (page 180). + +*Animal Heat and Motion.*--Most of the body's energy is expended as heat in +keeping warm. It is estimated that as much as five sixths of the whole +amount is used in this way. The proportion, however, varies with different +persons and is not constant in the same individual during different +seasons of the year. This heat is used in keeping the body at that +temperature which is best suited to carrying on the vital processes. All +parts of the body, through oxidation, furnish heat. Active organs, +however, such as the muscles, the brain, and the glands (especially the +liver), furnish the larger share. The blood in its circulation serves as a +_heat distributer_ for the body and keeps the temperature about the same +in all its parts (page 33). + +Next to the production of heat, in the consumption of the body's energy, +is the production of motion. This topic will be considered in the study of +the muscular system (Chapter XV). + +*Some Questions of Hygiene.*--The heat-producing capacity of the body +sustains a very important relation to the general health. A sudden chill +may result in a number of derangements and is supposed to be a +predisposing cause of _colds_. One's capacity for producing heat may be so +low that he is unable to respond to a sudden demand for heat, as in going +from a warm room into a cold one. As a consequence, the body is unable to +protect itself against unavoidable exposures. + +_Impairment of the heat-producing capacity_ is brought about in many ways. +Several diseases do this directly, or indirectly, to quite an extent. In +health too great care in protecting the body from cold is the most potent +cause of its impairment. Staying in rooms heated above a temperature of +70 deg. F., wearing clothing unnecessarily heavy, and sleeping under an excess +of bed clothes, all diminish the power of the body to produce heat. They +accustom it to producing only a small amount, so that it does not receive +sufficient of what might be called _heat-producing exercise_. Lack of +physical exercise in the open air, as well as too much time spent in +poorly lighted and ventilated rooms, tends also to reduce one's ability to +produce heat. Moreover, since most of the heat of the body comes from the +union of oxygen and food materials at the cells, a lack of either of these +will interfere with the production of heat. + +*Results of Exhaustion.*--Through overwork, or excesses in pleasurable +pursuits, one may make greater demands upon the energy of his body than it +can properly supply. The resulting condition, known as _exhaustion_, is +not only a matter of temporary inconvenience, but may through repetition +lead to a serious impairment of the health. It should be noted, in this +connection, that the energy of the body is spent in two general ways: +first, in carrying on the vital processes; and second, in the performance +of voluntary activities. Since, in all cases, there is a limit to one's +energy, it is easily possible to expend so much in the voluntary +activities that the amount left is not sufficient for the vital processes. +This leads to various disturbances and, among other things, renders the +body less able to supply itself with energy. + +*The Problem of Increasing One's Energy.*--Since the energy supply is kept +up through the food and the oxygen, it might be inferred that the +introduction of these substances into the body in larger amounts would +increase the energy at one's disposal. This does not necessarily follow. +Oxidation at the cells is preceded by digestion, absorption, circulation, +and assimilation. It is followed and influenced by the removal of wastes +from the body. A careful study of the problem leads to the conclusion that +while the energy supply to the body does depend upon the introduction of +the proper amounts of food and oxygen, it also depends upon the efficiency +of the vital processes. The maximum amount of energy may, therefore, be +expected when the body is in a condition of perfect health. Hence, one +desiring to increase the amount of his energy must give attention to all +those conditions that improve the health. + +*Effect of Stimulants on the Energy Supply.*--In the effort to get out of +the body as much as possible of work or of pleasure, various stimulants, +such as alcohol, tobacco, and strong tea and coffee, have been used. +Though these have the effect of giving a temporary feeling of strength and +of enabling the individual in some instances to accomplish results which +he could not otherwise have brought about, the general effect of their use +is to lessen, rather than to increase, the sum total of bodily power. The +student, for example, who drinks strong coffee in order to study late at +night is able to command less energy on the day following. While enabling +him to draw upon his reserve of nervous power for the time being, the +coffee deprives him of sleep and needed rest. + +The danger of stimulants, so far as energy is concerned, is this: they +tend to exhaust the bodily reserve so that there is not sufficient left +for properly running the vital processes. Evidences of their weakening +effect are found in the feeling of discomfort and lassitude which result +when stimulants to which the body has become accustomed are withdrawn. Not +until one gets back his bodily reserve is he able to work normally and +effectively. Increase in bodily energy comes through health and not +through the use of stimulants. + +*Summary.*--The body requires a continuous supply of energy. To obtain this +supply, materials possessing potential, or stored-up, energy are +introduced into it. The free oxygen of the air and the substances known as +foods, on account of the chemical relations which they sustain to each +other, contain potential energy and are utilized for supplying the body. +So long as the foods are not oxidized, the energy remains in the potential +form, but in the process of oxidation the potential energy is changed to +kinetic energy and made to do the work of the body. + +*Exercises.*--1. In what different ways does the body use energy? + +2. Show that a stone lying against the earth has no energy, while the same +stone above the earth has energy. + +3. How does potential energy differ from kinetic energy? + +4. What kind of energy is possessed by a bent bow? By a revolving wheel? +By a coiled spring? By the wind? By gunpowder? + +5. How does decomposing water with electricity store energy? + +6. Account for the energy possessed by the oxygen of the air and food +substances. + +7. Trace the energy supply of the body back to the sun. + +8. Why must both oxygen and food be introduced into the body in order to +supply it with energy? + +9. How may overwork and overexercise diminish the energy supply of the +body? + +10. How may one increase the amount of his energy? + + + +PRACTICAL WORK + + +*Suggested Experiments.*--1. The change of kinetic into potential energy +may be shown by stretching a piece of rubber, by lifting a weight, and by +separating the armature from a magnet. + +2. The change of potential into kinetic energy may be shown by letting +weights fall to the ground, by releasing the end of a piece of stretched +rubber, and by burning substances. + +3. The change of one form of kinetic energy to another may be illustrated +by rubbing together two pieces of wood until they are heated, by ringing a +bell, and by causing motion in air or in water by heating them. If +suitable apparatus is at hand, the transformation of electrical energy +into heat, light, sound, and mechanical motion can easily be shown. + +4. A weight connected by a cord with some small machine and made to run +it, will help the pupil to grasp the general principles in the storage of +energy through gravity. A vessel of water on a high support from which the +water is siphoned on to a small water wheel will serve the same purpose. + +5. The storing of energy by chemical means may be illustrated by +decomposing potassium chlorate with heat or by decomposing water by means +of a current of electricity. + +6. Study the transfer of energy from the body to surrounding objects, as +in moving substances and lifting weights. + +Fill a half gallon jar two thirds full of water and carefully take the +temperature with a chemical thermometer. Hold the hand in the water for +four or five minutes and take the temperature again. Inference. + + + + +CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION + + +In our study so far we have been concerned mainly with the introduction of +materials into the body. We are now to consider the removal of materials +from the body. The structures most directly concerned in this work are +known as + +*Glands.*--As generally understood, glands are organs that prepare special +liquids in the body and pour them out upon free surfaces. These liquids, +known as _secretions_, are used for protecting exposed parts, lubricating +surfaces that rub against each other, digesting food, and for other +purposes. They differ widely in properties as well as in function, but are +all alike in being composed chiefly of water. The water, in addition to +being necessary to the work of particular fluids, serves in all cases as a +carrier of solid substances which are dissolved in it. + +*General Structure of Glands.*--While the various glands differ greatly in +size, form, and purpose, they present striking similarities in structure. +All glands contain the following parts: + +1. Gland, or secreting, cells. These are _specialized_ cells for the work +of secretion and are the active agents in the work of the gland. They are +usually cubical in shape. + +2. A basement membrane. This is a thin, connective tissue support upon +which the secreting cells rest. + +3. A network of capillary and lymph vessels. These penetrate the tissues +immediately beneath the secreting cells. + +4. A system of nerve fibers which terminate in the secreting cells and in +the walls of the blood vessels passing to the glands. + +These structures--secreting cells, basement membrane, capillary and lymph +vessels, and nerve fibers--form the essential parts of all glands. The +capillaries and the lymph vessels supply the secreting cells with fluid, +and the nerves control their activities. + +*Kinds of Glands.*--Glands differ from one another chiefly in the +arrangement of their essential parts.(73) The most common plan is that of +arranging the parts around a central cavity formed by the folding or +pitting of an exposed surface. Many such glands are found in the mucous +membrane, especially that lining the alimentary canal, and are most +numerous in the stomach, where they supply the gastric juice. If these +glands have the general form of tubes, they are called _tubular_ glands; +if sac-like in shape, they are called _saccular_ glands. Both the tubular +and the saccular glands may, by branching, form a great number of similar +divisions which are connected with one another, and which communicate by a +common opening with the place where the secretion is used. This forms a +_compound_ gland which, depending on the structure of the minute parts, +may be either a _compound tubular_ or a _compound saccular_ gland. The +larger of the compound saccular glands are also called _racemose_ glands, +on account of their having the general form of a cluster, or raceme, +similar to that of a bunch of grapes. The general structure of the +different kinds of glands is shown in Fig. 85. + + [Fig. 85] + + +Fig. 85--*Diagram illustrating evolution of glands.* _A._ Simple secreting +surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve. + _B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound + tubular gland. _E._ Compound saccular gland. _F._ A compound racemose +gland with duct passing to a free surface. _G._ Relation of food canal to + different forms of glands. The serous coat has a secreting surface. + + +*Nature of the Secretory Process.*--At one time the gland was regarded +merely as a kind of filter which separated from the blood the ingredients +found in its secretions. Recent study, however, of several facts relating +to secretion has led to important modifications of this view. The +secretions of many glands are known to contain substances that are not +found in the blood, or, if present, are there in exceedingly small +amounts. Then again the cells of certain glands have been found to undergo +marked changes during the process of secretion. If, for example, the cells +of the pancreas be examined after a period of rest, they are found to +contain small granular bodies. On the other hand, if they are examined +after a period of activity, the granules have disappeared and the cells +themselves have become smaller (Fig. 86). The granules have no doubt been +used up in forming the secretion. These and other facts have led to the +conclusion that secretion is, in part, the separation of materials without +change from the blood, and, in part, a process by which special substances +are prepared and added to the secretion. According to this view the gland +plays the double role of a _filtering apparatus_ and of a _manufacturing +organ_. + + [Fig. 86] + + +Fig. 86--*Secreting cells from the pancreas* (after Langley). _A._ After a + period of rest. _B._ After a short period of activity. C. After a period + of prolonged activity. In _A_ and _B_ the nuclei are concealed by the + granules that accumulate during the resting period. + + +*Kinds of Secretion.*--In a general way all the liquids produced by glands +may be considered as belonging to one or the other of two classes, known +as the _useful_ and the _useless_ secretions. To the first class belong +all the secretions that serve some purpose in the body, while the second +includes all those liquids that are separated as waste from the blood. The +first are usually called _true secretions_, or secretions proper, while +the second are called _excretions_. The most important glands producing +liquids of the first class are those of digestion (Chapter X). + +*Excretory Work of Glands.*--The process of removing wastes from the body +is called _excretion_. While in theory excretion may be regarded as a +distinct physiological act, it is, in fact, leaving out the work of the +lungs, but a phase of the work of glands. From the cells where they are +formed, the waste materials pass into the lymph and from the lymph they +find their way into the blood. They are removed from the blood by glands +and then passed to the exterior of the body. + +*The Necessity for Excretion* is found in the results attending oxidation +and other chemical changes at the cells (page 107). Through these changes +large quantities of materials are produced that can no longer take any +part in the vital processes. They correspond to the ashes and gases of +ordinary combustion and form wastes that must be removed. The most +important of these substances, as already noted (page 110), are carbon +dioxide, water, and urea.(74) A number of mineral salts are also to be +included with the waste materials. Some of these are formed in the body, +while others, like common salt, enter as a part of the food. They are +solids, but, like the urea, leave the body dissolved in water. + +Waste products, if left in the body, interfere with its work (some of, +them being poisons), and if allowed to accumulate, cause death. Their +removal, therefore, is as important as the introduction of food and oxygen +into the body. The most important of the excretory glands are + +*The Kidneys.*--The kidneys are two bean-shaped glands, situated in the +back and upper portion of the abdominal cavity, one on each side of the +spinal column. They weigh from four to six ounces each, and lie between +the abdominal wall and the peritoneum. Two large arteries from the aorta, +called the _renal arteries_, supply them with blood, and they are +connected with the inferior vena cava by the _renal veins_. They remove +from the blood an exceedingly complex liquid, called the _urine_, the +principal constituents of which are water, salts of different kinds, +coloring matter, and urea. The kidneys pass their secretion by two slender +tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87). + + [Fig. 87] + + + Fig. 87--*Relations of the kidneys.* (Back view.) 1. The kidneys. 2. + Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries. + 7. Renal veins. + + +*Structure of the Kidneys.*--Each kidney is a compound tubular gland and is +composed chiefly of the parts concerned in secretion. The ureter serves as +a duct for removing the secretion, while the blood supplies the materials +from which the secretion is formed. On making a longitudinal section of +the kidney, the upper end of the ureter is found to expand into a +basin-like enlargement which is embedded in the concave side of the +kidney. The cavity within this enlargement is called the _pelvis of the +kidney_, and into it project a number of cone-shaped elevations from the +kidney substance, called the _pyramids_ (Fig. 88). + +From the summits of the pyramids extend great numbers of very small tubes +which, by branching, penetrate to all parts of the kidneys. These are the +_uriniferous tubules_, and they have their beginnings at the outer margin +of the kidney in many small, rounded bodies called the _Malpighian +capsules_ (_A_, Fig. 88). Each capsule incloses a cluster of looped +capillaries and connects with a single tubule (Fig. 89). From the capsule +the tubule extends toward the concave side of the kidney and, after +uniting with similar tubules from other parts, finally terminates at the +pyramid. Between its origin and termination, however, are several +convolutions and one or more loops or turns. After passing a distance many +times greater than from the surface to the center of the kidney, the +tubule empties its contents into the expanded portion of the ureter. + + [Fig. 88] + + + Fig. 88--*Sectional view of kidney.* 1. Outer portion or cortex. 2. + Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section + enlarged to show the tubules and their connection with the capsules. + + + [Fig. 89] + + +Fig. 89--*Malpighian capsule* highly magnified (Landois). _a._ Small artery + entering capsule and forming cluster of capillaries within. _e._ Small +vein leaving capsule and branching into _c_, a second set of capillaries, + _h._ Beginning of uriniferous tubule. + + +The uriniferous tubules are lined with secreting cells. These differ +greatly at different places, but they all rest upon a basement membrane +and are well supplied with capillaries. These cells provide one means of +separating wastes from the blood (Fig. 90). + + [Fig. 90] + + + Fig. 90--*Diagram illustrating renal circulation.* 1. Branch from renal +artery. 2. Branch from renal vein. 3. Small artery branches, one of which + enters a Malpighian capsule (5). 6. Small vein leaving the capsule and +branching into the capillaries (7) which surround the uriniferous tubules. +4. Small veins which receive blood from the second set of capillaries. 8. + Tubule showing lining of secreting cells. + + +*Blood Supply to the Kidneys.*--The method by which the kidneys do their +work is suggested by the way in which the blood circulates through them. +The renal artery entering each kidney divides into four branches and these +send smaller divisions to all parts of the kidney. At the outer margin of +the kidney, called the _cortex_, the blood is passed through _two sets of +capillaries_. The first forms the clusters in the Malpighian capsules and +receives the blood directly from the smallest arteries. The second forms a +network around the uriniferous tubules and receives the blood which has +passed from the capillary clusters into a system of small veins (Fig. 90). +From the last set of capillaries the blood is passed into veins which +leave the kidneys where the artery branches enter, uniting there to form +the main renal veins. + +*Work of the Kidneys.*--Why should the blood pass through two systems of +capillaries in the kidneys? This is because the separation of waste is +done in part by the Malpighian capsules and in part by the uriniferous +tubules. Water and salts are removed chiefly at the capsules, while the +remaining solid constituents of the urine pass through the secreting cells +that line the tubules. It was formerly believed that the kidneys obtained +their secretion by a process of filtration from the blood, but this belief +has been gradually modified. The prevailing view now is that the processes +of filtration and secretion are both carried on by the kidneys,--that the +capillary clusters in the Malpighian bodies serve as delicate filters for +the separation of water and salts, while the secreting cells of the +tubules separate substances by the process of secretion. + +On account of the large volume of blood passing through the kidneys this +liquid is still a bright red color as it flows into the renal veins (Fig. +90). The kidney cells require oxygen, but the amount which they remove +from the blood is not sufficient to affect its color noticeably. The blood +in the renal veins, having given up most of its impurities and still +retaining its oxygen, is considered the purest blood in the body. + +*Urea* is the most abundant solid constituent of the urine and is the +chief waste product arising from the oxidation of nitrogenous substances +in the body. Although secreted by the cells lining the uriniferous +tubules, it is not formed in the kidneys. The secreting cells simply +separate it from the blood where it already exists. The muscles also have +been suggested as a likely source of urea, for here the proteids are +broken down in largest quantities; but the muscles produce little if any +urea. Its production has been found to be the _work of the liver_. In the +muscular tissue, and in the other tissues as well, the proteids are +reduced to a lower order of compounds, such as the compounds of ammonia, +which pass into the blood and are then taken up by the liver. By the +action of the liver cells these are converted into urea and this is turned +back into the blood. From the blood the urea is separated by the secreting +cells of the kidneys. + +*Work of the Liver.*--The liver, already described as an organ of digestion +(page 152), assists in the work of excretion both by changing waste +nitrogenous compounds into urea and by removing from the blood the wastes +found in the bile. While the chief work of the liver is perhaps not that +of excretion, its functions may here be summarized. The liver is, first of +all, a _manufacturing organ_, producing, as we have seen, three distinct +products--bile, glycogen, and urea. On account of the nature of the urea +and the bile, the liver is properly classed as an _excretory organ_; but +in the formation of the glycogen it plays the part of a _storage organ_. +Then, on account of the use made of the bile after it is passed into the +food canal, the liver is also classed as a _digestive organ_. These +different functions make of the liver an organ of the first importance. + +*Excretory Work of the Food Canal.*--The glands connected with the food +canal, other than the liver, while secreting liquids that aid in +digestion, also separate waste materials from the blood. These are passed +into the canal, whence they leave the body with the undigested portions of +the food and the waste from the liver. Though the nature and quantity of +the materials removed by these glands have not been fully determined, +recent investigations have tended to enhance the importance attached to +this mode of excretion. + +*The Perspiratory Glands.*--The perspiratory, or sweat, glands are located +in the skin. They belong to the type of simple tubular glands and are very +numerous over the entire surface of the body. A typical sweat gland +consists of a tube which, starting at the surface of the cuticle, +penetrates to the under portion of the true skin and there forms a +ball-shaped coil. The coiled extremity, which forms the secreting portion, +is lined with secreting cells and surrounded by a network of capillaries. +The portion of the tube passing from the coil to the surface serves as a +duct (Figs. 91 and 121). + + [Fig. 91] + + + Fig. 91--*Diagram of section through a sweat gland.* _a._ Outer layer of + skin or cuticle. _b._ Dermis or true skin. _d, e._ Sections of the tube +forming the coiled portion of the gland. _c._ Duct passing to the surface. + The other structures of the skin not shown. + + +The sweat glands secrete a thin, colorless fluid, called _perspiration_, +or sweat. This consists chiefly of water, but contains a small per cent of +salts and of urea. The excretory work of these glands seems not to be so +great as was formerly supposed, but they supplement in a practical way the +work of the kidneys and, during diseases of these organs, show an increase +in excretory function to a marked degree. The perspiration also aids in +the regulation of the temperature of the body (Chapter XVI). + +*Excretory Work of the Lungs.*--While the lungs cannot be regarded as +glands, they do a work in the removal of waste from the body which must be +considered in the general process of excretion. They are especially +adapted to the removal of gaseous substances from the blood, and it is +through them that most of the carbon dioxide leaves the body. The lungs +remove also a considerable quantity of water. This is of course in the +gaseous form, being known as water vapor. + +*Ductless Glands and Internal Secretion.*--Midway in function between the +glands that secrete useful liquids and those that remove waste materials +from the blood is a class of bodies, found at various places, known as the +_ductless glands._ They are so named from their having the general form of +glands and from the fact that they have no external openings or ducts. +They prepare special materials which are passed into the blood and which +are supposed to exert some beneficial effect either upon the blood or upon +the tissues through which the blood circulates. The most important of the +ductless glands are the thyroid gland, located in the neck; the suprarenal +bodies, situated one just over each kidney; and the thymus gland, a +temporary gland in the upper part of the chest. The spleen and the +lymphatic glands (page 68) are also classed with the ductless glands. The +liver, the pancreas, and (according to some authorities) the kidneys, in +addition to their external secretions, produce materials that pass into +the blood. They perform in this way a function like that of the ductless +glands. The work of glands in preparing substances that enter the blood is +known as _internal secretion._ + +*Quantity of Excretory Products.*--If the weight of the normal body be +taken at intervals, after growth has been attained, there will be found to +be practically no gain or loss from time to time. This shows that +materials are leaving the body as fast as they enter and that the tissues +are being torn down as fast as they are built up. It also shows that +substances do not remain in the body _permanently_, but only so long +perhaps as is necessary for them to give up their energy, or serve some +additional purpose in the ever changing protoplasm. The excretory organs +then remove from the body a quantity of material that is equal in weight +to the materials absorbed by the organs of digestion and respiration. This +is estimated for the average individual to be about five pounds daily. The +passage of waste from the body is summarized in Table III. + + TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY +Materials State How Formed Condition in How Removed + in the Body the Blood from the + Blood +Carbon Gas By the Dissolved in Separated +dioxide oxidation of the plasma from the + the carbon and in loose blood at the + of proteids, combination alveoli of + carbohydrates, with salts the lungs + and fats. in the and then + blood. forced + through the + air passages + into the + atmosphere. +Urea Solid By the Dissolved in Removed by + oxidation in the plasma. the + the liver of uriniferous + nitrogenous tubules of + compounds. the kidneys + and to a + small extent + by the + perspiratory + glands. +Water Liquid By the As water. Removed by + oxidation of all the + the hydrogen organs of + of proteids, excretion, + carbohydrates, but in the + and fats. largest + Amount formed quantities + in the body is by the + small. kidneys and + the skin. +Salts Solid Dissolved in By the + the plasma. kidneys, + liver, and + skin. + + + +HYGIENE + + +The separation of wastes from the body has such a close relation to the +health that all conditions affecting it should receive the most careful +attention. Their retention beyond the time when they should be discharged +undoubtedly does harm and is the cause of many bodily disorders. + +*Value of Water.*--As a rule the work of excretion is aided by drinking +_freely_ of pure water. As water is the natural dissolver and transporter +of materials in the body, it is generally conceded by hygienists and +physicians that the taking of plenty of water is a healthful practice. +People do not as a rule drink a sufficient amount of water, about three +pints per day being required by the average adult, in addition to that +contained in the food. Most of the water should, of course, be taken +between meals, although the sipping of a small amount during meals does +not interfere with digestion. As stated elsewhere, the taking of a cup of +water on retiring at night and again on rising in the morning is very +generally recommended. + +*Protection of Kidneys and Liver.*--The kidneys and liver are closely +related in their work and in many instances are injured or benefited by +the same causes. Both, as already stated (page 124), are liable to injury +from an _excess of proteid food_, especially meats, and also by a +condition of inactivity of the bowels (page 166). The free use of alcohol +also has an injurious effect on both of these organs.(75) On the other +hand, increasing the activity of the skin has a beneficial effect upon +them, especially the kidneys. Exercise and bathing, which tend to make the +skin more active, are valuable aids both in ridding the body of impurities +and in lessening the work of the other excretory organs. One having a +disease of the kidneys, however, needs to exercise great care in bathing +on account of the bad results which follow getting chilled. + +*Special Care after Certain Diseases.*--Certain diseases, as measles, +diphtheria, scarlet fever, and typhoid fever, sometimes have the effect of +weakening the kidneys (and other vital organs) and of starting disease in +them. When this occurs it is usually the result of exposure or of +over-exertion while the body is in a weakened condition. Severe chilling +at such a time, by driving blood from the surface to the parts within, +often causes inflammation of the kidneys. On recovering from any wasting +disease one should exercise great caution both in resuming his regular +work and in exposing his body to wet or cold. + +*Misunderstood Symptoms.*--Pains in the small of the back, an increase in +the secretions of the kidneys, and a sediment in the urine very naturally +suggest some disorder of the kidneys. It is a fact, however, that these +symptoms have little or no relation to the state of the kidneys and may +occur when the kidneys are in a perfectly healthy condition. The kidneys +are not located in the small of the back, but above this place, so that +pains in this region are evidently not from the kidneys, while the +increase in the flow of the urine may arise from a number of causes, one +of which is an increase of certain waste products passed into the blood. +The symptoms referred to are frequently the results of nervous exhaustion, +resulting from overstudy, worry, eye strain, or some other condition that +overtaxes the nervous system. When this is the case, relief is obtained +through resting the nerves. Actual disease of the kidneys can only be +determined through a chemical and microscopic examination of the urine. To +resort to some patent medicine for kidney trouble without knowing that +such trouble exists, as is sometimes done, is both foolish and unhygienic. + +*Alcoholic Beverages and the Elimination of Waste.*--Causing as it does +such serious diseases as cirrhosis of the liver and Bright's disease of +the kidneys (footnote, page 210), alcohol will greatly interfere in this +way with the elimination of waste. There is also evidence to the effect +that it interferes with waste elimination before the stage is reached of +causing disease of these organs. Researches have shown that alcohol +increases the amount of uric acid in the body and decreases the amount of +urea found in the urine. The conclusion to be drawn is that alcohol +interferes in some way with the change of the harmful uric acid into the +comparatively harmless urea--an interference which in some instances +results in great harm. It has also been shown that malted liquors, such as +beer and ale, contain substances which, like the caffein of tea and coffee +(page 167), are readily converted into uric acid.(76) Wines contain acids +which may also act injuriously. The harm which such substances do is, of +course, additional to that caused by the alcohol. + +*Summary.*--As a result of the oxidations and other changes at the cells, +substances are produced that can no longer serve a purpose in the body. +They are of the nature of waste, and their continuous removal from the +body is as necessary to the maintenance of life as the introduction of +food and oxygen. The organs whose work it is to remove the waste, +excepting the lungs, are glands; and the material which they remove are of +the nature of secretions. From the cells, the waste passes through the +lymph in the blood. From the blood it is separated by the excretory organs +and passed to the exterior of the body. + +*Exercises.*--1. What general purposes are served by the glands in the +body? + +2. What are the parts common to all glands? What purpose is served by each +of these parts? + +3. How do tubular glands differ in structure from saccular glands? What is +a racemose gland? Why so called? + +4. Describe the nature of the secretory process. + +5. What conditions render necessary the formation of waste materials in +the body? Why must these be removed? + +6. How do the waste materials get from the cells to the organs of +excretion? + +7. Show by a drawing the connections of the kidneys with the large blood +vessels and the bladder. Name parts of drawing. + +8. In what do the uriniferous tubes have their beginning? In what do they +terminate? With what are they lined? + +9. Why should the blood pass through two sets of capillaries in the +kidneys? + +10. Bright's disease of the kidneys affects the uriniferous tubes and +interferes with their work. What impurity is then left in the blood? + +11. Trace water and salts from the Malpighian capsules to the bladder, +naming parts through which they pass. + +12. Trace carbon dioxide from the cells to the outside atmosphere. + +13. How does the quantity of material introduced into the body compare +with that which is removed by the organs of excretion? + +14. Name two ways of lessening the work of the kidneys. + +15. Why is the drinking of plenty of pure water a healthful practice? + + + +PRACTICAL WORK + + +*To suggest the Double Work of Glands.*--Prepare a simple filter by fitting +a piece of porous paper into a glass funnel. Through this pass pure water +and also water having salt dissolved in it and containing some sediment, +as sand. The water and the dissolved salt pass through, while the sediment +remains on the filter. Now substitute a fresh piece of paper in the funnel +and drop on its surface a little solid coloring matter, such as cochineal. +Again pass the liquid through the funnel. This time it comes through +colored, the color being added by the filter. Compare the filter and +materials filtered to the gland and the materials concerned in secretion +(blood, the liquid secreted, substances added by the gland, etc.). + + [Fig. 92] + + + Fig. 92--*The physiological scheme.* Diagram suggesting the essential + relation of the bodily activities. See Summary of Part I, page 215, and + Summary of Part II, page 413. + + + +SUMMARY OF PART I + + +The body is an organization of different kinds of cells; it grows through +the growth and reproduction of these cells; and its life as a whole is +maintained by providing such conditions as will enable the cells to keep +alive. Of chief importance in the work of the body is a nutrient fluid +which supplies the cells with food and oxygen and relieves them of waste. +A moving portion of this fluid, called the blood, serves as a transporting +agent, while another portion, called the lymph, passes the materials +between the blood and the cells. Through their effects upon the blood and +the lymph, the organs of circulation, respiration, digestion, and +excretion minister in different ways to the cells, and aid in the +maintenance of life. By their combined action two distinct movements are +kept up in the body, as follows: + +1. An _inward_ movement which carries materials from the outside of the +body toward the cells. + +2. An _outward_ movement which carries materials from the cells to the +outside of the body. + +Passing _inward_ are the oxygen and food materials _in a condition to +unite with each other_ and thereby change their potential into kinetic +energy. Passing _outward_ are the oxygen and the elements that formed the +food materials _after having united_ at the cells and liberated their +energy. + +As a final and all-important result, there is kept up a _continuous series +of chemical changes_ in the cells. These liberate the energy, provide +special substances needed by the cells, and preserve the life of the body +(Fig. 92). + +In the chapters which follow, we are to consider the problem of adjusting +the body to and of bringing it into proper relations with its +surroundings. + + + + + +PART II: MOTION, COORDINATION, AND SENSATION + + + + +CHAPTER XIV - THE SKELETON + + +One necessary means of establishing proper relations between the body and +its surroundings is _motion_.(77) Not only can the body move itself from +place to place, but it is able to move surrounding objects as well. In the +production of motion three important systems are employed--the muscular +system, the nervous system, and a system of mechanical devices which are +found mainly in the skeleton. The muscular system supplies the energy for +operating the mechanical devices, while the nervous system controls the +movements.(78) Although the skeleton serves other purposes, such as giving +shape to the body and protecting certain organs, its main use is that of +an aid in the production of motion. + +*Skeleton Tissues.*--The tissues employed in the construction of the +skeleton are the osseous, the cartilaginous, and the connective tissues. +These are known as the supporting tissues of the body. They form the +bones, supply the elastic pads at the ends of the bones, and furnish +strong bands, called ligaments, for fastening the bones together. The +skeleton forms about 16 per cent of the weight of the body. Its tissues, +being of a more durable nature than the rest of the body, do not so +readily decay. Especially is this true of the osseous tissue, which may be +preserved indefinitely, after removal from the body, by simply keeping it +dry. + +*The Bones.*--The separate units, or parts, of which the skeleton is +constructed are called bones. They are the hard structures that can be +felt in all parts of the body, and they comprise nearly the entire amount +of material found in the prepared skeleton. As usually estimated, the +bones are 208 in number. They vary greatly in size and shape in different +parts of the body. + +*Composition and Properties of Bones.*--The most noticeable and important +properties of the bones are those of hardness, stiffness, and toughness. +Upon these properties the uses of the bones depend. These properties may, +in turn, be shown to depend upon the presence in osseous tissue of two +essentially different kinds of substance, known as the _animal matter_ and +the _mineral matter_. If a bone is soaked in an acid, the mineral matter +is dissolved out, and as a result it loses its properties of hardness and +stiffness. (See Practical Work.) This is because the mineral matter +supplies these properties, being composed of substances which are hard and +closely resemble certain kinds of rock. The chief materials forming the +mineral matter are calcium phosphate and calcium carbonate. + +On the other hand, burning a bone destroys the animal matter. When this is +done the bone loses its toughness, and becomes quite brittle. The property +of toughness is, therefore, supplied by the animal matter. This consists +mainly of a substance called _ossein_, which may be dissolved out of the +bones by boiling them. Separated from the bones it is known as _gelatine_. +The blood vessels and nerves in the bones, and the protoplasm of the bone +cells, are also counted in with the animal matter. + + [Fig. 93] + + + Fig. 93--*Section of a long bone* (_tibia_), showing the gross structure. + + +If a dry bone from a full-grown, but not old, animal be weighed before and +after being burned, it is found to lose about one third of its weight. +From this we may conclude that about one third of the bone by weight is +animal matter and two thirds is mineral matter. This proportion, however, +varies with age, the mineral matter increasing with advance of years. + +*Gross Structure of Bones.*--The gross structure of the bones is best +learned by studying both dry and fresh specimens. (See Practical Work.) +The ends of the bones are capped by a layer of smooth, elastic cartilage, +while all the remaining surface is covered by a rather dense sheath of +connective tissue, called the _periosteum_. Usually the central part of +the long bones is hollow, being filled with a fatty substance known as the +_yellow marrow_. Around the marrow cavity the bone is very dense and +compact, but most of the material forming the ends is porous and spongy. +These materials are usually referred to as the _compact substance_ and the +_cancellous_, or _spongy, substance_ of the bones (Fig. 93). + +The arrangement of the compact and spongy substance varies with the +different bones. In the short bones (wrist and ankle bones, vertebrae, +etc.) and also in the flat bones (skull bones, ribs, shoulder blades, +etc.) there is no cavity for the yellow marrow, all of the interior space +being filled with the spongy substance. The _red marrow_, relations of +which to the red corpuscles of the blood have already been noted (page +27), occupies the minute spaces in the spongy substance. + + [Fig. 94] + + + Fig. 94--*Cross section of bone showing minute structure.* Magnified. 1. + Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which +pass the canaliculi. 4. A lacuna. Observe arrangement of lacunae at surface + and in deeper portion. + + +*Minute Structure of Bone.*--A microscopic examination of a thin slice of +bone taken from the compact substance shows this to be porous as well as +the spongy substance. Two kinds of small channels are found running +through it in different directions, known as the Haversian canals and the +canaliculi (Fig. 94). These serve the general purpose of distributing +nourishment through the bone. The _Haversian canals_ are larger than the +canaliculi and contain small nerves and blood vessels, chiefly capillaries +(Fig. 95). They extend lengthwise through the bone. The _canaliculi_ are +channels for conveying lymph. They pass out from the Haversian canals at +right angles, going to all portions of the compact substance except a thin +layer at the surface. In the surface layer of the bone the canaliculi are +in communication with the periosteum. + + [Fig. 95] + + + Fig. 95--*Section showing Haversian canal and contents*, highly magnified + (after Schaefer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve + fibers. 4. Lymph vessel. + + +*The Bone Cells.*--Surrounding the Haversian canals are thin layers of bone +substance called the _laminae_, and within these are great numbers of +irregular bodies, known as the _lacunae_. The walls of the lacunae are hard +and dense, but within each is an open space. In this lies a flattened +body, having a nucleus, which is recognized as the _bone cell_, or the +bone corpuscle (Fig. 96). It appears to be the work of the bone cells to +deposit mineral matter in the walls surrounding them and in this way to +supply the properties of hardness and stiffness to the bones. The +canaliculi connect with the lacunae in all parts of the bone, causing them +to appear under the microscope like so many burs fastened together by +their projecting spines (Fig. 94). + + [Fig. 96] + + + Fig. 96--*Bone cell* removed from the lacuna and very highly magnified. + (From Quain's _Anatomy_.) + + +*How the Bone Cells are Nourished.*--The bone cells, like all the other +cells of the body, are nourished by the lymph that escapes from the blood. +This passes through the canaliculi to the cells in the different parts of +the bone, as follows: + +1. The cells in the surface layer of the bone receive lymph from the +capillaries in the periosteum.(79) It gets to them through the short +canaliculi that run out to the surface. + +2. The cells within the interior of the bone receive their nourishment +from the small blood vessels in the Haversian canals. Lymph from these +vessels is conveyed to the cells through the canaliculi that connect with +the Haversian canals. + +*Plan and Purpose of the Skeleton.*--The framework of the body is such as +to adapt it to a _movable_ structure. Obviously the different parts of the +body cannot be secured to a foundation, as are those of a stationary +building, but must be arranged after a plan that is conducive to motion. A +moving structure, as a wagon or a bicycle, has within it some strong +central part to which the remainder is joined. The same is true of the +skeleton. That part to which the others are attached is a long, bony axis, +known as the _spinal column_. Certain parts, as the ribs and the skull, +are attached directly to the spinal column, while others are attached +indirectly to it. The arrangement of all the parts is such that the spinal +column is made the central, cohering portion of the skeleton and also of +the whole body. + +Besides the general arrangement of the parts of the skeleton, there is +such a grouping of the bones in each of its main divisions as will enable +them to serve definite purposes. In most places they form mechanical +devices for supplying special movements, and in certain places they +provide for the support or protection of important organs. In most cases +there is a definite combination of different bones, forming what is called +the bone group. + + [Fig. 97] + + + Fig. 97--The human skeleton. + + +*Bone Groups.*--On account of the close relation between the bones of the +same group, they cannot profitably be studied as individual bones, but +each must be considered as a part of the group to which it belongs. By +first making out the relation of a given bone to its group, its value to +the whole body can be determined. The most important of the groups of +bones are as follows: + +1. _The Spinal Column._--This group consists of twenty-four similarly +shaped bones, placed one above the other, called the _vertebrae_, and two +bones found below the vertebrae, known as the sacrum and the coccyx (Fig. +98). These twenty-six bones supply the central axis of the body, support +the head and upper extremities, and inclose and protect the spinal cord. + + [Fig. 98] + + + Fig. 98--The spinal column. + + +The upper seven vertebrae form the neck and are called the _cervical_ +vertebrae. They are smaller and have greater freedom of motion than the +others. The first and second cervical vertebrae, known as the _atlas_ and +the _axis_, are specially modified to form a support for the head and +provide for its movements. The head rests upon the atlas, forming with it +a hinge joint (used in nodding to indicate "yes"); and the atlas turns +upon an upward projection of the axis forming a pivot joint (used in +shaking the head to indicate "no"). + +The next twelve vertebrae, in order below the cervical, are known as the +_thoracic_ vertebrae. They form the back part of the framework of the +thorax and have little freedom of motion. The five vertebrae below the +thoracic are known as the _lumbar_ vertebrae. These bones are large and +strong and admit of considerable motion. Below the last lumbar vertebra is +a wedge-shaped bone which has the appearance of five vertebrae fused +together. This bone, known as the _sacrum_, connects with the large bones +which form the pelvic girdle. Attached to the lower end of the sacrum is a +group of from two to four small vertebrae, more or less fused, called the +_coccyx_. + + [Fig. 99] + + + Fig. 99--*Two views of a lumbar vertebra.* _A._ From above. _B._ From the + side. 1. Body. 2, 3, 4, 5. Projections from the neural arch. + + +*The Joining of the Vertebrae.*--A typical vertebra consists of a heavy, +disk-shaped portion in front, called the _body_, which is connected with a +ring-like portion behind, called the _neural arch_. The body and the +neural arch together encircle a round opening which is a part of the canal +that contains the spinal cord (Fig. 99). From the neural arch are seven +bony projections, or processes, three of which serve for the attachment of +muscles and ligaments, while the other four, two above and two below, are +for the interlocking of the vertebrae with each other. The separate +vertebrae are joined together in the spinal column, as follows: + +_a._ Between the bodies of adjacent vertebrae are disks of elastic +cartilage. Each disk is about one fourth of an inch thick and is grown +tight onto the face of the vertebra above and also onto the face of the +vertebra below. By means of these disks a very close connection is secured +between the vertebrae on the front side of the column. + +_b._ On the back of the column, the downward projections from the neural +arch of each vertebra above fit into depressions found in the neural arch +of the vertebra below. This _interlocking_ of the vertebrae, which is most +marked in the lumbar region, strengthens greatly the back portion of the +column. + +_c._ To further secure one bone upon the other, numerous ligaments pass +from vertebra to vertebra on all sides of the column. + +2. _The Skull._--The skull is formed by the close union of twenty-two +irregular bones. These fall naturally into two subgroups--the cranium and +the face (Fig. 100). The _cranium_ consists of eight thin, curved bones +which inclose the space, called the _cranial cavity_, that holds the +brain. The _face group_, consisting of fourteen bones, provides cavities +and supports for the different organs of the face, and supplies a movable +part (the inferior maxillary) which, with the bones above (superior +maxillary), forms the machine for masticating the food. + + [Fig. 100] + + +Fig. 100--*The skull (Huxley).* The illustration shows most of the bones of + the skull. + + +3. _The Thorax._--This group contains twenty-four bones of similar form, +called _ribs_, and a straight flat bone, called the _sternum_, or +breastbone (Fig. 101). The ribs connect with the spinal column behind, and +all but the two lowest ones connect with the sternum in front, and, by so +doing, inclose the thoracic cavity. As already stated (page 85), the bones +of the thorax form a mechanical device, or machine, for breathing. The +ribs are so arranged that the volume of the thorax is increased by +elevating them and diminished by depressing them, enabling the air to be +forced into and out of the lungs. + + [Fig. 101] + + + Fig. 101--*Bone groups of trunk.* + + +4. _The Shoulder and Pelvic Girdles._--These groups form two bony +supports--one at the upper and the other at the lower portion of the +trunk--which serve for the attachment of the arms and legs (Fig. 101). The +_shoulder girdle_ is formed by four bones--two clavicles, or collar bones, +and two scapulae, or shoulder blades. The clavicle on either side connects +with the upper end of the sternum and serves as a _brace_ for the +shoulder, while the scapula forms a socket for the humerus (the large bone +of the arm) and supplies many places for the attachment of muscles. + +The _pelvic girdle_ consists of two large bones of irregular shape, called +the _innominate_ bones. They connect behind with the sacrum and in front +they connect, through a small pad of cartilage, with each other. On the +inside of the girdle is a smooth, basin-shaped support for the contents of +the abdomen, but on the outside the bones are rough and irregular and +provide many places for the attachment of muscles and ligaments. Each +innominate bone has a deep, round socket into which the end of the femur +(the long bone of the leg) accurately fits. + +5. _The Arm and Hand Groups._--A long bone, the _humerus_, connects the arm +with the shoulder and gives form to the upper arm. In the forearm are two +bones, the _radius_ and the _ulna_, which connect at one end with the +humerus and at the other with the bones of the wrist (Fig. 102). + + [Fig. 102] + + + Fig. 102--*Bone groups of arm and leg.* + + +A group of eight small, round bones is found in the wrist, known as the +_carpal_ bones. These are arranged in two rows and are movable upon one +another. Five straight bones, the _metacarpals_, connect with the wrist +bones and form the framework for the palm of the hand. Attached to the +metacarpals are the bones of the fingers and thumb. These form an +interesting group of fourteen bones, called the _phalanges of the fingers_ +(Fig. 102). + +The bones of the hand provide a mechanical device, or machine, for +grasping, and the arm serves as a device for moving this grasping machine +from place to place. The work of the arm, in this respect, is not unlike +that of a revolving crane upon the end of which is a grab-hook. The hand +without the arm to move it about would be of little use. + +6. _The Leg and Foot Groups._--These correspond in form and arrangement to +the bones of the arm and hand. Since, however, the leg and foot are used +for purposes different from those of the arm and hand, certain differences +in structure are to be found. The _patella_, or kneepan, has no +corresponding bone in the arm; and the _carpus_, or ankle, which +corresponds to the wrist, contains seven instead of eight bones. The bones +of the foot and toes are the same in number as those of the hand and +fingers, but they differ greatly in size and form and have less freedom of +motion. The _femur_, which gives form to the thigh, is the longest bone of +the body. The _tibia_, or shin bone, and the _fibula_, the slender bone by +its side, give form to the lower part of the leg (Fig. 102). + +The legs are mechanical devices (walking machines) for moving the body +from place to place. The feet serve both as supports for the body and as +levers for pushing the body forward. By their attachment to the legs they +may be placed in all necessary positions for supporting and moving the +body. + +The different bone groups are shown in Fig. 97 and named in Table IV. + +*Adaptation to Special Needs.*--When any single bone is studied in its +relation to the other members of the group to which it belongs or with +particular reference to its purpose in the body, its adaptation to some +special place or use is at once apparent. Each bone serves some special +purpose, and to this purpose it is adapted by its form and structure. Long +bones, like the humerus and femur, are suited to giving strength, form, +and stiffness to certain parts, while irregular bones, like the vertebrae +and the pelvic bones, are fitted for supporting and protecting organs. +Others, like the wrist and ear bones, make possible a peculiar kind of +motion, and still others, like the ribs, are adapted to more than one +purpose. The vast differences in shape, size, structure, and surface among +the various bones are but the conditions that adapt them to particular +forms of service in the body. + +TABLE IV - THE PRINCIPAL BONES AND THEIR GROUPING IN THE BODY + + I. AXIAL SKELETON + + A. _Skull_, 28. + + 1. Cranium, 8. + + _ a._ Frontal, forehead 1 + _ b._ Parietal 2 + _ c._ Temporal, temple 2 + _ d._ Occipital 1 + _ e._ Sphenoid 1 + _ f._ Ethmoid 1 + + 2. Face, 14. + + _ a._ Inferior maxillary 1 + _ b._ Superior maxillary 2 + _ c._ Palatine, palate 2 + _ d._ Nasal bones 2 + _ e._ Vomer 1 + _ f._ Inferior turbinated 2 + _ g._ Lachrymal 2 + _ h._ Malar, cheek bones 2 + + 3. Bones of the Ears, 6. + + _ a._ Malleus 2 + _ b._ Incus 2 + _ c._ Stapes 2 + + B. _Spinal Column_, 26. + + 1. Cervical, or neck, vertebrae 7 + 2. Dorsal, or thoracic, vertebrae 12 + 3. Lumbar vertebrae 5 + 4. Sacrum 1 + 5. Coccyx 1 + + C. _Thorax_, 25. + + 1. Ribs 24 + 2. Sternum 1 + + D. _Hyoid_, 1 (at base of tongue). + + II. APPENDICULAR SKELETON + + A. _Shoulder girdle_ 4. + + 1. Clavicle, collarbone. 2 + 2. Scapula, shoulder blade 2 + + B. _Upper extremities_, 60. + + 1. Humerus 2 + 2. Radius 2 + 3. Ulna 2 + 4. Carpal, wrist bones 16 + 5. Metacarpal 10 + 6. Phalanges of fingers 28 + + C. _Pelvic girdle_, 2. + + 1. Osinnominatum 2 + + D. _Lower extremities_, 60. + + 1. Femur, thigh bone 2 + 2. Tibia, shin bone 2 + 3. Fibula 2 + 4. Patella, kneepan 2 + 5. Tarsal, ankle bones 14 + 6. Metatarsal, instep bones 10 + 7. Phalanges of toes 28 + + + +ARTICULATIONS + + +Any place in the body where two or more bones meet is called an +articulation, or joint. At the place of meeting the bones are firmly +attached to each other, thereby securing the necessary coherence of the +skeleton. The large number of bones, and consequently of articulations, +are necessary for the different movements of the body and also on account +of the manner in which the skeleton develops, or grows. Articulations are +classed with reference to their freedom of motion, as _movable_, _slightly +movable_, and _immovable_ articulations. + +Most of the _immovable_ articulations are found in the skull. Here +irregular, tooth-like projections from the different bones enable them to +interlock with one another, while they are held firmly together by a thin +layer of connective tissue. The wavy lines formed by articulations of this +kind are called _sutures_ (Fig. 100). + +The best examples of joints that are _slightly_, but not freely, _movable_ +are found in the front of the spinal column. The cartilaginous pads +between the vertebrae permit, by their elasticity, of a slight bending of +the column in different directions. These movements are caused, not by one +bone gliding over another, but by compressions and extensions of the +cartilage. Between the vertebrae in the back of the spinal column, however, +there is a slight movement of the bone surfaces upon one another. + +*Structure of the Movable Joints.*--By far the most numerous and important +of the joints are those that are freely movable. Such joints are strongly +constructed and endure great strain without dislocation, and yet their +parts move over each other easily and without friction. The ends of the +bones are usually enlarged and have specially formed projections or +depressions which fit into corresponding depressions or elevations on the +bones with which they articulate. In addition to this the articular +surfaces are quite smooth and dense, having no Haversian canals, and they +are covered with a layer of cartilage. Strong ligaments pass from one bone +to the other to hold each in its place (_A, _Fig. 103). Some of these +consist simply of bands, connecting the joint on its different sides, +while others form continuous sheaths around the joint. + + [Fig. 103] + + + Fig. 103--*Outside and inside view of knee joint.* 1. Tendons. 2. +Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is + lined, except upon the articular surfaces, by the synovial membrane. + + +The interior of the joint, except where the bone surfaces rub upon each +other, is covered with a serous lining, called the _synovial membrane_ +(_B_, Fig. 103). This secretes a thick, viscid liquid, the _synovial +fluid_, which prevents friction. The synovial membrane does not cover the +ends of the bones, but passes around the joint and connects with the bones +at their edges so as to form a closed sac in which the fluid is retained. + +*Kinds of Movable Joints.--*The different kinds of movable joints are the +ball and socket joint, the hinge joint, the pivot joint, the condyloid +joint, and the gliding joint. These are constructed and admit of motion, +as follows: + +1. In the _ball and socket_ joint the ball-shaped end of one bone fits +into a cup-shaped cavity in another bone, called the socket. The best +examples of such joints are found at the hips and shoulders. The ball and +socket joint admits of motion in all directions. + +2. In the _hinge_ joint the bones are grooved and fit together after the +manner of a hinge. Hinge joints are found at the elbows and knees and also +in the fingers. The hinge joint gives motion in but two directions--forward +and backward. + +3. A _pivot_ joint is formed by the fitting of a pivot-like projection of +one bone into a ring-like receptacle of a second bone, so that one, or the +other, is free to turn. A good example of the pivot joint is found at the +elbow, where the radius turns upon the humerus. Another example is the +articulation of the atlas with the axis vertebra as already noted. The +pivot joint admits of motion around an axis. + +4. The _condyloid_ joint is formed by the fitting of the ovoid +(egg-shaped) end of one bone into an elliptical cavity of a second bone. +Examples of condyloid joints are found at the knuckles and where the wrist +bones articulate with the radius and ulna. They move easily in two +directions, like hinge joints, and slightly in other directions. + +5. _Gliding_ joints are formed by the articulation of plain (almost flat) +surfaces. Examples of gliding joints are found in the articulations +between the bones of the wrist and those of the ankle. They are the +simplest of the movable joints and are formed by one bone gliding, or +slipping, upon the surface of another. + +*The Machinery of the Body.*--A machine is a contrivance for directing +energy in doing work. A sewing machine, for example, so directs the energy +of the foot that it is made to sew. Through its construction the machine +is able to produce just that form of motion needed for its work, and no +other forms, so that energy is not wasted in the production of useless +motion. The places in machines where parts rub or turn upon each other are +called _bearings_, and extra precautions are taken in the construction and +care of the bearings to prevent friction. + +The body cannot properly be compared to any single machine, but must be +looked upon as a complex organization which employs a number of different +kinds of machines in carrying on its work. The majority of these machines +are found in the skeleton. The bones are the parts that are moved, and the +joints serve as bearings. Connected with the bones are the muscles that +supply energy, and attached to the muscles are the nerves that control the +motion. Other parts also are required for rendering the machines of the +body effective in doing work. These are supplied by the tissues connected +with the bones and the muscles. + + + +HYGIENE OF THE SKELETON + + +Of chief concern in the hygiene of the skeleton is the proper _adjustment_ +of its parts. The efficiency of any of the body machines is impaired by +lack of proper adjustment. Not only this, but because of the fact that the +skeleton forms the groundwork of the whole body--muscles, blood vessels, +nerves, everything in fact, being arranged with reference to it--any lack +of proper adjustment of the bones interferes generally with the +arrangement and work of tissues and organs. The displaced bones may even +compress blood vessels and nerves and interfere, in this way, with the +nourishment and control of organs remote from the places where the +displacements occur. For these reasons the proper adjustment of the +different parts of the skeleton supplies one of the essential conditions +for preserving the health. + +*Hygienic Importance of the Spinal Column.*--What has been said about the +adjustment of the skeleton in general applies with particular force to the +spinal column. The spinal column serves both as the central axis of the +body and as the container of the spinal cord. Thirty-one pairs of nerves +pass between the vertebrae to connect the spinal cord with different parts +of the body, and two important arteries (the vertebral) pass through a +series of small openings in the bones of the neck to reach the brain. +Unnatural curves of the spine throw different parts of the body out of +their natural positions, diminish the thoracic and abdominal cavities, +and, according to the belief of certain physicians, compress the nerves +that pass from the cord to other parts of the body. Slightly misplaced +vertebrae in the neck, by compressing the vertebral arteries, may also +interfere with the supply of blood + + [Fig. 104] + + + Fig. 104--A tendency toward spinal curvature (after Mosher) + + + [Fig. 105] + + + Fig. 105--Effect on spinal column of improper position in writing. (From + Pyle's _Personal Hygiene._) + + +*How the Skeleton becomes Deformed*--We are accustomed to look upon the +skeleton as a rigid framework which can get out of its natural form only +through severe strain or by violence. This view is far from being correct. +On account of their necessary freedom of motion, the bones, especially +those of the spinal column, are easily slipped from their normal +positions; and where improper attitudes are frequently assumed, or +continued through long periods of time, the skeleton gradually becomes +deformed (Fig. 104). For example, the habit of always sleeping on the same +side with a high pillow may develop a bad crook in the neck; and the ugly +curves, assumed so frequently in writing (80) (Fig. 105), and also in +standing, when the weight is shifted too much on one foot, may become +permanent. Then the habit of reclining in a chair with the hips resting on +the front of the seat often deforms the back and causes a drooping of the +shoulders. In fact, slight displacements of the vertebrae come about so +easily _through incorrect positions_, that they may almost be said to +"occur of themselves" where active measures are not taken to preserve the +natural form of the body. The very few people who have perfectly formed +bodies show to what an extent has been overlooked an essential law of +hygiene. + +*Prevention of Skeletal Deformities.*--Those deformities of the skeleton +that are acquired through improper positions are prevented by giving +sufficient attention to the positions assumed in sitting, standing, and +sleeping, and also to the posture in various kinds of work. In sitting the +trunk should be erect and the hips should touch the back of the chair. One +should not lounge in the ordinary chair. In standing the body should be +erect, the shoulders back and down, the chest pushed slightly up and +forward, and the chin slightly depressed, while the weight should, as a +rule, rest about equally on the two feet. The habit of leaning against +some object when standing (the pupil in reciting often leans on his desk) +should be avoided. In sleeping the pillow should be of the right thickness +to support the head on a level with the spinal column and should not be +too soft. If one sleeps on his back, no pillow is required. It is best not +to acquire the habit of sleeping always on the same side. + +Where one is compelled by his work to assume harmful positions, these +should be corrected by proper exercises, and by cultivating opposing +positions during the leisure hours. Much is to be accomplished through +those forms of physical exercise which develop the muscles whose work it +is to keep the body in an upright position. + +*School Furniture.*--It has long been observed that school children are +more subject to curvature of the spine and other deformities of the +skeleton than the children who do not attend school. While this is due +largely to faulty positions assumed by the pupils at their work, it has +been suggested that the school furniture may be in part to blame for these +positions. Investigations of this problem have shown that most of the +school desks and seats in use in our public schools are unhygienically +constructed, in that they _force_ pupils into unnatural positions. School +seats should support the pupil in a natural position, both in the use of +his books and in writing, and there are many arguments in favor of the +so-called "adjustable" school furniture. Fig. 106 shows the seat and desk +designed by the Boston, Mass., Schoolhouse Commission after much study and +experimenting and used in the Boston schools. This furniture, which +provides a seat adjustable for height, having a back rest also adjustable +for height, and a desk which is likewise provided with a vertical +adjustment, supplies all essential hygienic requirements. It is to be +hoped that school furniture of this character may in the near future come +into general use. + + [Fig. 106] + + + Fig. 106--Adjustable seat and desk used in schools of Boston, Mass. + + +*Correction of Skeletal Deformities.*--It is, of course, easier to prevent +deformities of the skeleton by giving attention to proper positions, than +to correct them after they have occurred. It should also be noted that +severe deformities cannot be corrected by the individual for himself, but +these must come under the treatment of specialists in this line of medical +work. In mild cases of spinal curvature, drooping of the head, and round +shoulders, the individual _can_ benefit his condition. By working to +"substitute a correct attitude for the faulty one,"(81) he can by +persistence bring about marked improvements. It is better, however, to +have the advice and aid of a physical director, where this is possible. It +should also be borne in mind that the correction of skeletal deformities +requires effort through a long period of time, especially where the +deformities are pronounced; and one lacking the will power to persist will +not secure all the results which he seeks. + +*"Setting Up" Exercises.*--The splendid carriage of students from military +schools shows what may be accomplished in securing erectness of form where +proper attention is given to this matter. The military student gets his +fine form partly through his exercises in handling arms, but mainly +through his so-called "setting up" drill. As a suggestion to one desiring +to improve the form of his body, a modification of the usual "setting up" +drill is here given: + +1. Standing erect, with the heels together, the feet at an angle of 45 deg., +and hands at the sides, bring the arms to a horizontal position in front, +little fingers touching and nails down. From this position raise the hands +straight over the head, bringing the palms gradually together. Then with a +backward sweeping movement, return the hands again to the sides. Repeat +several times. + +2. With the feet as in the above exercise, bring the hands and the arms to +a level with the shoulders, palms down, elbows bent, middle fingers of the +two hands touching, and the extended thumbs touching the chest. Keeping +the palms down and the arms on a level with the shoulders, extend the +hands as far sideward and backward as possible, returning each time to the +first position. As the hands move out, inhale deeply (through the nose), +and as they are brought back, exhale quickly (through the mouth). Repeat +several times. + +3. With the arms at the sides and the feet side by side and touching, +bring the hands in a circular movement to a vertical position over the +head, and lock the thumbs. Keeping the knees straight and the thumbs +locked, bend forward, letting the hands touch the ground if possible, and +then bring the body and hands again to the vertical position. Then by a +backward sweeping movement, return the hands again to the sides. Repeat. + +While these exercises may be practiced whenever convenient, it is best to +set apart some special time each day for them, as on retiring at night or +on rising in the morning. + +*Hygienic Footwear.*--A necessary aid to erectness of position in standing +and walking is a properly fitting shoe. Heels that are too high tilt the +body unnaturally forward, and shoes that cause any kind of discomfort in +walking lead to unnatural positions in order to protect the feet. Shoes +should fit snugly, being neither too large nor too small. Many shoes, +however, are unhygienically constructed, and no attempt should be made to +wear them. Certainly is this true of styles that approach the "French +heel" or the "toothpick toe" (Fig. 107). However, many styles of shoes are +manufactured that are both hygienic and neat fitting. Rubber heels, on +account of their elasticity, are to be preferred to those made of leather. + + [Fig. 107] + + + Fig. 107--Heels and toes of unhygienic and of hygienic footwear. + + +*The Skeleton in Childhood and Old Age.*--Certain peculiarities are found +to exist in the bones of children and of old people which call for special +care of the skeleton during the first and last periods of life. The bones +of children are soft, lacking mineral matter, and are liable to become +bent For this reason, children who are encouraged to walk at too early an +age may bend the thigh bones, causing the too familiar "bow-legs." These +bones may also be bent by having children sit on benches and chairs which +are too high for the feet to reach the floor, and which do not provide +supports for the feet. Wholesome food, fresh air, sunlight, and exercise +are also necessary to the proper development of the bones of children. +Where these natural conditions are lacking, as in the crowded districts of +cities, children often suffer from a disease known as "rickets," on +account of which their bones are unnaturally soft and easily bent. + +On account of the accumulation of mineral matter, the bones of elderly +people become brittle and are easily broken, and from lack of vigor of the +bone cells they heal slowly after such injuries occur. This makes the +breaking of a bone by an aged person a serious matter. Old people should, +as far as possible, avoid liabilities to falls, such as going rapidly up +and down stairs, or walking on icy sidewalks, and should use the utmost +care in getting about. In old people also the cartilage between the bones +softens, increasing the liability of getting misshaped. Special attention, +therefore, should be given to erectness of form, and to such exercises as +tend to preserve the natural shape of the body. + +*Treatment of Fractures.*--A fractured bone always requires the aid of a +surgeon, and no time should be lost in securing his services. In the +meantime the patient should be put in a comfortable position, and the +broken limb supported above the rest of the body. Though the breaking of a +bone is not, as a rule, a serious mishap, it is necessary that the very +best skill be employed in setting it. Any failure to bring the ends of the +broken bone into their normal relations permanently deforms the limb and +interferes with its use. + +*Dislocations and Sprains.*--Dislocations, if they be of the larger joints, +also require the aid of the surgeon in their reduction and sometimes in +their subsequent treatment. Simple dislocations of the finger joints, +however, may be reduced by pulling the parts until the bones can be +slipped into position. + +_A sprain_, which is an overstrained condition of the ligaments +surrounding a joint, frequently requires very careful treatment. When the +sprain is at all serious, a physician should be called. Because of the +limited supply of blood to the ligaments, they are slow to heal, and the +temptation to use the joint before it is fully recovered is always great. +Massage(82) judiciously applied to a sprained joint, by bringing about a +more rapid change in the blood and the lymph, is beneficial both in +relieving the pain, and in hastening recovery. + +*Summary.*--The skeleton, or framework of the body, is a structure which is +movable as a whole and in most of its parts. It preserves the form of the +body, protects important organs, and supplies the mechanical devices, or +machines, upon which the muscles act in the production of motion. The +skeleton is adapted to its purposes through the number and properties of +the bones, and through the cartilage and connective tissue associated with +the bones. The places where the different bones connect one with another +are known as joints, and most of these admit of motion. The preservation +of the natural form of the skeleton is necessary, both for its proper +action and for the health of the body. + +*Exercises.*--1. State the main purpose of the skeleton. What is the +necessity for so many bones in its construction? + +2. How may the per cent of animal and of mineral matter in a bone be +determined? + +3. What properties are given the bones by the animal matter? What by the +mineral matter? + +4. Locate the bone cells. What is their special function? + +5. State the plan by which nourishment is supplied to the bone cells in +different parts of the bone. + +6. Give the uses of the periosteum. + +7. State the purpose of the Haversian canals. Of the canaliculi. + +8. Give functions of the spinal column. + +9. Name the different materials used in the construction of a joint and +the purpose served by each. + +10. Name four mechanical devices, or machines, found in the skeleton and +state the purpose served by each. + +11. Name one or more of the body machines not located in the skeleton. + +12. Of what advantage is the peculiar shape of the lower jaw? Of the ribs? +Of the bones of the pelvic girdle? + +13. State the importance of preserving the natural form of the skeleton. +How are unnatural curves produced in the spinal column? + +14. How may slight deformities of the skeleton be corrected? + +15. What different systems are employed in the body in the production of +motion? What is the special function of each? + + + +PRACTICAL WORK + + +To obtain clear ideas of the form and functions of the bones, a careful +examination of a prepared and mounted skeleton is necessary. Many of the +bones, however, may be located and their general form made out from the +living body. Bones of the lower animals may also be studied to advantage. + +*Experiments to show the Composition of Bone.*--1. Examine a slender bone, +like that in a chicken's leg. Note that it resists bending and is +difficult to break. Note also that it is elastic--that, when slightly bent, +it will spring back. + +2. Soak such a bone over night in a mixture of one part hydrochloric acid +and four parts water. Then ascertain by bending, stretching, and twisting +what properties the bone has lost. The acid has dissolved out the mineral +matter. + +3. Burn a small piece of bone in a clear gas flame, or on a bed of coals, +until it ceases to blaze and turns a white color. Can the bone now be bent +or twisted? What properties has it lost and what retained? What substance +has been removed from the bone by burning? + +*Observation on the Gross Structure of Bone.*--1. Procure a long, dry bone. +(One that has lain out in the field until it has bleached will answer the +purpose excellently.) Test its hardness, strength, and stiffness. Saw it +in two a third of the distance from one end, and saw the shorter piece in +two lengthwise. Compare the structure at different places. Find rough +elevations on the outside for the attachment of muscles, and small +openings into the bone for the entrance of blood vessels and nerves. Make +drawings to represent the sections. + +2. Procure a fresh bone from the butcher shop. Note the difference between +it and the dry bone. Examine the materials surrounding the sides and +covering the ends of the bone. Saw through the enlarged portion at the end +and examine the red marrow. Saw through the middle of the bone and observe +the yellow marrow. + +*To show the Minute Structure of the Bone.*--Prepare a section of bone for +microscopic study as follows: With a jeweler's saw cut as thin a slice as +possible. Place this upon a good-sized whetstone, not having too much +grit, and keeping it wet rub it under the finger, or a piece of leather, +until it is thin enough to let the light shine through. The section may +then be washed and examined with the microscope. If the specimen is to be +preserved for future study, it may be mounted in the usual way, but with +_hard_ balsam. Prepare and study both transverse and longitudinal +sections, making drawings. The sections should be prepared from bones that +are thoroughly dry but which have not begun to decay. + +*To show the Structure of a Joint.*--Procure from a butcher the joint of +some small animal (hog or sheep). Cut it open and locate the cartilage, +synovial membrane, and ligaments. Observe the shape and surface of the +rubbing parts and the strength of the ligaments. + + + + +CHAPTER XV - THE MUSCULAR SYSTEM + + +As already stated, the skeleton, the nervous system, and the muscular +system are concerned in the production of motion. The skeleton and the +nervous system, however, serve other purposes in the body, while the +muscular system is devoted exclusively to the production of motion. For +this reason it is looked upon as the special _motor_ system. The muscular +tissue is the most abundant of all the tissues, forming about 41 per cent +of the weight of the body. + +*Properties of Muscles.*--The ability of muscular tissue to produce motion +depends primarily upon two properties--the property of irritability and the +property of contractility. _Irritability_ is that property of a substance +which enables it to respond to a stimulus, or to act when acted upon. +_Contractility_ is the property which enables the muscle when stimulated +to draw up, thereby becoming shorter and thicker (a condition called +contraction), and when the stimulation ceases, to return to its former +condition (of relaxation). The property of contractility enables the +muscles to produce motion. Irritability is a condition necessary to their +control in the body. + +*Kinds of Muscular Tissue.*--Three kinds of muscular tissue are found in +the body. These are known as the _striated_, or striped, muscular tissue; +the _non-striated_, or plain, muscular tissue; and the _muscular tissue of +the heart_. These are made up of different kinds of muscle cells and act +in different ways to cause motion. The striated muscular tissue far +exceeds the others in amount and forms all those muscles that can be felt +from the surface of the body. The non-striated muscle is found in the +walls of the food canal, blood vessels, air passages, and other tubes of +the body; while the muscular tissue of the heart is confined entirely to +that organ. + +*Striated Muscle Cells.*--The cells of the striated muscles are slender, +thread-like structures, having an average length of 1-1/2 inches (35 +millimeters) and a diameter of about 1/400 of an inch (60 {~GREEK SMALL LETTER MU~}). Because of +their great length they are called fibers, or fiber cells. They are marked +by a number of dark, transverse bands, or stripes, called striations,(83) +which seem to divide them into a number of sections, or disks (Fig. 108). +A thin sac-like covering, called the _sarcolemma_, surrounds the entire +cell and just beneath this are a number of nuclei.(84) + + [Fig. 108] + + +Fig. 108--*A striated muscle cell* highly magnified, showing striations and + nuclei. Attached to the cell is the termination of a nerve fiber. + + +Within the sarcolemma are minute fibrils and a semiliquid substance, +called the _sarcoplasm_. At each end the cell tapers to a point from which +the sarcolemma appears to continue as a fine thread, and this, by +attaching itself to the inclosing sheath, holds the cell in place. Most of +the muscle cells receive, at some portion of their length, the termination +of a nerve fiber. This penetrates the sarcolemma and spreads out upon a +kind of disk, having several nuclei, known as the _end plate_. + +*The "Muscle-organ."*--We must distinguish between the term "muscle" as +applied to the muscular tissue and the term as applied to a working group +of muscular tissue, which is an organ. In the muscle, or muscle-organ, is +found a definite grouping of muscle fibers such as will enable a large +number of them to act together in the production of the same movement. An +examination of one of the striated muscles shows the individual fibers to +lie parallel in small bundles, each bundle being surrounded by a thin +layer of connective tissue. (See Practical Work.) These small bundles are +bound into larger ones by thicker sheaths and these in turn may be bound +into bundles of still larger size (Fig. 109). The sheaths surrounding the +fiber bundles are connected with one another and also with the outer +covering of the muscle, known as + + [Fig. 109] + + + Fig. 109--*Diagram* of a section of a muscle, showing the perimysium and + the bundles of fiber cells. + + + [Fig. 110] + + +Fig. 110--*A muscle-organ in position.* The tendons connect at one end with + the bones and at the other end with the fiber cells and perimysium. (See + text.) + + +*The Perimysium.*--The plan of the muscle-organ is revealed through a study +of the perimysium. This is not limited to the surface of the muscle, as +the name suggests, but properly includes the sheaths that surround the +bundles of fibers. Furthermore, the surface perimysium and that within the +muscle are both continuous with the strong, white cords, called _tendons_, +that connect the muscles with the bones. By uniting with the bone at one +end and blending with the perimysium and fiber bundles at the other, the +tendon forms a very secure attachment for the muscle. The perimysium and +the tendon are thus the means through which the fiber cells in any +muscle-organ are made to _pull together_ upon the same part of the body +(Fig. 110). + +*Purpose of Striated Muscles.*--The striated muscles, by their attachments +to the bones, supply motion to all the mechanical devices, or machines, +located in the skeleton. Through them the body is moved from place to +place and all the external organs are supplied with such motion as they +require. Because of the attachment of the striated muscles to the +skeleton, and their action upon it, they are called _skeletal_ muscles. As +most of them are under the control of the will, they are also called +_voluntary_ muscles. They are of special value in adapting the body to its +surroundings. + +*Structure of the Non-striated Muscles.*--The cells of the non-striated +muscles differ from those of the striated muscles in being decidedly +spindle-shaped and in having but a single well-defined nucleus (Fig. 111). +Furthermore, they have no striations, and their connection with the nerve +fibers is less marked. They are also much smaller than the striated cells, +being less than one one-hundredth of an inch in length and one +three-thousandth of an inch in diameter. + +In the formation of the non-striated muscles, the cells are attached to +one another by a kind of muscle cement to form thin sheets or slender +bundles. These differ from the striated muscles in several particulars. +They are of a pale, whitish color, and they have no tendons. Instead of +being attached to the bones, they usually form a distinct layer in the +walls of small cavities or of tubes (Fig. 111). Since they are controlled +by the part of the nervous system which acts independently of the will, +they are said to be _involuntary_. They contract and relax slowly. + + [Fig. 111] + + + Fig. 111--*Non-striated muscle cells.* _A._ Cross section of small artery + magnified, showing (1) the layer of non-striated cells. _B._ Three + non-striated cells highly magnified. + + +*Work of the Non-striated Muscles.*--The work of the non-striated muscles, +both in purpose and in method, is radically different from that of the +striated. They do not change the _position_ of parts of the body, as do +the striated muscles, but they alter the _size_ and _shape_ of the parts +which they surround. Their purpose, as a rule, is to move, or control the +movement of, materials within cavities and tubes, and they do this by +means of the _pressure_ which they exert. Examples of their action have +already been studied in the propulsion of the food through the alimentary +canal and in the regulation of the flow of blood through the arteries +(pages 159 and 49). While they do not contract so quickly, nor with such +great force as the striated muscles, their work is more closely related to +the vital processes. + +*Structure of the Heart Muscle.*--The cells of the heart combine the +structure and properties of the striated and the non-striated muscle +cells, and form an intermediate type between the two. They are +cross-striped like the striated cells, and are nearly as wide, but are +rather short (Fig. 112). Each cell has a well-defined nucleus, but the +sarcolemma is absent. They are placed end to end to form fibers, and many +of the cells have branches by which they are united to the cells in +neighboring fibers. In this way they interlace more or less with each +other, but are also cemented together. They contract quickly and with +great force, but are not under control of the will. Muscular tissue of +this variety seems excellently adapted to the work of the heart. + + [Fig. 112] + + +Fig. 112--*Muscle cells from the heart*, highly magnified (after Schaefer). + + +*The Muscular Stimulus.*--The inactive, or resting, condition of a muscle +is that of relaxation. It does work through contracting. It becomes +active, or contracts, only when it is being acted upon by some force +outside of itself, and it relaxes again when this force is withdrawn. Any +kind of force which, by acting on muscles, causes them to contract, is +called a _muscular stimulus_. Electricity, chemicals of different kinds, +and mechanical force may be so applied to the muscles as to cause them to +contract. These are _artificial_ stimuli. So far as known, muscles are +stimulated _naturally_ in but one way. This is through the nervous system. +The nervous system supplies a stimulus called the _nervous impulse_, which +reaches the muscles by the nerves, causing them to contract. By means of +nervous impulses, all of the muscles (both voluntary and involuntary) are +made to contract as the needs of the body for motion require. + +*Energy Transformation in the Muscle.*--The muscle serves as a kind of +engine, doing work by the transformation of potential into kinetic energy. +Evidences of this are found in the changes that accompany contraction. +Careful study shows that during any period of contraction oxygen and food +materials are consumed, waste products, such as carbon dioxide, are +produced, and heat is liberated. Furthermore, the _blood supply to the +muscle_ is such that the materials for providing energy may be carried +rapidly to it and the products of oxidation as rapidly removed. Blood +vessels penetrate the muscles in all directions and the capillaries lie +very near the individual cells (Fig. 113). Provision is made also, through +the nervous system, for _increasing_ the blood supply when the muscle is +at work. From these facts, as well as from the great force with which the +muscle contracts, one must conclude that the muscle is a _transformer of +energy_--that within its protoplasm, chemical changes take place whereby +the potential energy of oxygen and food is converted into the kinetic +energy of motion. + + [Fig. 113] + + + Fig. 113--*Capillaries* of muscles. + + +*Plan of Using Muscular Force.*--Two difficulties have to be overcome in +the using of muscular force in the body. The first of these is due to the +fact that the muscles exert their force _only when they contract_. They +can pull but not push. Hence, in order to bring about the opposing +movements(85) of the body, each muscle must work against some force that +produces a result directly opposite to that which the muscle produces. +Some of the muscles (those of breathing) work against the elasticity of +certain parts of the body; others (those that hold the body in an upright +position), to some extent against gravity; and others (the non-striated +muscle in arteries), against pressure. But in most cases, _muscles work +against muscles_. + + [Fig. 114] + + + Fig. 114--*The muscle pair* that operates the forearm. For names of these + muscles, see Fig. 119. + + +The striated, or skeletal, muscles are nearly all arranged after the +last-named plan. As a rule a pair of muscles is so placed, with reference +to a joint, that one moves the part in one direction, and the other moves +it in the opposite direction. From the kinds of motion which the various +muscle pairs produce, they are classified as follows: + +1. _Flexors and Extensors._--The flexor muscles bend and the extensors +straighten joints (Fig. 114). + +2. _Adductors and Abductors._--The adductors draw the limbs into positions +parallel with the axis of the body and the abductors draw them away. + +3. _Rotators_ (two kinds).--The rotators are attached about pivot joints +and bring about twisting movements. + +4. _Radiating and Sphincter Muscles. _--The radiating muscles open and the +sphincter muscles close the natural openings of the body, such as the +mouth. + +The pupil should locate examples of the different kinds of muscle pairs in +his own body. + +*Exchange of Muscular Force for Motion.*--The second difficulty to be +overcome in the use of muscular force in the body is due to the fact that +the muscles contract through _short_ distances, while it is necessary for +most of them to move portions of the body through _long_ distances. It may +be easily shown that the longest muscles of the body do not shorten more +than three or four inches during contraction. To bring about the required +movements of the body, which in some instances amount to four or five +feet, requires that a large proportion of the muscular force be exchanged +for motion. The machines of the skeleton, while providing for motion in +definite directions, also provide the means whereby _strong forces_, +acting through _short distances_, are made to produce movements of _less +force_, through _long distances_. The mechanical device employed for this +purpose is known as + +*The Lever.*--The lever may be described as a stiff bar which turns about a +fixed point of support, called the _fulcrum_. The force applied to the bar +to make it turn is called the _power_, and that which is lifted or moved +is termed the _weight_. The weight, the power, and the fulcrum may occupy +different positions along the bar and this gives rise to the three kinds +of levers, known as levers of the first class, the second class, and the +third class (Fig. 115). In levers of the _first class_ the fulcrum +occupies a position somewhere between the power and the weight. In the +_second class_ the weight is between the fulcrum and the power. In the +_third class_ the power is between the fulcrum and the weight. + + [Fig. 115] + + + Fig. 115--*Classes of levers. I.* Two levers of first class showing +fulcrums in different positions. II. Lever of second class. III. Lever of + third class. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._ + Weight-arm. + + +*Application to the Body.*--In the body the bones serve as levers; the +turning points, or fulcrums, are found at the joints; the muscles supply +the power; and parts of the body, or things to be lifted, serve as +weights. For these levers to _increase_ the motion of the muscles, it is +necessary that the muscles be attached to the bones _near the joints_, and +that the parts to be moved be located at some distance from the joints. In +other words the (muscle) power-arm must be _shorter_ than the (body) +weight-arm.(86) + +Examining Fig. 116, it is seen that the distances moved by the power and +weight vary as their respective distances from the fulcrum. That is to +say, if the weight is twice as far from the fulcrum as the power, it will +move through twice the distance, and if three times as far, through three +times the distance. Thus the muscles, by acting through short distances +(on the short arms of levers), are able to move portions of the body +(located on the long arms) through long distances. Can all three classes +of levers be used in this way in the body? + + [Fig. 116] + + +Fig. 116--*Motion producing levers.* Diagrams show relative distances moved +by the power and weight in levers having the power nearer the fulcrum than + is the weight. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight. + + +*Classes of Levers found in the Body.*--Practically all of the levers of +the body belong either to the first class or the third class. In both of +these the muscle power can be applied to the short arm of the lever, +thereby moving the body weight through a longer distance than the muscle +contracts (Fig. 116). In the levers of the second class, however, the +weight occupies this position, being situated _between_ the power and +fulcrum (Fig. 117). The weight, therefore, _cannot_ move farther than the +power in this lever. It must always move a shorter distance. While such a +lever is of great advantage in lifting heavy weights outside of the body, +it cannot be used for increasing the motion of the muscles. For this +reason no well-defined levers of the second class are present in the +body.(87) + + [Fig. 117] + + + Fig. 117--*Weight lifting levers.* Diagrams show relative distances moved + by the power and weight in levers having the weight nearer the fulcrum + than is the power. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight. + + + [Fig. 118] + + + Fig. 118--*Diagram of the foot lever.* _F._ Fulcrum at ankle joint. _W._ + Body weight expressed as pressure against the earth. While the muscle +power acts through the distance _ab_, the fulcrum support (body) is forced + through the distance _FE_. + + +*Loss of Muscular Force.*--Using a small spring balance for measuring the +power, a light stick for a lever, and a small piece of metal for a weight, +and arranging these to represent some lever of the body (as the forearm), +it is easily shown that the gain in motion causes a corresponding loss in +muscular power. (See Practical Work.) If, for example, the balance is +attached two inches from the fulcrum and the weight twelve inches, the +pull on the balance is found to be six times greater than the weight that +is being lifted. If other positions are tried, it is found that the power +exerted in each case is as many times greater than the weight as the +weight-arm is times longer than the power-arm. + +Applying this principle to the levers of the body, it is seen that the +gain in motion is at the expense of muscular force, or, as we say, +_muscular force is exchanged for motion_. This exchange is greatly to the +advantage of the body; for while the ability to lift heavy weights is +important, the ability to move portions of the body rapidly and through +long distances is much more to be desired. + +*Important Muscles.*--There are about five hundred separate muscles in the +body. These vary in size, shape, and plan of attachment, to suit their +special work. Some of those that are prominent enough to be felt at the +surface are as follows: + +_Of the head_: The _temporal_, in the temple, and the _masseter_, in the +cheek. These muscles are attached to the lower jaw and are the chief +muscles of mastication. + +_Of the neck_: The _sterno-mastoids_, which pass between the mastoid +processes, back of the ears, and the upper end of the sternum. They assist +in turning the head and may be felt at the sides of the neck (Fig. 119). + +_Of the upper arm_: The _biceps_ on the front side, the _triceps_ behind, +and the _deltoid_ at the upper part of the arm beyond the projection of +the shoulder. + + [Fig. 119] + + + Fig. 119--Back and front views of important muscles. + + +_Of the forearm_: The _flexors_ of the fingers, on the front side, and the +_extensors_ of the fingers, on the back of the forearm (Fig. 119). + +_Of the hand_: The _adductor pollicis_ between the thumb and the palm. + +_Of the trunk_: The _pectoralis major_, between the upper front part of +the thorax and the shoulder; the _trapezius_, between the back of the +shoulders and the spine; the _rectus abdominis_, passing over the abdomen +from above downward; and the _erector spinae_, found in the small of the +back. + +_Of the hips_: The _glutens maximus_, fastened between the lower back part +of the hips and the upper part of the femur. + +_Of the upper part of the leg_: The _rectus femoris_, the large muscle on +the front of the leg which connects at the lower end with the kneepan. + +_Of the lower leg_: The _tibialis anticus_ on the front side, exterior to +the tibia, and the _gastrocnemius_, the large muscle in the calf of the +leg. This is the largest muscle of the body, and is connected with the +heel bone by the _tendon of Achilles_ (Fig. 119). + +The use of these muscles is, in most instances, easily determined by +observing the results of their contraction. + + + +HYGIENE OF THE MUSCLES + + +The hygiene of the muscles is almost expressed by the one word _exercise_. +It is a matter of everyday knowledge that the muscles are developed and +strengthened by use, and that they become weak, soft, and flabby by +disuse. The effects of exercise are, however, not limited to the large +muscles attached to the skeleton, but are apparent also upon the +involuntary muscles, whose work is so closely related to the vital +processes. While it is true that exercise cannot be applied directly to +the involuntary muscles, it is also true that exercise of the voluntary +muscles causes a greater activity on the part of those that are +involuntary and is indirectly a means of exercising them. + +*Exercise and Health.*--In addition to its effects upon the muscles +themselves, exercise is recognized as one of the most fundamental factors +in the preservation of the health. Practically every process of the body +is stimulated and the body as a whole invigorated by exercise properly +taken. On the other hand, a lack of exercise has an effect upon the entire +body somewhat similar to that observed upon a single muscle. It becomes +weak, lacks energy, and in many instances actually loses weight when +exercise is omitted. This shows exercise to supply an actual need and to +be in harmony with the nature and plan of the body. + +*How Exercise benefits the Body.*--In accounting for the healthful effects +of exercise, it must be borne in mind that the body is essentially a +motion-producing structure. Furthermore, its plan is such that the +movements of its different parts aid indirectly the vital processes. The +student will recall instances of such aid, as, for example, the assistance +rendered by muscular contractions in the circulation of the blood and +lymph, due to the valves in veins and lymph vessels, and the assistance +rendered by abdominal movements in the propulsion of materials through the +food canal. A fact not as yet brought out, however, is that _exercise +stimulates nutritive changes in the cells_, thereby imparting to them new +vigor and vitality. While this effect of exercise cannot be fully +accounted for, two conditions that undoubtedly influence it are the +following: + +1. Exercise causes the blood to circulate more rapidly. + +2. Exercise increases the movement of the lymph through the lymph vessels. + +The increase in the flow of the blood and the lymph causes changes to take +place more rapidly in the liquids around the cells, thereby increasing the +supply of food and oxygen, and hastening the removal of waste. + +*One should plan for Exercise.*--Since exercise is demanded by the nature +and plan of the body, to neglect it is a serious matter. People do not +purposely omit exercise, but from lack of time or from its interference +with the daily routine of duties, the needed amount is frequently not +taken. Especially is this true of students and others who follow sedentary +occupations. People of this class should plan for exercise as they plan +for the other great needs of the body--food, sleep, clothing, etc. It is +only by making a sufficient amount of muscular work or play a regular part +of the daily program that the needs of the body for exercise are +adequately supplied. + +*Amount and Kind of Exercise.*--The amount of exercise required varies +greatly with different individuals, and definite recommendations cannot be +made. For each individual also the amount should vary with the physical +condition and the other demands made upon the energy. One in health should +exercise sufficiently to keep the muscles firm to the touch and the body +in a vigorous condition. + +Of the many forms of exercise from which one may choose, the question is +again one of individual adaptability and convenience. While the different +forms of exercise vary in their effects and may be made to serve different +purposes, the consideration of these is beyond the scope of an elementary +text. As a rule one will not go far wrong by following his inclinations, +observing of course the conditions under which exercise is taken to the +best advantage. + +*General Rules for Healthful Exercise.*--That exercise may secure the best +results from the standpoint of health, a number of conditions should be +observed: 1. It should not be excessive or carried to the point of +exhaustion. Severe physical exercise is destructive to both muscular and +nervous tissues. 2. It should, if possible, be of an interesting nature +and taken in the open air. 3. It should be counter-active, that is, +calling into play those parts of the body that have not been used during +the regular work.(88) 4. It should be directed toward the weak rather than +toward the strong parts of the body. 5. When one is already tired from +study, or other work, it should be taken with moderation or omitted for +the time being. (For exercise of the heart muscle and the muscular coat of +the blood vessels see pages 55 and 57.) + +*Massage.*--In lieu of exercise taken in the usual way, similar effects are +sometimes obtained by a systematic rubbing, pressing, stroking, or +kneading of the skin and the muscles by one trained in the art. This +process, known as massage, may be gentle or vigorous and is subject to a +variety of modifications. Massage is applied when one is unable to take +exercise, on account of disease or accident, and also in the treatment of +certain bodily disorders. A weak ankle, wrist, or other part of the body, +or even a bruise, may be greatly benefited by massage. The flow of blood +and lymph is stimulated, causing new materials to be passed to the +affected parts and waste materials to be removed. Massage, however, should +never be applied to a boil, or other infected sore. The effect in this +case would be to spread the infection and increase the trouble. + +*Summary.*--Motion is provided for in the body mainly through the muscle +cells. These are grouped into working parts, called muscles, which in turn +are attached to the movable parts of the body. The striated muscles, as a +rule, are attached to the mechanical devices found in the skeleton, and +bring about the voluntary, movements. The non-striated muscles surround +the parts on which they act, and produce involuntary movements. Both, +however, are under the control of the nervous system. To bring about the +opposing movements of the body, the striated muscles are arranged in +pairs; and to increase their motion, the bones are used as levers. +Physical exercise is necessary both for the development of the muscles and +for the health and vigor of the entire body. + +*Exercises.*--1. Compare the striated and non-striated muscles with +reference to structure, location, and method of work. + +2. In what respects is the muscular tissue of the heart like the striated, +and in what respects like the non-striated, muscular tissue? + +3. If muscles could push as well as pull, would so many be needed in the +body? Why? + +4. Locate muscles that work to some extent against elasticity and gravity. + +5. Locate five muscles that act as flexors; five that act as extensors; +two that act as adductors; and two as abductors. Locate sphincter and +radiating muscles. + +6. By what means does the nervous system control the muscles? + +7. Give proofs of the change of potential into kinetic energy during +muscular contraction. + +8. Define the essential properties of muscular tissue and state the +purpose served by each. + +9. Describe a lever. For what general purpose are levers used in the body? +What other purpose do they serve outside of the body? + +10. Why are levers of the second class not adapted to the work of the +body? + +11. Name the class of lever used in bending the elbow; in straightening +the elbow; in raising the knee; in elevating the toes; and in biting. Why +is one able to bite harder with the back teeth than with the front ones +when the same muscles are used in both cases? + +12. Measure the distance from the middle of the palm of the hand to the +center of the elbow joint. Find the attachment of the tendon of the biceps +muscle to the radius and measure its distance to the center of the elbow +joint. From these distances calculate the force with which the biceps +contracts in order to support a weight of ten pounds on the palm of the +hand. + +13. How does exercise benefit the health? How does a short walk "clear the +brain" and enable one to study to better advantage? + +14. When exercisers taken for its effects upon the health, what conditions +should be observed? + + + +PRACTICAL WORK + + +The reddish muscle found in a piece of beef is a good example of striated +muscle. The clear ring surrounding the intestine of a cat (shown by cross +section) and the outer portion of the preparation from the cow's stomach, +sold at the butcher shop under the name of _tripe_, are good examples of +non-striated muscular tissue. The heart of any animal, of course, shows +the heart muscle. + +*To show the Structure of Striated Muscle.*--Boil a tough piece of beef, as +a cut from the neck, until the connective tissue has thoroughly softened. +Then with some pointed instrument, separate the main piece into its fiber +bundles and these in turn into their smallest divisions. The smallest +divisions obtainable are the muscle cells or fibers. + +*To show Striated Fibers.*--Place a small muscle from the leg of a frog in +a fifty-per-cent solution of alcohol and leave it there for half a day or +longer. Then cover with water on a glass slide, and with a couple of fine +needles tease out the small muscle threads. Protect with a cover glass and +examine with a microscope, first with a low and then with a high power. +The striations, sarcolemma, and sometimes the nuclei and nerve plates, may +be distinguished in such a preparation. + +*To show Non-striated Cells.*--Place a clean section of the small intestine +of a cat in a mixture of one part of nitric acid and four parts of water +and leave for four or five hours. Thoroughly wash out the acid with water +and separate the muscular layer from the mucous membrane. Cover a small +portion of the muscle with water on a glass slide and tease out, with +needles, until it is as finely divided as possible. Examine with a +microscope, first with a low and then with a high power. The cells appear +as very fine, spindle-shaped bodies. + +*To illustrate Muscular Stimulus and Contraction.*--Separate the muscles at +the back of the thigh of a frog which has just been killed and draw the +large sciatic nerve to the surface. Cut this as high up as possible and, +with a sharp knife and a small pair of scissors, dissect it out to the +knee. Now cut out entirely the large muscle of the calf of the leg (the +gastrocnemius), but leave attached to it the nerve, the lower tendon, and +the bones of the knee. Mount on an upright support, as shown in Fig. 120, +and fasten the tendon to a lever below by a thread or small wire hook: + + [Fig. 120] + + + Fig. 120--*Apparatus* for demonstrating properties of muscles. + + +1. Lay the nerve over the ends of the wires from a small battery which are +attached to the support at _A_, and arrange a second break in the circuit +at _B_. At this place the battery circuit is made and broken either by a +telegraph key or by simply touching and separating the wires. Note that +the muscle gives a single contraction, or twitch, both when the current is +made and when it is broken. + +2. Remove the current and pinch the end of the nerve, noting the result. +With very fine wires, connect the battery directly to the ends of the +muscle. Stimulate by making and breaking the current as before. In this +experiment the muscle cells are stimulated by the direct action of the +current and not by the current acting on the nerve. + +3. With the wires attached to either the muscle or the nerve, make and +break the current in rapid succession. This causes the muscle to enter +into a second contraction before it has relaxed from the first, and if the +shocks follow in rapid succession, to continue in the contracted state. +This condition, which represents the method of contraction of the muscles +in the body, is called _tetanus_. + +NOTE.--In these experiments a twitching of the muscle is frequently +observed when no stimulus is being applied. This is due to the drying out +of the nerve and is prevented by keeping it wet with a physiological salt +solution. (See footnote, page 38.) + +*To show the Action of Levers.*--With a light but stiff wooden bar, a +spring balance, and a wedge-shaped fulcrum, show: + +1. The position of the weight, the fulcrum, and the power in the different +classes of levers, and also the weight-arm and the power-arm in each case. + +2. The direction moved by the power and the weight respectively in the use +of the different classes of levers. + +3. That when the power-arm and weight-arm are equal, the power equals the +weight and moves through the same distance. + +4. That when the power-arm is longer than the weight-arm, the weight is +greater, but moves through a shorter distance than the power. + +5. That when the weight-arm is longer than the power-arm, the power is +greater and moves through a shorter distance than the weight. + +*To show the Loss of Power in the Use of the Body Levers.*--Construct a +frame similar to, but larger than, that shown in Fig. 120, (about 12 +inches high), and hang a small spring balance (250 grams capacity) at the +place where the muscle is attached. Fasten the end of a lever to the +upright piece, at a point on a level with the end of the balance hook. +(The nail or screw used for this purpose must pass loosely through the +lever, and serve as a pivot upon which it can turn.) The lever should +consist of a light piece of wood, and should have a length at least three +times as great as the distance from the hook to the turning point. Connect +the balance hook with the lever by a thread or string, and then hang upon +it a small body of known weight. Note the amount of force exerted at the +balance in order to support the weight at different places on the lever. +At what point is the force just equal to the weight? Where is it twice as +great? Where three times? Show that the force required to support the +weight increases proportionally as the weight-arm and as the distance +through which the weight may be moved by the lever. Apply to the action of +the biceps muscle in lifting weights on the forearm. + +*A Study of the Action of the Biceps Muscle.*--Place the fingers upon the +tendon of the biceps where it connects with the radius of the forearm. +With the forearm resting upon the table, note that the tendon is somewhat +loose and flaccid, but that with the slightest effort to raise the forearm +it quickly tightens. Now transfer the fingers to the body of the muscle, +and sweep the forearm through two or three complete movements, noting the +changes in the length and thickness of the muscle. Lay the forearm again +on the table, back of hand down, and place a heavy weight (a flatiron or a +hammer) upon the hand. Note the effort required to raise the weight, and +then shift it along the arm. Observe that the nearer it approaches the +elbow the lighter it seems. Account for the difference in the effort +required to raise the weight at different places. Does the effort vary as +the distance from the tendon? + + + + +CHAPTER XVI - THE SKIN + + +Protective coverings are found at all the exposed surfaces of the body. +These vary considerably at different places, each being adapted to the +conditions under which it serves. The most important ones are the _skin_, +which covers the entire external surface of the body; the _mucous +membrane_, which lines all the cavities that communicate by openings with +the external surface; and the _serous membrane_, which, including the +synovial membranes, lines all the closed cavities of the body. In addition +to the protection which it affords, the skin is one of the means by which +the body is brought into proper relations with its surroundings. It is +because of this function that we take up the study of the skin at this +time. + +*The Skin* is one of the most complex structures of the body, and serves +several distinct purposes. It is estimated to have an area of from 14 to +16 square feet, and to have a thickness which varies from less than one +eighth to more than one fourth of an inch. It is thickest on the palms of +the hands and the soles of the feet, the places where it is most subject +to wear. It is made up of two distinct layers--an outer layer called the +_epidermis_, or cuticle, and an inner layer called the _dermis_, or cutis +vera (Fig. 121). + +*The Dermis.*--This is the thicker and heavier of the two layers, and is +made up chiefly of connective tissue. The network of tough fibers which +this tissue supplies, forms the essential body of the dermis and gives to +it its power of resistance. It is on account of the connective tissue that +the skins of animals can be converted into leather by tanning. A variety +of structures, including blood and lymph vessels, oil and perspiratory +glands, hair follicles, and nerves, are found embedded in the connective +tissue (Fig. 122). These aid in different ways in the work of the skin. + + [Fig. 121] + + + Fig. 121--*Section of skin* magnified, _a, b._ Epidermis, _b._ Pigment + layer. _c._ Papillae, _d._ Dermis. _e._ Fatty tissue. _f, g, h._ Sweat + gland and duct. _i, k._ Hair and follicle. _l._ Oil gland. + + +On the outer surface of the dermis are numerous elevations, called +_papillae_. These average about one one-hundredth of an inch in height, and +one two hundred and fiftieth of an inch in diameter. They are most +numerous on the palms of the hands, the soles of the feet, and the under +surfaces of the fingers and toes. At these places they are larger than in +other parts of the body, and are closely grouped, forming the parallel +curved ridges which cover the surfaces. Each papilla contains a loop of +capillaries and a small nerve, and many of them are crowned with touch +corpuscles (page 342). + + [Fig. 122] + + + Fig. 122--*Diagram* of section of skin showing its different structures. + + +*The Epidermis* is much thinner than the dermis. It is made up of several +layers of cells which are flat and scale-like at the surface, but are +rounded in form where the epidermis joins the dermis. The epidermis has +the appearance of being _moulded onto_ the dermis, filling up the +depressions between the papillae and having corresponding irregularities +(Fig. 121). No blood vessels are found in the epidermis, its nourishment +being derived from the lymph which reaches it from the dermis. Only the +part next to the dermis is made up of _living_ cells. These are active, +however, in the formation of new cells, which take the place of those that +are worn off at the surface. Some of the cells belonging to the inner +layer of epidermis contain _pigment granules_, which give the skin its +color (Fig. 121). The epidermis contains no nerves and is therefore +non-sensitive. The hair and the nails are important modifications of the +epidermis. + +*A Hair* is a slender cylinder, formed by the union of epidermal cells, +which grows from a kind of pit in the dermis, called the _hair follicle_. +The oval and somewhat enlarged part of the hair within the follicle is +called the _root_, or _bulb_, and the uniform cylinder beyond the follicle +is called the _shaft_. Connected with the sides of the follicles are the +_oil_, or _sebaceous, glands_ (Figs. 121 and 122). These secrete an oily +liquid which keeps the hair and cuticle soft and pliable. Attached to the +inner ends of the follicles are small, involuntary muscles whose +contractions cause the roughened condition of the skin that occurs on +exposure to cold. + +*A Nail* is a tough and rather horny plate of epidermal tissue which grows +from a depression in the dermis, called the _matrix_. The back part of the +nail is known as the _root_, the middle convex portion as the _body_, and +the front margin as _the free edge_ (Fig. 123). Material for the growth of +the nail is derived from the matrix, which is lined with active epidermal +cells and is richly supplied with blood vessels. Cells added to the root +cause the nail to grow in length (forward) and cells added to the under +surface cause it to grow in thickness. The cuticle adheres to the nail +around its entire circumference so that the covering over the dermis is +complete. + + [Fig. 123] + + + Fig. 123--*Section of end of finger* showing nail in position. + + +*Functions of the Skin.*--The chief function of the skin is that of +protection. It is able to protect the body on account of the tough +connective tissue in the dermis, the non-sensitive cells of the epidermis, +and also by the touch corpuscles and their connecting nerve fibers. This +protection is of at least three kinds, as follows: + +1. _From mechanical injuries_ such as might result from contact with hard, +rough, or sharp objects. The main quality needed for resisting mechanical +injuries is _toughness_, and this is supplied both by the epidermis and by +the connective tissue of the dermis. + +2. _From chemical injuries_ caused by contact with various chemical +agents, as acids, alkalies, and the oxygen of the air. The epidermis, +being of such a nature as to resist to a considerable extent the action of +chemical agents, affords protection from these substances. (89) + +3. _From disease germs_ which are everywhere present. The epidermis is the +main protective agent against attacks of germs, but should the epidermis +be broken, they meet with further resistance from the fluids of the dermis +and the white corpuscles of the blood. + +4. _From an excessive evaporation of liquid from the surface of the body_. +In the performance of this function, the skin is an important means of +keeping the tissues soft and the blood and lymph from becoming too +concentrated. + +*Other Functions of the Skin.*--Through the perspiratory glands the skin is +an _organ of excretion_. While the secretion from a single gland is small, +the waste that leaves the body through all of the perspiratory glands is +considerable (90) (page 206). By means of the nerves terminating in the +touch corpuscles, the skin serves as the _organ of touch_, or feeling +(Chapter XX). To a slight extent also the skin may absorb liquid +substances, these being taken up by the blood and lymph vessels, and +perform a respiratory function, throwing off carbon dioxide. But the most +important function of the skin, in addition to protection, is that of +serving as + +*An Organ of Adaptation.*--Forming, as it does, the boundary between the +body and its physical environment, the skin is perhaps the most important +agent through which the body is adapted to its immediate surroundings. +Evidence of this is found in the great variety of influences which are +able to affect the body through their action upon the nerves in the skin, +and in the changes which the epidermis undergoes on exposure. The latter +function is especially marked in the lower animals, the coverings of +epidermal tissue (hair, scales, feathers, etc.) adapting each species to +the physical conditions under which it lives. In man the most striking +example of adaptation through the skin is seen in the variations in the +quantity of blood circulating through it, corresponding to the changes in +the temperature outside of the body. These variations are of great +importance, having to do with the + +*Maintenance of the Normal Temperature.*--It is necessary to the +continuance of life that the temperature of the body be kept at a nearly +uniform degree, called the _normal temperature_, which is about 98.6 deg. F. +The maintenance of the normal temperature depends mainly upon four +conditions: the chemical changes at the cells, the circulation of the +blood, the nervous system, and _the skin_. The chemical changes produce +the heat, the blood in its circulation distributes the heat over the body, +and the nervous system controls the heat-producing and distributing +processes (page 320). The skin is the chief means by which the body gets +rid of an excess of heat and, by so doing, avoids overheating. (91) + +*How the Skin cools the Body.*--The skin is a means of ridding the body of +an excess of heat in at least two ways: + +1. _By the conduction and radiation of heat from its surface_ as from a +stove. This goes on all the time, but varies with the amount of heat +brought to the surface by the blood. + +2. _By the evaporation of the perspiration._ It is a well-established and +easily demonstrated principle that liquids in evaporating use up heat.(See +Practical Work.) It is also a matter of everyday experience that the +perspiration has a cooling effect upon the body and that its flow +increases with the amount of heat to be gotten rid of. The quantity of +perspiration secreted, and of heat disposed of through its evaporation, +also varies with the amount of blood circulating through the skin. + +*Temperature Regulation by the Skin.*--Variations in the quantity of blood +circulating through the skin enable this organ to throw off just the right +amount of heat for keeping the body at the normal temperature. If it is +necessary for the body to rid itself of an excess of heat, the quantity of +blood circulating in the skin is increased. This brings the blood near the +surface, where more heat can be radiated and where it may cause an +increase in the perspiration. On the other hand, if the body is in danger +of losing too much heat, the circulation diminishes in the skin and +increases in the internal organs. This stops the rapid loss of heat from +the surface. The skin in this work is of course made to cooperate with +other parts of the body. That it is not the only organ concerned in +regulating the escape of heat is seen in the results that follow +sensations either of chilliness or of heat at the surface. + +*Effects of Heat and Cold Sensations.*--Sensations, or feelings, of heat +and cold are made possible through the nerves which connect the brain with +the _temperature corpuscles_, found in the skin (page 343). As the warm +blood recedes from the skin, a sensation of cold is felt, but when the +blood returns, there is again the feeling of warmth. The sensation of cold +prompts one to seek a warmer place, or to put on more clothing; while the +sensation of heat, if it be oppressive, leads to activities of an opposite +kind. Prompted in this way by the sensations from the skin, one +voluntarily supplies the external conditions, such as clothing and heat, +that affect the body temperature. + +*Alcohol and the Regulation of Temperature.*--Alcohol, through its effect +upon the nervous system, interferes seriously with the regulation of the +body temperature. By dilating the capillaries, it increases the +circulation in the skin and leads to an undue loss of heat. At the same +time the excess of blood in the skin causes a _feeling of warmth_ which +has led to the erroneous belief that alcohol is a heat producer. If taken +on a cold day, it deceives one about his true condition and leads to a +wasting of heat when it should be carefully economized. Not only is +alcohol of no value in maintaining the body temperature, but if taken +during severe exposure to cold, it becomes a menace to life itself. +Arctic, explorers and others exposed to severe cold have found that they +withstand cold far better when no alcohol at all is used.(92) + + + +HYGIENE OF THE SKIN + + +Much of the hygiene of the skin is included in the problems of keeping it +warm and clean. It is kept warm by clothing; bathing is the method of +keeping it clean. + +*Clothing* should be warm and loose-fitting. Woolen fabrics are to be +preferred in winter to cotton because, being poorer conductors of heat, +they afford better protection from the cold. But wool fails to absorb the +perspiration rapidly from the skin and to pass it to the outside where it +is evaporated. This, together with its tendency to irritate, makes woolen +clothing somewhat objectionable for wearing next to the skin. This +objection, however, is obviated by woolen underwear which is lined by a +thin weaving of cotton. + +*Bathing.*--The solid material from the perspiration, which is left on the +skin, together with the oil from the oil glands and the dirt from the +outside, tends to close up the pores and develop offensive odors. Keeping +the skin clean is, for these reasons, necessary from both a health and a +social standpoint. While one should always keep clean, the frequency of +the bath will depend upon the season, the occupation of the individual, +and the nature and amount of the perspiration. As to the kind of bath to +be taken and the precautions to be observed, no specific rules can be laid +down. These must be determined by the facilities at hand and by the health +and natural vigor of the bather. Severe chilling of the body should be +avoided, especially by those in delicate health. If a hot bath is taken, +one should dash cold water over the body on finishing. One should then +quickly dry and rub the body with a coarse towel. The dash of cold water +closes the pores of the skin and lessens the liability of taking cold. + +*The Tonic Bath.*--The cold bath has been found to have a beneficial effect +upon the general health beyond its effect upon the skin. When taken with +care as to the length of time and the degree of cold, decided tonic +effects are observed on the circulation and on the nervous system. The +rapid changes of temperature vigorously exercise the non-striated muscles +of the blood vessels (page 57) and the nerves controlling them. The +irritability of the nervous system in general is also lessened. For this +reason the cold bath is one of the best means of keeping both mind and +body in good condition during the warm months. Sponging off the body with +cold or tepid water before retiring is also an excellent aid in securing +sound sleep during the hot summer nights. + +Danger from the cold bath arises through the shock to the nervous system +and the loss of heat from the body. It is avoided by using water whose +temperature is not too low and by limiting the time spent in the bath. A +brisk rubbing with a coarse towel should always follow the cold bath. +People past middle age are, as a rule, not benefited by the cold bath; and +those in delicate health, especially if inclined toward rheumatism, are +likely to be affected injuriously by it. + +*Care of the Complexion.*--A good complexion is a natural accompaniment of +good health and depends primarily upon two conditions--a clear skin and an +active circulation of the blood through it. Clearness of the skin depends +largely upon the elimination of waste material from the body, and where +the solid wastes are not effectively removed through the natural channels +(the liver, kidneys, and bowels), blotches, sallowness of the skin, and +skin eruptions are likely to result. In seeking to clear the complexion, +attention must be given to all those agencies that favor the elimination +of waste, and especially should there be a free and thorough evacuation of +the bowels each day. The general health should also be looked after, +attention being given to exercise, fresh air, proper food,(93) sufficient +sleep, etc. + +Bathing is the chief means employed for increasing the circulation in the +skin, although exercise which is sufficiently vigorous to cause one to +perspire freely is a valuable aid. A daily bath of warm or hot water, +finished off with a dash of cold, followed by a thorough rubbing of the +entire surface, and this by a kneading of the skin with the thumbs and +fingers, will in most cases bring about the desired results. A little +olive oil, thoroughly worked into the skin during the kneading process, is +beneficial where one lacks flesh or where the skin is dry and thin. The +olive oil is also beneficial where the baths are exhausting or render one +susceptible to cold. In rubbing and kneading, the skin should not be +bruised or irritated. + +The much advertised "complexion beautifiers" which are applied directly to +the face frequently have the effect of clogging the pores and of causing +eruptions of the skin. On the other hand, certain authorities state that +the cold cream preparations may be of advantage in giving the skin a +desired softness, and that when judiciously used (the face being cleansed +after each application) they do no harm. Of the different kinds of face +powder those prepared from rice are considered the least injurious. + +*Treatment of Skin Wounds.*--Skin wounds which may not be serious in +themselves frequently become so through getting infected with germs. Blood +poisoning often results from such infections, one of the worst forms being +_tetanus_, or lockjaw. A wound should be kept clean, and if it shows signs +of infection, it should be washed with some antiseptic solution. Or, it +may be cleansed with pure warm water and then covered with some antiseptic +ointment,(94) of which there are a number on the market. A weak solution +of carbolic acid (one part acid to twenty-five parts of water) makes an +excellent antiseptic wash. It may be used not only for cleansing wounds, +but also in counteracting the poisonous effects that follow the bites of +insects. + +A wound resulting from the bite of an animal (cat or dog), even though +slight, should receive more serious attention, and as soon as possible +after the occurrence. Such wounds should be cauterized, and for this +purpose pure carbolic, acid (undiluted with water) may be used. A wooden +toothpick is dipped into the acid and this is worked about in the wound. +The acid is then washed out with warm water. A deep wound from a rusty +nail or a thorn should be treated in the same manner and should be kept +open, not being allowed to heal at the surface first. If one has reason to +believe he has been bitten by a mad dog, the wound should be cauterized as +above, and a physician should be summoned at once. Deep wounds from +explosives, or other causes, should also receive the attention of the +physician. Many cases of lockjaw result every year from wounds inflicted +by the toy pistols, firecrackers, etc., used in our Fourth of July +celebrations. These are due to the embedding in the skin or flesh of small +solid particles on which are lockjaw germs. Wounds of this nature should, +of course, receive the attention of the physician. + +*Care of the Nails.*--Relief from a blood blister under the nail is secured +by boring a small hole through the nail with the sharp point of a +sterilized penknife (page 38). This simple bit of surgery not only +relieves the pain, but is frequently the only means of saving the nail. +Ingrown toe nails are relieved by scraping a broad strip in the middle of +the nail until very thin. This relieves the pressure, preventing the sides +of the nail from being forced into the toe. While the finger nails should +be trimmed in a curve, corresponding to the end of the finger, it is +recommended that the toe nails be cut straight across (Fig. 124), as this +method diminishes the pressure from the shoe and keeps the nails from +ingrowing. Shoes that pinch the toes should, of course, not be worn (page +238). + + [Fig. 124] + + + Fig. 124--Proper method of trimming nails of toes. + + +*Care of the Hair.*--Occasional washing of the hair is beneficial, but too +much wetting causes decay of the hair roots, which leads to its falling +out. The worst enemy of the hair is dandruff. A method of removing +dandruff which is highly recommended is that of rubbing olive oil into the +scalp and later of removing this with a cleansing shampoo. The olive oil +is placed on the scalp with a medicine dropper and thoroughly rubbed in +with the fingers. After three or four hours the hair is washed with soap +and water (any good toilet soap will do) and rinsed with pure water. The +hair is then dried, the surplus water being removed with a coarse towel. +Where the dandruff is very troublesome, this treatment may be given once +or twice a week; but in mild cases once a month is sufficient. Massage of +the scalp, by increasing the circulation at the hair roots, is beneficial, +but irritation by a fine-tooth comb, a stiff hair brush, or by other means +should be avoided. Frequent brushing and combing, however, are necessary +both for the good appearance of the hair and for spreading the oil +secreted by the glands at the hair roots. + +*Summary.*--The skin forms the external covering of the body and also +serves additional purposes. It is a most important agency in adapting the +body to its physical surroundings, as shown by the part which it plays in +the regulation of the body temperature. The skin should be kept clean and +active, and skin wounds, even though small, should be guarded against +infection. + +*Exercises.*--1. Name an example of each of the protective coverings of the +body. + +2. Compare the dermis and the epidermis with reference to thickness, +composition, and function. + +3. To what is the color of the skin due? How is the color of the skin +affected by the sunlight? + +4. What modifications of the epidermis are found on our bodies? What are +found on the body of a chicken? + +5. What different kinds of protection are provided by the skin? + +6. How does the perspiration cool the body? + +7. What change occurs in the circulation in the skin when the body is +becoming too cold? When becoming too warm? What is the purpose of these +changes? + +8. How does alcohol cause one to _feel_ warm when he may be losing too +much of his heat? + +9. What precaution should be observed by one in poor health, in taking a +bath? + +10. How may the cold bath be a means of improving the general health? + + + +PRACTICAL WORK + + +*Observations on the Skin and its Appendages.*--Examine the palm of the +hand with a lens. Note the small ridges which correspond to the rows of +papillae beneath the cuticle. In these find small pits, which are the +openings of the sweat glands. + +2. Examine the epidermis on the back of the hand and palm. At which place +is it thickest and most resisting? Is it of uniform thickness over the +palm? Try picking it with a pin at the thickest place, noting if pain is +felt. Inference? + +3. Examine a finger nail. Is the free edge or the root the thickest? Trim +closely the thumb nail and the nail of the middle finger of one hand and +try to pick up a pin, or other minute object, from a smooth, hard surface. +The result indicates what use of the nails? Suggest other uses. + +4. Examine with a microscope under a low power hairs from a variety of +animals, as the horse, dog, cat, etc., noting peculiarities of form and +surface. + +*To illustrate Cooling Effects of Evaporation.*--1. Wet the back of the +hand and move it through the air to hasten evaporation. Observe that, as +the hand dries, a sensation of cold is felt. Repeat the experiment, using +ether, alcohol, or gasolene instead of the water, noting the differences +in results. These liquids evaporate faster than water. + +2. Wet the bulb of a thermometer with alcohol or water. Move it through +the air to hasten evaporation. Note and account for the fall of the +mercury. + + + + +CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM + + +*Cooerdination and Adjustment.*--If we consider for a moment the movements +of the body, we cannot fail to note the cooeperation of organs, one with +another. In the simple act of whittling a stick one hand holds the stick +and the other the knife, while the movements of each hand are such as to +aid in the whittling process. Examples of cooeperation are also found in +the taking of food, in walking, and in the performance of different kinds +of work. Not only is cooeperation found among the external organs, but our +study of the vital processes has shown that the principle of cooeperation +is carried out by the internal organs as well. The fact that all the +activities of the body are directed toward a common purpose makes the +cooeperation of its parts a necessity. The term "cooerdination" is employed +to express this cooeperation, or working together, of the different parts +of the body. + +A further study of the movements of the body shows that many of them have +particular reference to things outside of it. In going about one naturally +avoids obstructions, and if anything is in the way he walks around or +steps over it. Somewhat as a delicate instrument (the microscope for +example) is altered or adjusted, in order to adapt it to its work, the +parts of the body, and the body as a whole, have to be _adjusted_ to their +surroundings. This is seen in the attitude assumed in sitting and in +standing, in the position of the hands for different kinds of work, in the +variations of the circulation of the blood in the skin, and in the +movements for protecting the body.(95) + +*Work of the Nervous System.*--How are the different activities of the body +controlled and cooerdinated? How is the body adjusted to its surroundings? +The answer is found in the study of the nervous system. Briefly speaking, +the nervous system controls, cooerdinates, and adjusts the different parts +of the body by fulfilling two conditions: + +1. It provides a complete system of connections throughout the body, +thereby bringing all parts into communication. + +2. It supplies a means of controlling action (the so-called impulse) which +it passes along the nervous connections from one part of the body to +another. + +The present chapter deals with the first of these conditions; the chapter +following, with the second. + +*The Nerve Skeleton.*--If all the other tissues are removed, leaving only +the nervous tissue, a complete skeleton outline of the body still remains. +This nerve skeleton, as it has been called, has the general form of the +framework of bones, but differs from it greatly in the fineness of its +structures and the extent to which it represents every portion of the +body. An examination of a nerve skeleton, or a diagram of one (Fig. 125), +shows the main structures of the nervous system and their connection with +the different parts of the body. + +Corresponding to the skull and the spinal column is a central nervous +axis, made up of two parts, the _brain_ and the _spinal cord_. From this +central axis white, cord-like bodies emerge and pass to different parts of +the body. These are called _nerve trunks_, and the smaller branches into +which they divide are called _nerves_. The nerves also undergo division +until they terminate as fine thread-like structures in all parts of the +body. The distribution of nerve terminations, however, is not uniform, as +might be supposed, but the skin and important organs like the heart, +stomach, and muscles are the more abundantly supplied. On many of the +nerves are small rounded masses, called _ganglia_, and from many of these +small nerves also emerge. At certain places the nerves and ganglia are so +numerous as to form a kind of network, known as a _plexus_. + + [Fig. 125] + + + Fig. 125--*Diagram of nerve skeleton.* The illustration shows the extent + and general arrangement of the nervous tissue. _A._ Brain. _B._ Spinal + cord. _N._ Nerve trunks and nerves. _G._ Ganglia. + + +It is through these structures--brain and spinal cord, nerve trunks and +nerves, ganglia and nerve terminations--that connections are established +between all parts of the body, but more especially between the surface of +the body and the organs within. + +*The Neurons, or Nerve Cells.*--While a hasty examination of the nerve +skeleton is sufficient to show the connection of the nervous system with +all parts of the body, no amount of study of its gross structures reveals +the nature of its connections or suggests its method of operation. Insight +into the real nature of the nervous system is obtained only through a +study of its minute structural elements. These, instead of being called +cells, as in the case of the other tissues, are called _neurons_. The use +of this term, instead of the simpler one of nerve cell, is the result of +recent advances in our knowledge of the nervous system.(96) + + [Fig. 126] + + + Fig. 126--*Diagram of a mon-axonic neuron* (greatly enlarged except as to + length). The central thread in the axon is the axis cylinder. + + +The neurons are in all respects cells. They differ widely, however, from +all the other cells of the body and are, in some respects, the most +remarkable of all cells. They are characterized by minute extensions, or +prolongations, which in some instances extend to great distances. Though +the neurons in certain parts of the body differ greatly in form and size +from those in other parts of the body, most of them may be included in one +or the other of two classes, known as _mon-axonic_ neurons and _di-axonic_ +neurons. + +*Mon-axonic Neurons.*--Neurons of this class consist of three distinct +parts, known as the cell-body, the dendrites, and the axon (Fig. 126). + +The _cell-body_ has in itself the form of a complete cell and was at one +time so described. It consists of a rounded mass of protoplasm, containing +a well-defined nucleus. The protoplasm is similar to that of other cells, +but is characterized by the presence of many small granules and has a +slightly grayish color. + +The _dendrites_ are short extensions from the cell-body. They branch +somewhat as the roots of a tree and form in many instances a complex +network of tiny rootlets. Their protoplasm, like that of the cell-body, is +more or less granular. The dendrites increase greatly the surface of the +cell-body, to which they are related in function. + +The _axon_, or nerve fiber, is a long, slender extension from the +cell-body, which connects with some organ or tissue. It was at one time +described as a distinct nervous element, but later study has shown it to +be an outgrowth from the cell-body. The mon-axonic neurons are so called +from their having but a single axon. + +*Di-axonic Neurons.*--Neurons belonging to this class have each a +well-defined cell-body and two axons, but no parts just like the dendrites +of mon-axonic neurons. The cell-body is smooth and rounded, and its axons +extend from it in opposite directions (Fig. 127). + + [Fig. 127] + + + Fig. 127--*Diagram of a di-axonic neuron.* The diagram shows only the + conducting portion of the axon, or axis cylinder. + + +*Structure of the Axon.*--The axon, or nerve fiber, has practically the +same structure in both classes of neurons, being composed in most cases of +three distinct parts. In the center, and running the entire length of the +axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). The +axis cylinder is present in all axons and is the part essential to their +work. It may be considered as an extension of the protoplasm from the +cell-body. Surrounding the axis cylinder is a thick, whitish-looking +layer, known as the _medullary sheath_, and around this is a thin +covering, called the _primitive sheath_, or neurilemma. The medullary +sheath and the primitive sheath are not, strictly speaking, parts of the +nerve cell, but appear to be growths that have formed around it. Certain +of the axons have no primitive sheath and others are without a medullary +sheath.(97) + +*Form and Length of Axons.*--Where the axons terminate they usually +separate into a number of small divisions, thereby increasing the number +of their connections. Certain axons are also observed to give off branches +before the place of termination is reached (Fig. 131). These collateral +branches, by distributing themselves in a manner similar to the main +fiber, greatly extend the influence of a single neuron. + +In the matter of length, great variation is found among the axons in +different parts of the body. In certain parts of the brain, for example, +are fibers not more than one one-hundredth of an inch in length, while the +axons that pass all the way from the spinal cord to the toes have a length +of more than three feet. Between these extremes practically all variations +in length are found. + +*Arrangements of the Neurons.*--Nowhere in the body do the neurons exist +singly, but they are everywhere connected with each other to form the +different structures observed in the nerve skeleton. Two general plans of +connection are to be observed, known as the anatomical and the +physiological, or, more simply speaking, as the "side-by-side" and +"end-to-end" plans. The side-by-side plan is seen in that disposition of +the neurons which enables them to form the nerves and the ganglia, as well +as the brain and spinal cord. The end-to-end connections are necessary to +the work which the neurons do. + +*Side-by-side Connections.*--On separating the ganglia and nerves into +their finest divisions, it is found that the nerves consist of axons, +while the ganglia are made up mainly of cell-bodies and dendrites. The +axons lie side by side in the nerve, being surrounded by the same +protective coverings, while the cell-bodies form a rounded mass or +cluster, which is the ganglion (Fig. 128). But the axons, in order to +connect with the cell-bodies, must terminate within the ganglion, so that +they too form a part of it. To some extent, also, axons pass through +ganglia with which they make no connection. The neurons in the brain and +spinal cord also lie side by side, but their arrangement is more complex +than that in the nerves and ganglia. + + [Fig. 128] + + + Fig. 128--*Diagrams illustrating arrangement of neurons.* _A, B._ Ganglia + and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of + _A_ are end-to-end connections of different neurons; in the ganglion of + _B_ are the cell-bodies of di-axonic neurons. _C._ Section of a nerve + trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles + of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small + artery and vein. + + +The side-by-side arrangement of the neurons shows clearly the structure of +the ganglia and nerves. The nerve is seen to be a bundle of axons, or +nerve fibers, held together by connective tissue, while the ganglion is +little more than a cluster of cell-bodies. Their connection is necessarily +very close, for the same group of neurons will form, with their axons, the +nerve, and, with their cell-bodies, the ganglion (Fig. 128). + +*End-to-end Connections.*--These consist of loose end-to-end unions of the +fiber branches of certain neurons with the dendrites of other neurons. The +purpose of such connections is to provide the means of communication +between different parts of the body. There appears to be no actual uniting +of the fiber branches with the dendrites, but they come into relations +sufficiently close to establish _conduction pathways_, and these extend +throughout the body (Fig. 129). They connect all parts of the body with +the brain and spinal cord, while connections within the brain and cord +bring the parts into communication with each other. + + [Fig. 129] + + +Fig. 129--*Diagram of a nerve path* starting at the skin, extending through +the spinal cord, and passing out to muscles. A division of this path also + reaches the brain. + + +*Nature of the Nervous System.*--The nervous system represents the sum +total of the neurons in the body. In some respects it may be compared to +the modern telephone system. The neurons, like the electric wires, connect +different places with a central station (the brain and spinal cord), and +through the central station connections are established between the +different places in the system. As the separate wires are massed together +to form cables, the neurons are massed to form the gross structures of the +nervous system. The nervous system, however, is so radically different +from anything found outside of the animal body that no comparison can give +an adequate idea of it. We now pass to a study of the gross structures +observed in the nerve skeleton. + +*Divisions of the Nervous System.*--While all of the nervous structures are +very closely blended, forming one complete system for the entire body, +this system presents different divisions which may, for convenience, be +studied separately. As physiologists have become better acquainted with +the human nervous system, different schemes of classification have been +proposed. The following outline, based upon the location of the different +parts, presents perhaps the simplest view of the entire group of nervous +structures: + + [Table] + +*The Central Division.*--This division of the nervous system lies within +the cranial and spinal cavities, and consists of the brain and the spinal +cord. The brain occupying the cranial cavity and the spinal cord in the +spinal cavity connect with each other through the large opening at the +base of the skull to form one continuous structure. The brain and cord are +the most complicated portions of the nervous system, and the ones most +difficult to understand. + + [Fig. 130] + + + Fig. 130--*Diagram of divisions of brain.* + + +*The Brain.*--The brain, which is the largest mass of nervous tissue in the +body, weighs in the average sized man about 50 ounces, and in the average +sized woman about 44 ounces.(98) It may be roughly divided into three +parts, which are named from their positions (in lower animals) the +forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain +consists almost entirely of a single part, known as + +*The Cerebrum.*--The cerebrum comprises about seven eighths of the entire +brain, and occupies all the front, middle, back, and upper portions of the +cranial cavity, spreading over and concealing, to a large extent, the +parts beneath. The surface layer of the cerebrum is called the _cortex_. +This is made up largely of cell-bodies, and has a grayish appearance.(99) +The cortex is greatly increased in area by the presence everywhere of +ridge-like _convolutions_, between which are deep but narrow depressions, +called _fissures_. The interior of the cerebrum consists mainly of nerve +fibers, or axons, which give it a whitish appearance. These fibers connect +with the cell-bodies in the cortex (Fig. 131). + +The cerebrum is a double organ, consisting of two similar divisions, +called the _cerebral hemispheres_. These are separated by a deep groove, +extending from the front to the back of the brain, known as the _median +fissure_. The hemispheres, however, are closely connected by a great band +of underlying nerve fibers, called the _corpus callosum_. + + [Fig. 131] + + +Fig. 131--*Microscope drawing* of a neuron from cerebral cortex. _a._ Short + segment of the axis cylinder with collateral branches. + + +At the base of the cerebrum three large masses of cell-bodies are to be +found. One of these, a double mass, occupies a central position between +the hemispheres, and is called the _optic thalami_. The other two occupy +front central positions at the base of either hemisphere, and are known as +the _corpora striata_, or the striate bodies. + +*The Midbrain* is a short, rounded, and compact body that lies immediately +beneath the cerebrum, and connects it with the hindbrain. On account of +the great size of the cerebrum, the midbrain is entirely concealed from +view when the other parts occupy their normal positions. However, if the +cerebrum is pulled away from the hindbrain, it is brought into view +somewhat as in Fig. 130. + +The midbrain carries upon its back and upper surface four small rounded +masses of cell-bodies, called the _corpora quadrigemina_. The upper two of +these bodies are connected with the eyes; the lower two appear to have +some connection with the organs of hearing. On the front and under +surface, the midbrain separates slightly as if to form two pillars, which +are called the _crura cerebri_, or cerebral peduncles. These contain the +great bundles of nerve fibers that connect the cerebrum with the parts of +the nervous system below. + +*The Hindbrain* lies beneath the back portion of the cerebrum, and +occupies the enlargement at the base of the skull. It forms about one +eighth of the entire brain, and is composed of three parts--the cerebellum, +the pons, and the bulb. + +*The Cerebellum* is a flat and somewhat triangular structure with its +upper surface fitting into the triangular under surface of the back of the +cerebrum. It is divided into three lobes--a central lobe and two lateral +lobes--and weighs about two and one half ounces. In its general form and +appearance, as well as in the arrangement of its cell-bodies and axons, +the cerebellum resembles the cerebrum. It differs from the cerebrum, +however, in being more compact, and in having its surface covered with +narrow, transverse ridges instead of the irregular and broader +convolutions (Fig. 132). + +*The Pons*, or pons Varolii, named from its supposed resemblance to a +bridge, is situated in front of the cerebellum, and is readily recognized +as a circular expansion which extends forward from that body. It consists +largely of bands of nerve fibers, between which are several small masses +of cell-bodies. The fibers connect with different parts of the cerebellum +and with parts above. + + [Fig. 132] + + +Fig. 132--*Human brain* viewed from below. _C._ Cerebrum. _Cb._ Cerebellum. + _M._ Midbrain. _P._ Pons. _B._ Bulb. I-XII. Cranial nerves. + + +*The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of +the spinal cord within the cranial cavity. It is somewhat triangular in +shape, and lies immediately below the cerebellum. It contains important +clusters of cell-bodies, as well as the nerve fibers that pass from the +spinal cord to the brain. + +*The Spinal Cord.*--This division of the central nervous system is about +seventeen inches in length and two thirds of an inch in diameter. It does +not extend the entire length of the spinal cavity, as might be supposed, +but terminates at the lower margin of the first lumbar vertebra.(100) It +connects at the upper end with the bulb, and terminates at the lower +extremity in a number of large nerve roots, which are continuous with the +nerves of the hips and legs (Fig. 133). Two deep fissures, one in front +and the other at the back, extend the entire length of the cord, and +separate it into two similar divisions. These are connected, however, +along their entire length by a central band consisting of both gray and +white matter. + + [Fig. 133] + + + Fig. 133--*Spinal cord*, showing on one side the nerves and ganglia with + which it is closely related in function. _A._ Bulb. _B._ Cervical +enlargement. _C._ Lumbar enlargement. _D._ Termination of cord. _E._ Nerve + roots that occupy the spinal cavity below the cord. _P._ Pons. _D.G._ + Dorsal root ganglia. _S.G._ Sympathetic ganglia. _N._ Nerve trunks to + upper and lower extremities. + + +The arrangement of the neurons of the spinal cord is just the reverse of +that in the cerebrum--the center being occupied by a double column of +cell-bodies, which give it a grayish appearance, while the fibers occupy +the outer portion of the cord, giving it a whitish appearance. + +The spinal cord is not uniform in thickness, but tapers slightly, though +not uniformly, from the upper toward the lower end. At the places where +the nerves from the arms and legs enter the cord two enlargements are to +be found, the upper being called the _cervical_ and the lower the _lumbar +enlargement_. These, on account of the difference in length between the +cord and the spinal cavity, are above--the lower one considerably above--the +places where the limbs which they supply join the trunk (Fig. 133). + +*Arrangement of the Neurons of the Brain and Cord.*--The cell-bodies in the +brain and spinal cord are collected into groups, and their fibers extend +from these groups to places that may be near or remote. Guided by the +white and gray colors of the nervous tissue, and also by the structures +revealed by the microscope, physiologists have made out three general +schemes in the grouping of cell-bodies, as follows: + +1. _That of surface distribution_, the cell-bodies forming a thin but +continuous layer over a given surface. This is the plan in the cerebrum +and cerebellum, and here are found devices for increasing the surface: the +cerebrum having convolutions, the cerebellum transverse ridges. + +2. _That of collections of cell-bodies into rounded masses._ Such masses +are found in the bulb, the pons, the midbrain, and the base of the +cerebrum. + +3. _That of arrangement in a continuous column._ This is the plan in the +spinal cord. It matters not at what place the spinal cord be cut, a +central area of gray matter, resembling in form the capital letter H, is +always found. + +The fibers connecting with the cell-bodies in the brain and spinal cord +are gathered into bundles or tracts, and these pass through different +parts somewhat as follows: + +1. _In the cerebrum_ they extend in three general directions, forming +three classes of fibers. The first connect different localities in the +same hemisphere, and are known as _association_ fibers (_A_, Fig. 134). +The second make connection between the two hemispheres, and form the +corpus callosum. These are known as _commissural_ fibers (_C_, Fig. 134). +The third connect the cerebrum with the parts of the nervous system below, +and are called _projection_ fibers (_P_, Fig. 134). + +2. _In the cerebellum_ both association and commissural fibers are found. +Bands of fibers, passing upward toward the cerebrum and downward toward +the cord, connect this part of the brain with other parts of the nervous +system. + + [Fig. 134] + + +Fig. 134--*Semi-diagrammatic representation of a section through the right +cerebral hemisphere*, showing fiber tracts. _A._ Association fibers. _C._ +Commissural fibers. _P._ Projection fibers. The cell-bodies with which the + fiber bundles connect are in the surface layer or cortex. + + +3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that +connect these parts with the cerebrum(101) and cerebellum above; second, +that pass into and become a part of the nerves of the body; and third, +that connect the opposite sides of these parts together. + +*The Peripheral Division.*--The peripheral division of the nervous system +includes all the nervous structures found outside of the brain and spinal +cord. These consist of the cranial, spinal, and sympathetic nerves, and of +various small ganglia, all of which are closely connected with the central +system. + +*Spinal Nerves and Dorsal-root Ganglia.*--The spinal nerves comprise a +group of thirty-one pairs, which connect the spinal cord with different +parts of the trunk, with the upper, and with the lower extremities. Each +nerve joins the cord by two roots, these being named from their positions +the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The +two roots blend together within the spinal cavity to form a single nerve +trunk, which passes out between the vertebrae. On the dorsal root of each +spinal nerve is a small ganglion which is named, from its position, the +_dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.) + +*Double Nature of Spinal Nerves.*--Charles Bell, in 1811, made the +remarkable discovery that each spinal nerve is double in function. He +found the portion connecting with the cord by the dorsal root to be +concerned in the _production of feeling_ and the portion connecting by the +ventral root to be concerned in the _production of motion_. In keeping +with these functions, the two divisions of the nerve are made up of +different kinds of fibers, as follows: + +1. The dorsal-root divisions, of the fibers of di-axonic neurons, the +cell-bodies of which form the dorsal-root ganglia (Fig. 135). + +2. The ventral-root divisions, of the fibers of mon-axonic neurons, the +cell-bodies of which are in the gray matter of the cord. + +The first convey impulses to the cord and are called _afferent_ +neurons;(102) the second convey impulses from the cord and are known as +_efferent_ neurons. Thus, by forming a part of the nerve pathways between +the skin and the brain, the dorsal divisions of these nerves aid in the +production of feeling; and by completing pathways to the muscles, the +ventral divisions aid in the production of motion (Figs. 129, 135, and +141). + + [Fig. 135] + + + Fig. 135--*Connection of spinal nerves with the cord.* On the right is + shown a nerve pathway from the skin to the muscle. A division of this + pathway reaches the brain. + + +*The Cranial Nerves.*--From the under front surface of the brain, twelve +pairs of nerves emerge and pass to the head, neck, and upper portions of +the trunk. These, the cranial nerves, have names suggestive of their +function or distribution and, in addition, are given numbers which +indicate the order in which they leave the brain (Fig. 136). Unlike the +spinal nerves, the cranial nerves present great variety among themselves, +scarcely any two of them being alike in function or in their connection +with different parts of the body. Several of them have to do with the +special senses, and are for this reason very important. They connect the +brain with the different parts of the head, neck, and trunk, as follows: + +1. The first pair (_olfactory_ nerves; nerves of smell; afferent) connect +with the mucous membrane of the nostrils (Fig. 136). + +2. The second pair (_optic_ nerves; nerves of sight; afferent) connect +with the retina of the eyes. + +3. The third, fourth, and sixth pairs (_motores oculi;_ control muscles of +the eyes; efferent) connect with the internal and external muscles of the +eyeballs (Fig. 136). + + [Fig. 136] + + +Fig. 136--*Diagram suggesting the distribution and functions of the cranial + nerves* (Colton). See also Fig. 132. + + +4. The fifth pair (_trigeminal_ nerves; nerves of feeling to the face, of +taste to the front of the tongue, and of control of muscles of +mastication; afferent and efferent) connect with the skin of the face, the +mucous membrane of the mouth, the teeth, and the muscles of mastication. + +5. The seventh pair (_facial_ nerves; control muscles that give the facial +expressions; efferent) connect with the muscles just beneath the skin of +the face. + +6. The eighth pair (_auditory_ nerves; nerves of hearing; afferent) +connect with the internal ear. + +7. The ninth pair (_glossopharyngeal_ nerves; nerves of taste to back of +tongue and of muscular control of pharynx; afferent and efferent) connect +with the back surface of the tongue and with the muscles of the pharynx. + +8. The tenth pair (_vagus_, or pneumogastric, nerves; nerves of feeling +and of muscular control; afferent and efferent) connect with the heart, +larynx, lungs, and stomach. They have the widest distribution of any of +the cranial nerves. + +9. The eleventh pair (_spinal accessory_ nerves; control muscles of neck; +efferent) connect with the muscles of the neck. + +10. The twelfth pair (_hypoglossal_ nerves; control muscles of the tongue; +efferent) connect with the muscles of the tongue. + +*Sympathetic Ganglia and Nerves.*--The sympathetic ganglia are found in +different parts of the body, and vary in size from those which are half an +inch in diameter to those that are smaller than the heads of pins. The +largest and most important ones are found in two chains which lie in +front, and a little to either side, of the spinal column, and extend from +the neck to the region of the pelvis (Figs. 125 and 133). The number of +ganglia in each of these chains is about twenty-four. They are connected +on either side by the right and left sympathetic nerves which extend +vertically from ganglion to ganglion. In addition to the ganglia forming +these chains, important ones are found in the head (outside of the cranial +cavity) and in the plexuses of the thorax and the abdomen. + +The sympathetic ganglia receive nerves from the central division of the +nervous system, but connect with glands, blood vessels, and the intestinal +walls through fibers from their own cell-bodies. Some of these latter +fibers join the spinal nerves, and some blend with each other to form +small sympathetic nerves. + +*Protection of Brain and Spinal Cord.*--On account of their delicate +structure, the brain and spinal cord require the most complete protection. +In the first place, they are surrounded by the bones of the head and +spinal column; these not only shield them from the direct effects of +physical force, but by their peculiar construction prevent, to a large +degree, the passage of jars and shocks to the parts within. In the second +place, they are surrounded by three separate membranes, as follows: + +1. The _dura_, or dura mater, a thick, dense, and tough membrane which +lines the bony cavities and forms supporting partitions. + +2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous +blood vessels, that covers the surface of the brain and cord. + +3. The _arachnoid_, a membrane of loose texture, that lies between the +dura and the pin. + +Finally, within the spaces of the arachnoid is a lymph-like liquid which +completely envelops the brain and the cord, and which, by serving as a +watery cushion, protects them from jars and shocks. Thus the brain and +cord are directly shielded by bones, by membranes, and by the liquid which +surrounds them. They are also protected from jars resulting from the +movements of the body by the general elasticity of the skeleton. + +*Summary.*--The nervous system establishes connections between all parts of +the body, and provides a stimulus by means of which they are controlled. +It is made up of a special form of cells, called neurons. The neurons form +the different divisions of the nervous system, and also serve as the +active agents in carrying on its work. Through a side-by-side method of +joining they form the nerves, ganglia, spinal cord, and brain; and by a +method of end-to-end joining they connect places remote from each other, +and provide for nervous movements through the body. The nervous system, +may in some respects be compared to a complicated system of telephony, in +which the chains of neurons correspond to the wires, and the brain and +spinal cord to the central station. + +Exercises.--1. Give the meaning of the term "cooerdination." Supply +illustrations. + +2. What two general conditions are supplied in the body by the nervous +system? + +3. Compare the skeleton outline of the nervous system with the bony +skeleton. + +4. Sketch outlines of mon-axonic and di-axonic neurons. + +5. Give two differences between the neurons and the other cells of the +body. + +6. Describe the two general methods of connecting neurons in the body. +What purpose is accomplished by each method? + +7. Name and locate the principal divisions of the nervous system. + +8. Draw an outline of the brain (side view), locating each of its +principal divisions. + +9. If a pencil were placed over the ear, what portions of the brain would +be above it and what below? + +10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons, +and the bulb. + +11. Locate and describe the cortex. State purpose of the convolutions. + +12. State the general differences between the cranial and the spinal +nerves. + +13. Locate and give the number of the dorsal-root ganglia. Locate and give +the approximate number of the sympathetic ganglia. + +14. Show how the two portions of the spinal nerves are formed--the one from +the mon-axonic and the other from the di-axonic neurons. + +15. Enumerate the different agencies through which the brain and spinal +cord are protected. + +16. What cranial nerves contain afferent fibers? What ones contain +efferent fibers? What ones contain both afferent and efferent fibers? + +17. In what respects is the nervous system similar to a system of +telephony? In what respects is it different? + + + +PRACTICAL WORK + + +Examine a model of the brain, identifying the different divisions and +noting the position and relative size of the different parts (Fig. 137). +Observe the convolutions of the cerebrum and compare these with the +parallel ridges of the cerebellum. If the model is dissectible, study the +arrangement of the cell-bodies (gray matter) and the distribution of the +fiber bundles (white matter). Note the connection of the cranial nerves +with the under side. + + [Fig. 137] + + + Fig. 137--Model for demonstrating the brain (dissectible). + + +A prepared nervous system of a frog (such as may be obtained from supply +houses) should also be examined. Observe the appearance and general +distribution of the nerves and their connection with the brain and spinal +cord. If such a preparation is not at hand, some small animal may be +dissected to show the main divisions of the nervous system, as follows: + +*Dissection of the Nervous System* (by the teacher).--For this purpose a +half-grown cat is generally the best available material. This should be +killed with chloroform and secured to a board as in the dissection of the +abdomen (page 169). Open the abdominal cavity and remove the contents, +tying the alimentary canal where it is cut, and washing out any blood +which may escape. Dissect for the nervous system in the following order: + +1. Cut away the front of the chest, exposing the heart and lungs. Find on +each side of the heart a nerve which passes by the side of the pericardium +to the diaphragm. These nerves assist in controlling respiration and are +called the _phrenic_ nerves. Find other nerves going to different parts of +the thorax. + +2. Remove the heart and lungs. Find in the back part of the thoracic +cavity, on each side of the spinal column, a number of small "knots" of +nervous matter joined together by a single nerve. These are sympathetic +ganglia. Where the neck joins the thorax, find two sympathetic ganglia +much larger than the others. + +3. Cut away the skin from the shoulder and upper side of the fore leg. By +separating the muscles and connective tissue where the leg joins the +thorax, find several nerves of considerable size. These connect with each +other, forming a network called the _brachial plexus_. From here nerves +pass to the thorax and to the fore leg. + +4. From the brachial plexus trace out the nerves which pass to different +parts of the fore leg. In doing this separate the muscles with the fingers +and use the knife only where it is necessary to expose the nerves. Note +that some of the branches pass into the muscles, while others connect with +the skin. + +5. Remove the skin from the upper portion of one of the hind legs and +separate the muscles carefully until a large nerve is found. This is one +of the divisions of the _sciatic_ nerve. Carefully trace it to the spinal +cord, cutting away the bone where necessary, and find the connections of +its branches with the cord. Then trace it toward the foot, discovering its +branches to different muscles and to the skin. + +6. Unjoint the neck and remove the head. Examine the spinal cord where +exposed. Cut away the bone sufficiently to show the connection between the +cord and one of the spinal nerves. On the dorsal root of one of the nerves +find a small ganglion. What is it called? + +7. Fasten the head to a small board and remove the scalp. Saw through the +skull bones in several directions. Pry off the small pieces of bones, +exposing the upper surface of the brain. Study its membranes, +convolutions, and divisions. + +8. With a pair of bone forceps, or nippers, break away the skull until the +entire brain can be removed from the cavity. Examine the different +divisions, noting the relative position and size of the parts. + +9. With a sharp knife cut sections through the different parts, showing +the positions of the "gray matter" and of the "white matter." + +NOTE.--If the entire class is to examine one specimen, it is generally +better to have the dissecting done beforehand and the parts separated and +tacked to small boards. This will permit of individual examination. +Sketches of the sciatic nerve, brachial plexus, and of sections through +the brain and spinal cord should be made. + +*Location of Nerves in the Body.*--Several of the nerves of the body lie +sufficiently near the surface to be located by pressure and are easily +recognized as sensitive cords. Slight pressure from the fingers reveals +the presence of nerves in the grooves of the elbow (the crazy bone), +between the muscles on the inner side of the arm near the shoulder, and in +the hollow part of the leg back of the knee. These are all large nerves. +Small nerves may be located in the same manner in the face and neck. + + + + +CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM + + +In the preceding chapter was pointed out the method by which the different +parts of the body are brought into communication by the neurons or nerve +cells. We are now to study the means whereby the neurons are made to +control and cooerdinate the different parts of the body and bring about the +necessary adjustment of the body to its surroundings. This work of the +neurons naturally has some relation to their properties. + +*Properties of Neurons.*--The work of the neurons seems to depend mainly +upon two properties--the property of irritability and the property of +conductivity. _Irritability_ was explained, in the study of the muscles +(page 243), as the ability to respond to a stimulus. It has the same +meaning here. The neurons, however, respond more readily to stimuli than +do the muscles and are therefore more irritable. Moreover, they are +stimulated by all the forces that induce muscular contraction and by many +others besides. They are by far the most irritable portions of the body. + +_Conductivity_ is the property which enables the effect of a stimulus to +be transferred from one part of a neuron to another. On account of this +property, an excitation, or disturbance, in any part of a neuron is +conducted or carried to all the other parts. Thus a disturbance at the +distant ends of the dendrites causes a movement toward the cell-body and, +reaching the cell-body, the disturbance is passed through it into the +axon. This movement through the neuron is called the _nervous impulse_. + +*Purpose of the Impulse. *--Though the nature of the nervous impulse is not +understood, (103) its purpose is quite apparent. It is the means employed +by the nervous system for controlling and cooerdinating the different parts +of the body. The arrangement of the neurons enables impulses to be started +in certain parts of the nervous system, and the property of conductivity +causes them to be passed _as stimuli _to other parts. This enables +excitation at one place to bring about action at another place. + +Acting as stimuli, the impulses seem able to produce two distinct effects: +first, to throw resting organs into action and to increase the activity of +organs already at work; and second, to diminish the rate, or check +entirely, the activity of organs. Impulses producing the first effect are +called _excitant_ impulses; those producing the second effect, +_inhibitory_ impulses. + +*Functions of the Parts of Neurons.*--The _cell-body_ serves as a nutritive +center from which the other parts derive nourishment. Proof of this is +found in the fact that when any part of the neuron is separated from the +cell-body, it dies, while the cell-body and the parts attached to the +cell-body may continue to live. In addition to this the cell-body probably +reenforces the nervous impulse. + +The _dendrites_ serve two purposes: first, they extend the surface of the +cell-body, thereby enabling it to absorb a greater amount of nourishment +from the surrounding lymph; second, they act as _receivers of stimuli_ +from other neurons. The same impulse does not pass from one neuron to +another. An impulse in one neuron, however, is able to excite the neuron +with which it makes an end-to-end connection, so that a series of impulses +is produced along a given nerve path (Fig. 129). + +The special _function of the axon_ is to transmit the impulse. By its +length, structure, and property of conductivity it is especially adapted +to this purpose. The axis cylinder, however, is the only part of the axon +concerned in the transmission. The primitive sheath and the medullary +layer protect the axis cylinder, and, according to some authorities, serve +to insulate it. The medullary sheath may also aid in the nourishment of +the axis cylinder. + +*Nerve Stimuli.*--While the properties of irritability and conductivity +supply a necessary cause for the production and transmission of nervous +impulses, these alone are not sufficient to account for their origin. An +additional cause is necessary--a force not found in the nerve protoplasm, +but one which, by its action on the protoplasm, makes it produce the +impulse. In this respect, the neuron does not differ essentially from the +cell of a muscle. Just as the muscle cell requires a stimulus to make it +contract, so does the neuron require a stimulus to start the impulse. +Hence, in accounting for the activities of the body, it is not sufficient +to say they are caused by nervous impulses. We must also investigate the +_nerve stimuli_--the means through which the nervous impulses are started. +Most of these are found outside of the body and are known as external +stimuli. + +*Action of External Stimuli.*--In the arrangement of the nervous system the +most favorable conditions are provided for the reception of external +stimuli. Not only do vast numbers of neurons terminate at the surface of +the body,(104) but they connect there with delicate structures, called +_sense organs_. The purpose of the sense organs is to _sensitize_ (make +sensitive) the terminations of the neurons. This they do by supplying +special structures through which the stimuli can act to the best advantage +upon the nerve endings. Moreover, there are different kinds of sense +organs, and these cause the neurons to be sensitive to different kinds of +stimuli. Acting through the sense organs adapted for receiving them, +light, sound, heat, cold, and odors all act as stimuli for starting +impulses. Indeed, the arrangement is so complete that the nervous system +is subjected to the action of external stimuli in some form practically +all the time. The work of the sense organs is further considered in +Chapters XX, XXI, and XXII. + +*How External Stimuli act on Internal Organs.*--For stimulating the neurons +not connected with the body surface we are dependent, so far as known, +upon the nervous impulses. An impulse started by the external stimulus +goes only so far as its neuron extends. But it serves as a stimulus for +the neuron with which the first connects and starts an impulse in this +connecting neuron, the point of stimulation being where the fiber +terminations of the first neuron make connection with the dendrites of the +second. This impulse in turn stimulates the next neuron, and so on, +producing a series of impulses along a given nerve path. In this way the +effect of an external stimulus may reach and bring about action in any +part of the body. This is in brief the general plan of inducing action in +the various organs of the body. This plan, however, is varied according to +circumstances, and at least three well-defined forms of action are easily +made out. These are known as _reflex action, voluntary action_, and +_secondary reflex action_. + +*Reflex Action.*--When some sudden or strong stimulus acts upon the nerve +terminations at the surface of the body, an immediate response is +frequently observed in some quick movement. The jerking away of the hand +on accidentally touching a hot stove, the winking of the eyes on sudden +exposure to danger, and the quick movements from slight electrical shocks +are familiar examples. The explanation of reflex action is that external +stimuli start impulses in neurons terminating at the surface of the body +and these, in turn, excite impulses in neurons which pass from the spinal +cord or brain to the muscles (Fig. 138). Since there is an apparent +turning back of the impulses by the cord or brain, the resulting movements +are termed _reflex_.(105) + + [Fig. 138] + + + Fig. 138--*Diagram illustrating reflex action of an external organ.* + + +*Reflex Action and the Mind.*--If one carefully studies the reflex actions +of his own body, he will find that they occur at the time, or even a +little before the time, that he realizes what has happened. If a feather +is brought in contact with the more sensitive parts of the face of a +sleeping person, there is a twitching of the skin and sometimes a movement +of the hand to remove the offending substance. Surgeons operating upon +patients completely under the influence of chloroform, and therefore +completely unconscious, have observed strong reflex actions. These and +other similar cases indicate clearly that reflex action occurs +_independently_ of the mind--that the mind neither causes nor controls it. +If a further proof of this fact were needed, it is supplied by experiments +upon certain of the lower animals,(106) which live for a while after the +removal of the brain. These experiments show that the nervous impulses +that produce reflex action need only pass through the spinal cord and do +not reach the cerebrum, the organ of the mind. + +*The Reflex Action Pathway.*--By study of the impulses that produce any +reflex action, a rather definite pathway may be made out, having the +following divisions: + +1. _From the surface of the body to the central nervous system_ (usually +the spinal cord). This, the _afferent_ division, is made up of di-axonic +neurons, and these have (in the case of the spinal nerves) their +cell-bodies in the dorsal root ganglia (page 295). They are acted upon by +external stimuli, while their impulses in turn act on the neurons in the +spinal cord. + +2. _Through the central system_ (spinal cord or base of brain). This, the +_intermediate_ division, may be composed of mon-axonic neurons, or it may +consist of branches from the afferent neurons. In the case of separate +neurons, these are acted upon by impulses from the afferent neurons, while +their impulses serve in turn as stimuli to other neurons within the cord +(Fig. 129). + +3. _From the central nervous system to the muscles._ This, the _efferent_ +division, is made up of mon-axonic neurons. Most of these have their +cell-bodies in the gray matter of the cord, while their fibers pass into +the spinal nerves by the ventral roots.(107) They may be stimulated by +impulses either from the intermediate neurons, or from branches of the +afferent neurons. Their impulses reach and stimulate the muscles. + +*Reflex Action in Digestion.*--The flowing of the saliva, when food is +present in the mouth, is an example of reflex action. In this case, +however, the organ excited to activity is a gland instead of a muscle. The +food starts the impulses, and these, acting through the bulb, reach and +stimulate the salivary glands. In a similar manner food excites the glands +that empty their fluids into the stomach and intestines, and stimulates +the muscular coats of these organs to do their part in the digestive +process. To a considerable extent, neurons having their cell-bodies in the +sympathetic ganglia are concerned in these actions (Fig. 139). + + [Fig. 139] + + + Fig. 139--Diagram illustrating reflex action in its relation to the food + canal. The nerve path in this case includes sympathetic neurons. + + +*Reflex Action in the Circulation of the Blood.*--On sudden exposure to +cold, the small arteries going to the skin quickly diminish in size, check +the flow of blood to the surface, and prevent too great a loss of heat. In +this case, impulses starting at the surface of the body are transmitted to +the bulb and then through the efferent neurons to the muscles in the walls +of the arteries. In a somewhat similar manner, heat leads to a relaxation +of the arterial walls and an increase in the blood supply to the skin. +Other changes in the blood supply to different parts of the body are also +of the nature of reflex actions. As in the work of digestion, neurons +having their cell-bodies in the sympathetic ganglia aid in the control of +the circulation. + +*Purposes of Reflex Action.*--The examples of reflex action so far +considered illustrate its two main purposes--(1) protection, and (2) a +means of controlling important processes. + +The pupil has but to study carefully the reflex actions of his own body +for a period, say of two or three weeks, in order to be convinced of their +protective value. He will observe that portions of his body have, on +exposure to danger, been moved to places of safety, while in some +instances, like falling, his entire body has been adjusted to new +conditions. He will also find that reflex action is quicker, and for that +reason offers in some cases better protection, than movements directed by +the mind. In digestion and circulation are found the best examples of the +control of important processes through reflex action. + +*Voluntary Action.*--It is observed that reflex action, in the sense that +it has so far been considered, is not the usual mode of action of the +external organs, but is, instead, a kind of emergency action, due to +unusual conditions and excitation by strong stimuli. Voluntary actions, on +the other hand, represent the ordinary, or normal, action of these organs. +They comprise the movements of the body of which we are conscious and +which are _controlled by the mind_. But while they are of a higher order +than reflex actions and are under _intelligent_ direction, they are +brought about in much the same manner. + +*Voluntary Action Pathways* differ in but one essential respect from those +of reflex action. They pass through the cerebrum, the organ of the mind +(Fig. 140). This is necessary in order that the mind may control the +action. From all portions of the body surface, afferent pathways may be +traced to the cerebrum; and from the cerebrum efferent pathways extend to +all the voluntary organs. A complex system of intermediate neurons, found +mostly in the brain, join the afferent with the efferent pathways. The +voluntary pathways are not distinct from, but include, reflex pathways, a +fact which explains why the same external stimulus may excite both reflex +and voluntary action (Fig. 141). + + [Fig. 140] + + + Fig. 140--*Diagram of a voluntary action pathway.* + + +*Choice in Voluntary Action.*--In reflex action a given stimulus, acting in +a certain way; produces each time the same result. This is not the case +with voluntary action, the difference being _due to the mind_. In these +actions the external stimulus first excites the mind, and the resulting +mental processes--perhaps as memory of previous experiences--supply a +variety of facts, any of which may act as stimuli to action. Before the +action takes place, however, some one fact must be singled out from among +the mental processes excited. This fact becomes the _exciting stimulus_ +and leads to action. It follows, therefore, that the action which finally +occurs is not necessarily the result of an immediate external stimulus, +but of a _selected_ stimulus--one which is the result of choice. + + [Fig. 141] + + +Fig. 141--*Diagram of voluntary action pathways* including reflex pathways. + + +Not only does the element of choice enter into the selection of the proper +stimulus, but it also enters into the time, nature, and intensity of the +action. For these reasons it is frequently impossible to trace voluntary +actions back to their actual stimuli. The pupil will recognize the element +of choice in such simple acts as picking up some object from the street, +complying with a request, and purchasing some article from a store. + +*Reflex and Voluntary Action Compared.*--Certain likenesses and +differences, already suggested in these two forms of action, may now be +more fully pointed out. Reflex and voluntary action are alike in that the +primary cause of each is some outside force or condition which has +impressed itself upon the nervous system. They are also alike in the +general direction taken by the impulses in producing the action. The +impulses are, first, from the surface of the body to the central nervous +system; second, through the central system; and third, from the central +nervous system to the active tissues of the body. + +Their chief differences are to be found, first, in the pathways followed +by the impulses, which are through the cerebrum (the organ of the mind) in +voluntary action, but in reflex action are only through the spinal cord or +the lower parts of the brain; and second, in the fact that voluntary +action is under the direction of the mind, while reflex action is not. It +would seem, therefore, that the statement sometimes made that "voluntary +action is reflex action plus the mind" is not far from correct. Mind, +however, is the important factor in this kind of action. + +*Secondary Reflex Action.*--Everyday experience teaches that any voluntary +action becomes easier by repetition. A given act performed a number of +times under conscious direction establishes a condition in the nervous +system that enables it to occur without that direction and very much as +reflex actions occur. Actions of this kind are known as secondary reflex +actions, or as _acquired reflexes_. Walking, writing, and numerous other +movements pertaining to the occupation which one follows are examples of +such reflexes. These activities are at first entirely voluntary, but by +repetition they gradually become reflex, requiring only the stimulus to +start them. + +The advantages to the body of its acquired reflexes are quite apparent. +The mind does not have to attend to the selection and direction of stimuli +and, to that extent, is left free for other work. A good example of this +is found in writing, where the mind apparently gives no heed to the +movements of the hand and is only concerned in what is being written. The +student will easily supply other illustrations of the advantages of +secondary reflex action. + +The development of secondary reflexes probably consists in the +establishment of fixed pathways for impulses through the nervous system. +Through the branching of the nerve fibers many pathways are open to the +impulses. But in repeating the same kind of action the impulses are guided +into particular paths, or channels. In time these paths become so well +established that the impulses flow along them without conscious direction +and it is then simply necessary that some stimulus starts the impulses. By +following the established pathways, these reach the right destination and +produce the desired result. According to this view, secondary reflex +action is but a higher phase of ordinary reflex action--a kind of reflex +action, the conditions of which have been established by the mind through +repetition. (See functions of the cerebellum, page 317.) + +*Habits.*--People are observed to act differently when exposed to the same +conditions, or when acted upon by the same stimuli. This is explained by +saying they have different habits. By _habits_ are meant certain general +modes of action that have been acquired by repetition. Certain acts +repeated again and again have established conditions in the nervous system +which enable definite forms of action to be excited, somewhat after the +manner of reflex action. On account of habits, therefore, the actions of +the individual are more or less _predisposed_. What he will do under +certain conditions may be foretold from his habits. Habits simply +represent, a higher order of secondary reflexes--those more closely +associated with the mental life and character than are the lower forms. + +Habits, in common with other forms of secondary reflex action, serve the +important purpose of _economizing the nervous energy_. However, if +pernicious habits are formed instead of those that are useful, they are +detrimental from both a moral and physical standpoint. Youth is recognized +as the period in which fundamental habits are formed and character is +largely determined. Therefore parents and teachers do wisely when they +insist upon the formation of right habits by the young. + +*Functions of Divisions of the Nervous System.*--The relationship between +the different parts of the nervous system is very close and one part does +not work independently of other parts. At the same time the general work +of the nervous system requires that its different divisions serve +different purposes: + +1. The peripheral divisions of the nervous system are concerned in the +transmission of impulses between the surface of the body and the central +system and between the central system and the active tissues. The nerves +are the carriers of the impulses. The ganglia contain the cell-bodies +which serve as nutritive centers; and, in the case of the sympathetic +ganglia, these cell-bodies are the places where the fiber terminations of +one neuron connect with, and stimulate, other neurons. + +2. The gray matter in the spinal cord, bulb, pons, and midbrain (through +the cell-bodies, fiber terminations, and short neurons which they contain) +completes the reflex action pathways between the surface of the body and +the voluntary muscles, and also between the surface of the body and the +organs of circulation and digestion. + +3. The white matter of the spinal cord, bulb, pons, and midbrain (by means +of the fibers of which they are largely composed) forms connections with, +and passes impulses between, the various parts of the central nervous +system. + +4. The bulb, because of certain special reflex-action pathways completed +through it, is the portion of the central nervous system concerned in the +control of respiration, circulation, and the secretion of liquids. + +*Work of the Sympathetic Ganglia and Nerves.*--The neurons which form these +ganglia aid in controlling the vital processes, especially digestion and +circulation. These neurons are controlled for the most part by fibers from +the bulb and spinal cord, and cannot for this reason be looked upon as +forming an independent system. Their chief purpose seems to be that of +spreading the influence of neurons from the central system over a wider +area than they would otherwise reach. For example, a single neuron passing +out from the spinal cord may, by terminating in a sympathetic ganglion, +stimulate a large number of neurons, each of which will in turn stimulate +the cells of muscles or of glands. Because of this function, the +sympathetic neurons are sometimes called _distributing_ neurons. + +*Functions of the Cerebellum.*--Efforts to discover some _special_ function +of the cerebellum have been in the main unsuccessful. Its removal from +animals, instead of producing definite results, usually interferes in a +mild way with a number of activities. The most noticeable results are a +general weakness of the muscles and an inability on the part of the animal +to balance itself. This and other facts, including the manner of its +connection with other parts of the nervous system, have led to the belief +that the cerebellum is the chief organ for the _reflex_ cooerdination of +muscular movements, especially those having to do with the balancing of +the body. In this connection it is subordinate to and under the control of +the cerebrum. Of the relations which the cerebellum sustains to the +cerebrum and to the different parts of the body, the following view is +quite generally held: + +In the development of secondary reflexes, as already described, conditions +are established in the cerebellum, such that given stimuli may act +_reflexively_ through it and produce definite results in the way of +muscular contraction. After the establishment of these conditions, +afferent impulses from the eyes, ears, skin, and other places, under the +general direction of the cerebrum, may cause such actions as the balancing +of the body, walking, etc., as well as the delicate and varied movements +of the hand. This view of its functions makes of the cerebellum the great +center of secondary reflex action. + +*Functions of the Cerebrum.*--While the work of the cerebrum is closely +related to that of the general nervous system, it, more than any other +part, exercises functions peculiar to itself. The cerebrum is the part of +the nervous system upon which our varied experiences leave their +impressions and through which these impressions are made to influence the +movements of the body. But the power to alter, postpone, or entirely +inhibit, nervous movements is but a part of the general work ascribed to +the cerebrum as _the organ of the mind_. Numerous experiments performed +upon the lower animals, together with observations on man, show the +cerebrum to be the seat of the mental activities, and to make possible, in +some way, the processes of consciousness, memory, volition, imagination, +emotion, thought, and sensation. + +*Localization of Cerebral Functions.*--Many experiments have been performed +with a view to determining whether the entire cerebrum is concerned in +each of its several activities or whether special functions belong to its +different parts. These experiments have been made upon the lower animals +and the results thus obtained compared with observations made upon injured +and imperfectly developed brains in man. The results have led to the +conclusion that certain forms of the work of the cerebrum are _localized_ +and that some of its parts are concerned in processes different from those +of others. + + [Fig. 142] + + + Fig. 142--*Location of cerebral functions.* Diagram of cerebrum, showing + most of the areas whose functions are known. + + +The work of locating the functions of different parts of the cerebrum +forms one of the most interesting chapters in the history of brain +physiology. The portions having to do with sight, voluntary motion, +speech, and hearing have been rather accurately determined, while +considerable evidence as to the location of other functions has been +secured. Much of the cerebral surface, however, is still undetermined +(Fig. 142). + + + +NERVOUS CONTROL OF IMPORTANT PROCESSES + + +*Circulation of the Blood.*--1. _Control of the Heart._--The ability to +contract at regular intervals has been shown to reside in the heart +muscle. Among other proofs is that furnished by cold-blooded animals, like +the frog, whose heart remains active for quite a while after its removal +from the body. These automatic contractions, however, are not sufficient +to meet all the demands made upon the circulation. The needs of the +tissues for the constituents of the blood vary with their activity, and it +is therefore necessary to vary frequently the force and rapidity of the +heart's contractions. Such changes the heart itself is unable to bring +about. + +For the purpose of controlling the rate and force of its contractions, the +heart is connected with the central nervous system by two kinds of fibers: + +_a._ Fibers that convey _excitant_ impulses to the heart to quicken its +movements. + +_b._ Fibers that convey _inhibitory_ impulses to the heart to retard its +movements. + +The cell-bodies of the excitant fibers are found in the sympathetic +ganglia, but fibers from the bulb connect with and control them. The +cell-bodies of the inhibitory fibers are located in the bulb, from where +their fibers pass to the heart as a part of the vagus nerve. + +In addition to the fibers above mentioned, are those that convey impulses +_from_ the heart to the bulb. These connect with neurons that in turn +connect with blood vessels and with them act reflexively, when the heart +is likely to be overstrained, to cause a dilation of the blood vessels. +This lessens the pressure which the heart must exert to empty itself of +blood. These fibers serve, in this way, as a kind of safety valve for the +heart. + +2. _Control of Arteries._--Changes in the rate and force of the heart's +contractions can be made to correspond only to the _general_ needs of the +body. When the blood supply to a particular organ is to be increased or +diminished, this is accomplished through the muscular coat in the +arteries. The connection of the arterial muscle with the sympathetic +ganglia and the method by which they vary the flow of blood to different +organs has already been explained (pages 311 and 49), so that only the +location of the controlling neurons need be noted here. These, like the +controlling neurons of the heart, have their cell-bodies in the bulb. It +thus appears that the entire control of the circulation is effected in a +reflex manner through the nerve centers in the bulb. These centers are +stimulated by conditions that relate to the movement of the blood through +the body. + +*Respiration.*--Efferent fibers connect the different muscles of +respiration with a cluster of cell-bodies in the bulb, called the +_respiratory center_. This center together with the nerves and muscles in +question form an automatic, or self-acting, mechanism similar in some +respects to that of the heart. Through the impulses passing from the +respiratory center to the muscles, a rhythmic action is maintained +sufficient to satisfy the usual needs of the body for oxygen. The demand +of the body for oxygen, however, varies with its activities, and to such +variations the respiratory center alone is unable to respond. The +regulating factor in the respiratory movements has been found to be the +condition of the blood with reference to the presence of oxygen and carbon +dioxide. If the blood contains much carbon dioxide and little oxygen, it +acts as a strong stimulus to the respiratory center, causing it, in turn, +to stimulate the respiratory muscles with greater intensity and frequency. +On the other hand, if the blood contains much oxygen and little carbon +dioxide, it acts only as a mild stimulus. This explains how physical +exercise increases the breathing, since the muscles at work consume more +oxygen than when resting and give more carbon dioxide and other wastes to +the blood. + +The respiratory center is also connected by afferent nerves with the +mucous membrane of the air passages. Irritation of the nerve endings in +this membrane causes impulses to pass to the center, and this leads, by +reflex action, to such modifications of the respiratory acts as sneezing +and coughing. There is also a connection between the cerebrum and the +respiratory center. This is shown by the fact that one can voluntarily +change the rate and force of the respiratory movements, and further by the +fact that emotions affect the breathing. + +*Regulation of the Body Temperature.*--As explained in the study of the +skin (page 270), the nervous system regulates the body temperature by +controlling the circulation of the blood through the skin and the internal +organs. This is accomplished by stimulating in a reflex manner the muscles +in the walls of certain arteries. To prevent the body from getting too +hot, muscles in the arteries going to the skin relax, thereby allowing +more blood to flow to the surface, where the heat can be disposed of +through radiation and through the evaporation of the perspiration. On the +other hand, if the body is in danger of losing too much heat, the muscles +in the walls of arteries going to the skin are made to contract and those +to internal organs relax, so that less blood flows to the skin and more to +the internal organs. In this way the nervous system adjusts the +circulation to suit the conditions of temperature outside of and within +the body and, in so doing, maintains the normal body temperature. + +*Summary.*--The nervous system is able to control, cooerdinate, and adjust +the different organs of the body through its intimate connection with all +parts and through a stimulus (the nervous impulse) which it supplies and +transmits. Nervous impulses, excited by external stimuli, follow definite +paths and cause activity in the different parts of the body. All such +pathways are through the central nervous system. In reflex action the +impulses are mainly through the spinal cord, but to some extent through +the bulb, pons, and midbrain. In voluntary action they pass through the +cerebrum--a condition that leads to important modifications in the results. +The cerebrum, in addition to controlling the voluntary movements, is able +to establish the necessary conditions for secondary reflex actions, such +as walking, writing, etc. Although certain of the divisions of the nervous +system exercise special functions, all parts of it are closely related. + +Exercises.--1. Give the function of each of the parts of a neuron. + +2. State the purpose of the nervous impulse. + +3. Show that the exciting cause of bodily action is outside of the nervous +system and, to a large extent, outside of the body. + +4. Describe the arrangement that enables stimuli outside of the body to +cause action within the body. + +5. Describe a reflex action and show how it is brought about. + +6. Distinguish between afferent, efferent, and intermediate neurons. + +7. Draw diagrams showing the impulse pathways in voluntary and in reflex +action. + +8. What purposes are served by the sympathetic neurons? + +9. Describe the method of control of the circulatory and digestive +processes. How do reflex actions protect the body? + +10. Compare voluntary and reflex action. In what sense are all the +activities of the body reflex? + +11. In what sense is walking voluntary? In what sense is it reflex? + +12. How does secondary reflex action lessen the work of the nervous +system? + +13. State the special functions of the nerves, ganglia, spinal cord, bulb, +cerebellum, and cerebrum. + +14. State the importance of the formation of correct habits. + + [Fig. 143] + + + Fig. 143--Nerve board for demonstrating nerve pathways. + + + +PRACTICAL WORK + + +*To demonstrate Nerve Pathways.*--A smooth board, 2x6 ft., is painted +black, and upon this is drawn in white a life-size outline of the body. +Pieces of cord of different colors and lengths are knotted to represent +mon-axonic and di-axonic neurons. These are then pinned or tacked to the +board in such a manner as to represent the connections in the different +kinds of nerve pathways. Fig. 143 shows such a board with connections for +a reflex action and a voluntary action of the same muscle. + +*Study of the "Knee Jerk" Reflex.*--A boy is seated on a chair with the +legs crossed. With a small pointer he is given a light, quick blow on the +upper margin of the patella at the point of connection of the tendon. The +stroke will usually be followed by a reflex movement of the foot. Does +this take place independently of the mind? (The one upon whom the +experiment is being performed should assume a relaxed condition and make +no effort either to cause or prevent the movement.) Can the movement be +inhibited (prevented)? Repeat the experiment, effort being made to prevent +the movement, but not by contracting opposing muscles. + +Other reflex actions adapted to class study are those of the eyes, such as +the closing of the lids on moving objects near them and the dilating of +the pupils when the eyes are shaded. The involuntary jerking of the head +on bringing the prongs of a vibrating tuning fork in contact with the end +of the nose is also a reflex action which can be studied to advantage. + +*To determine the Reaction Time.*--Have several pupils join hands, facing +outwards, making a complete circle, excepting one gap. Give a signal by +touching the hand of one pupil at the end of the line. Let this pupil +communicate the signal, by pressure of the other hand, to the next pupil +and so on around, having the last pupil raise the free hand at close of +the experiment. Note carefully the time, preferably with a stop watch, +required to complete the experiment and divide this by the number of +pupils, to get the average reaction time. The experiment may be repeated +with boys only and then with girls, comparing their average reaction time. + +*Reflex Action of the Salivary Glands.*--Place a small pinch of salt upon +the tongue and note the flow of saliva into the mouth. Try other +substances, as starch, bits of wood, and sugar. What appears to be the +natural stimulus for these glands? Compare with reflex actions of the +muscles. + + + + +CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM + + +The far-reaching effects and serious nature of disorders of the nervous +system are sufficient reasons for considering carefully those conditions +that make or mar its efficiency. Controlling all the activities of the +body and affecting through its own condition the welfare of all the +organs, the hygiene of the nervous system is, in a large measure, the +hygiene of the entire body. Moreover, it is known that some of our worst +diseases, including paralysis and insanity, are disorders of the nervous +system and are prevented in many instances by a proper mode of living. + +*The Main Problem.*--Many of our nervous disorders are undoubtedly due to +the age in which we live. Our modern civilization, with all its facilities +for human advancement and enjoyment, throws an extra strain upon the +nervous system. Educational and social standards are higher than ever +before and life in all its phases is more complex. Since we can hardly +change the conditions under which we live, and probably would not if we +could, we must learn to adapt or adjust ourselves to them so as to secure +for the nervous system such relief as it requires. This adjustment is +sometimes difficult, even when the actual needs of the nervous system are +known. + +The healthful action of the nervous system requires, on the one hand, +exercise, but on the other hand, a certain condition of quietude, or +_poise_--a state which is directly opposed to that of restlessness. The +conditions of modern life seem able to force upon the nervous system all +the exercise that it needs, and more (whether it be of the right kind or +not), so that the main problem of to-day seems to be that of conserving, +or economizing, the nervous energy and of preventing nervous waste. + +*Wasteful Forms of Nervous Activity.*--There are without doubt many forms +of activity that waste the vital forces of the body and lead to nervous +exhaustion. Take, for example, the rather common habit of worrying over +the trivial things of life. Certainly the nervous energy spent in this way +cannot be used in doing useful work, but must be counted as so much loss +to the body. One who would use his nervous system to the best advantage +must find some way of preventing waste of this kind.(108) + +Undue excitement, as well as pleasurable dissipations, also tend toward +nervous exhaustion. And while the fact is recognized that pleasurable +activities supply a necessary mental exercise, the limit of healthful +endurance must be watched and _excesses of all kinds avoided_. Intense +emotional states are found to be exhausting in the extreme; and the +suppression of such undesirable feelings as anger, fear, jealousy, and +resentment are of immense value in the hygiene of the nervous system. + +*The Habit of Self-control.*--Much of the needless waste of nervous energy, +including that of worrying over trivial matters, may be prevented through +the exercise of self-control. From the standpoint of the nervous system, +the present age differs from the past mainly in supplying a greater number +and variety of nerve stimuli. Self-control means the ability to suppress +activities that would result from undesirable stimuli and to direct the +bodily activities into channels that are profitable. Self-control, +therefore, is not only to be exercised on occasions of great emergency, +but in the everyday affairs of life as well. It is even more important +that the daily toiler at his task be able to keep the petty annoyances of +life from acting as irritants to his nervous system than that he keep cool +during some great calamity. The habit of self-control is acquired mainly +through the persistent effort to prevent any and all kinds of petty +annoyances from affecting the nerves or the temper. + +*Nervousness.*--Self-control is much more easily practiced by some than by +others. This is due partly to habit, but is also due to an actual +difference in the degree of sensitiveness, or irritability, of the nervous +systems of different people. One whose nervous system tends to respond too +readily to any and all kinds of stimuli is said to be "nervous." This +condition is in some instances inherited, but is in most cases due to the +wasteful expenditure of nervous energy or to the action of some drug upon +the body. Excess of mental work, too much reading, long-continued anxiety, +eye strain, and the use of tea, coffee, alcohol, tobacco, or other drugs, +including many of those taken as medicines, are known to cause +nervousness. Nervousness is not only a source of great annoyance, both to +one's self and to others, but is a menace to the general health. + +The first step toward securing relief from such a condition is the removal +of the cause. The habits should be inquired into and excesses of all kinds +discontinued. In some instances it may be necessary to _have the eyes +__examined_ and glasses fitted by a competent oculist.(109) The nervous +energy should be carefully economized and the habit of self-control +diligently cultivated. Special exercises that have for their purpose the +equalizing of the circulation and the strengthening of the blood vessels +of the neck and the brain also have beneficial effects. + +*Nervous Overstrain.*--Both mental and physical overwork tends to weaken +the nervous system and to produce nervousness. Where hard mental work is +long continued, or where it is carried on under excitement, a tense +nervous condition is developed which is decidedly weakening in its +effects. The causes which lead to such a condition, and in fact overwork +of all kinds, should if possible be avoided. Where this is not possible, +and in many cases it is not, the period of overwork should be followed by +one of rest, recreation, and plenty of sleep. To the overworked in body or +in mind, nothing is more important from a hygienic, as well as moral, +standpoint, than the right use of the _one rest day in seven_. The best +interests of our modern civilization _require_ that the Sabbath be kept as +a quiet, rest-giving day. + +*Disturbed Circulation of the Brain.*--Nervousness not infrequently is +accompanied by an increase in the circulation of the brain and disappears +when this condition is relieved. Though mental work and excitement tend +naturally to increase the circulation in the brain, this should subside +with rest and relief from excitement. When there is a tendency for this +condition to become permanent, effort should be made looking for relief. +Increasing the circulation in the lower extremities by hot or cold foot +baths, or by much walking, is found to be most beneficial. Special +exercises of the muscles of the neck are also recommended as a means of +relieving this condition.(110) + +*Hygienic Value of Work.*--Within reasonable limits, both mental and +physical work are conducive to the vigor of the nervous system. Through +work the energies of the body find their natural outlet, and this prevents +dissipation and the formation of bad habits. Even hard work does not +injure the nervous system, and severe mental exertion may be undergone, +provided the proper hygienic conditions are observed. The nervous +disorders suffered by brain workers are not, as a rule, due to the work +which the brain does, but to violation of the laws of health, especially +the law of exercise. Such persons should observe the general laws of +hygiene and especially should they practice daily those forms of physical +exercise that tend to counteract the effects of mental work. + +*Physical Exercise* properly taken is beneficial to the nervous system +through both direct and indirect effects. A large proportion of the nerve +cells have for their function the production of motion, and these are +called into play only through muscular activity. Then, as already +suggested, physical exercise counteracts the unpleasant effects of mental +work. Hard study causes an excess of blood to be sent to the brain and a +diminished amount to the arms and to the legs. Physical exercise +redistributes the blood and equalizes the circulation. Light exercise +should, therefore, follow hard study. The student before retiring at night +is greatly aided in getting to sleep and is put in a better condition for +the next day's work by ten to fifteen minutes of light gymnastics. A daily +walk of two or three miles is also an excellent means of counteracting the +effects of mental work. The brain worker should, however, avoid violent +exercise or the carrying of any kind of exercise to exhaustion. + +*Sleep*, and plenty of it, is one of the first requirements of the nervous +system. It is during sleep that the exhausted brain cells are replenished. +To shorten the time for sleep is to weaken the brain and to lessen its +working force. No one should attempt to get along with less than eight +hours of sleep each day and most people require more. Children require +more sleep than adults. Those under six years should have from eleven to +twelve hours of sleep per day. Children between six and ten years should +have at least ten hours. + +*Insomnia*, or sleeplessness, on account of its effects upon the nervous +system, is to be regarded as a serious condition, and hygienic means for +relieving it should be diligently sought. Having its cause in nervousness, +a disturbed circulation of the brain, or some form of nervous exhaustion, +it is benefited through relieving these conditions and in the manner +already described. Of course the external conditions for aiding sleep +should not be overlooked. The bed should be comfortable, and the room +should be cool, well ventilated, dark, and quiet. The inducing of sleep by +means of drugs is a dangerous practice and should never be resorted to +except under the direction of the physician. + +*Effects of Heat and Cold.*--Heat and cold both have their effects upon the +nervous system. Heat increases the nervous irritability, while cold acts +as a natural sedative to the nerves. A nervous person is made more nervous +by an overheated atmosphere, but derives beneficial effects from exposing +the body freely to cold air and water. The tonic cold bath (page 273), if +taken with the usual precautions, can be used to good advantage in +diminishing nervousness. The taking of outdoor exercise in cold weather +is, for the same reason, an excellent practice. + +*Effect of Emotional States.*--We have already noted the effect of certain +emotional states upon the digestion of the food (page 162). Emotional +states are also known to interfere with breathing and with the action of +the heart. Such effects are explained through the close relation of the +mind to the work of the nervous system in general. While certain emotional +states, such as fear, anger, melancholia, and the impulse to worry, +interfere seriously with the normal action of the nervous system, others, +such as contentment, cheerfulness, and joy, are decidedly beneficial in +their effects. How important, then, is the habit of suppressing the states +that are harmful and of cultivating those that are beneficial. From a +hygienic, as well as social, standpoint a cheerful, happy disposition is +worth all the effort necessary for its attainment. + +*The Nervous Condition of Children* should be a matter of deep concern on +the part of both parents and teachers. In the home, as well as in the +school, the child may be "pushed" until the nervous system receives +permanent injury. Exhaustion of nerve cells is produced through too many +and too vivid impressions being made upon the immature brain. The child +should be protected from undue excitement. He should have the benefit of +outdoor exercise and should be early inured to cold. He should be shielded +from the poisoning effects of tea, coffee, tobacco, alcohol, and other +drugs. He should have impressed upon him the habit of self-control. He +should not be indulged in foolish caprices or whims, but should be taught +to be content with plain, wholesome food and with the simple forms of +enjoyment. + +*Influences at School.*--School life is necessarily a great strain upon the +child. Night study added to the work of the day makes a heavy burden for +elementary pupils to bear. Though the legal school age is usually fixed at +six years, delicate children should be kept out of school until they are +seven or eight years old, provided they have good homes. In addition to +the excitation incident to studying and reciting lessons, conditions +frequently arise both in the schoolroom and upon the playground that +create a feeling of fear or dread in the minds of children. Quarrels and +feuds among the children and the bullying of big boys on the playground +may work untold harm. All conditions tending to develop fear, uneasiness, +or undue excitement on the part of children should receive the attention +of those in authority. + +*Excessive Reading* is a frequent cause of injury to the nervous systems +of children. This has a bad effect, both on account of too many +impressions being made upon the mind and also on account of the strain to +the eyes. Then if the reading consists mostly of light fiction, the mind +is directed away from the really important things of life. The reading of +children should be thoughtfully controlled, both as to quality and +quantity. Exciting stories should, as a rule, be excluded, but a taste for +biography, historical and scientific writings, and for the great works of +literature should be cultivated. Simple fairy tales which have a +recognized value in developing the imagination of the child need not be +omitted, but it is of vital importance that the "story-reading habit" be +not formed. + +*Effects of Drugs.*--Because of its delicacy of structure a number of +chemical compounds, or drugs, are able to produce injurious effects upon +the nervous system. Some of these are violent poisons, while others, in +small quantities, are mild in their action. Certain drugs, in addition to +their immediate effects, bring about changes in the nervous system which +cause an unnatural appetite, or craving, that leads to their continued +use. This is the case with alcohol, the intoxicating substance in the +usual saloon drinks, and with nicotine, the stimulating drug in tobacco. +The same is also true of morphine, chloral, and several other drugs used +as medicines. The _danger of becoming a slave_ to some useless and +pernicious habit should dissuade one from the use of drugs except in cases +of positive emergency. + +*Alcohol and the Nervous System.*--Alcohol, as already shown, injures +practically all portions of the body; but it has its worst effects upon +the nervous system. Through its action on this system, it interferes with +the circulation of the blood, produces a condition of "temporary insanity" +called intoxication, weakens the will, and eventually dethrones the +reason. Worst of all, it produces a condition of "chronic poisoning" which +manifests itself in an unnatural craving, and this causes it to be used by +the victim even when he knows he is "drinking to his own destruction." +Though its use in small quantities does not, as a rule, produce such +marked effects upon the nervous system, it develops the "craving," and +this is apt in time to lead to its use in larger quantities. But even if +this does not occur, the practice is objectionable for its unhygienic +effects in general.(111) Tippling with such mild solutions of alcohol as +light wine, beer, and hard cider is, for these reasons, a dangerous +pastime. + +*Alcohol and Crime.*--It is sometimes stated that no one who leaves alcohol +alone will be injured by it. This is true only of its direct effects; not +of its indirect effects. Whenever a crime is committed somebody is +injured, and alcohol is known to be a chief cause of crime. Alcohol causes +crime through the loss of self-control, seen especially in intoxication, +and also because of the moroseness and quarrelsomeness which it developes +in certain individuals. Indirectly it causes crime through the poverty +which it engenders and through its influence in bringing about social +conditions out of which crime develops. Everything considered, the free +use of alcohol is incompatible with the nervous health and moral tone of a +community. + +*Nicotine and the Nervous System.*--Nicotine is an oily substance which is +extracted from the tobacco plant. Its action on the nervous system is in +general that of a poison. Taken in small quantities, it is a mild +stimulant and, if the doses are repeated, a habit is formed which is +difficult to break. Tobacco is used mainly for the stimulating effect of +this drug. While not so serious in its results as the alcohol and other +drug habits, the use of tobacco is of no benefit, is a continual and +useless expense, and, in many instances, causes a derangement of the +healthy action of the body.(112) With the bad effects of the nicotine must +be included those of questionable substances added to the tobacco by the +manufacturer, either for their agreeable flavor or for adulteration. + +*Relation of Age to the Effects of Nicotine.*--The use of tobacco by the +young is especially to be deplored. In addition to the harmful effects +observed in those of mature years, nicotine interferes with the normal +development of the body and lays, in many instances, the foundation for +physical and mental weakness in later life. The cigarette is decidedly +harmful, especially when inhalation is practiced, its deadening effects +being in part due to the wrappers, some of which have been shown to +contain arsenic and other poisonous drugs. While dulling the intellect and +weakening the body, cigarette smoking also tends to make criminals of +boys.(113) Parents, teachers, school officers, and all who have the good +of mankind at heart should take every precaution, including that of +setting a good example, to prevent the formation of the tobacco habit by +those of immature years. + +*Habit versus Self-control.*--The power of self-control, already emphasized +for its importance in the economical expenditure of the nervous energy, is +of vital importance in its relation to the habits of the body. +Self-control is the chief safeguard against the formation of bad habits +and is the only means of redemption from such habits after they have once +been formed. The persistent cultivation of the power to control the +appetites and the passions, as well as all forms of activity which tend to +injure the body or debase the character, gives a tone to the nervous +system which increases the self-respect and raises the individual to a +_higher plane of life_. The worst habits _can_ be broken and good ones +formed in their stead, if only there is sufficient determination to +accomplish these results. Failure comes from not having the mind +thoroughly "made up" and from not having, back of the desire to do better, +"the strong will of a righteous determination." + +*Effects of External Conditions.*--While the inner life and habits have +most to do with the hygiene of the nervous system, a certain amount of +attention may properly be given to those conditions outside of the body +which affect directly or indirectly the state of this system. Noise, +disorder, and confusion act as nervous irritants, but quiet, order, and +system have the opposite effect. There is, therefore, much in the +management of the office, factory, schoolroom, or home that has to do with +the real hygiene of the nerves as well as with the efficiency of the work +that is being done. The suppression of distracting influences not only +enables the mind to be given fully to the work in hand, but actually +prevents waste of nervous energy. Although the responsibility for securing +the best conditions for work rests primarily with those in charge, it is +also true that each individual in every organization may contribute to the +order or disorder that prevails. + +*Social Relations.*--In considering the external conditions that affect the +nervous system, the fact must not be overlooked that man is a social being +and has to adjust himself to an established social order. His relations to +his fellow-men, therefore, affect strongly his nervous condition and +theirs also. For this reason the best hygiene of the nervous system is +based upon _moral_ as well as physical right living. Along with the power +of self-control and the maintenance of a correct nervous poise, there +should be a proper regard for the welfare of others. On account of the +ease with which one individual may disturb the nervous state of another, +those social forms and customs which tend to establish harmonious +relations among men are truly hygienic in their effects, and may well be +carried out in spirit as well as "in letter." + +It is also a fact that a given mental state in one person tends to excite +a like state in those with whom he associates. How important, then, that +each and all cultivate, as habits, the qualities of cheerfulness, +kindness, and good-will, instead of the opposite states of mind. +Especially in the family, and other groups of closely associated +individuals, should the nervous effect of one member upon the others be +considered and every effort made to secure and maintain harmonious +relations. + +*The High Ideal.*--Everything considered, the conditions most favorable to +the healthfulness of the nervous system are in harmony with what our +greatest teachers have pointed to as the higher plane of living. On this +account a true conception of the value and meaning of life is of the +greatest importance. _An ever present, strong desire to live a vigorous, +but simple and noble, life_ will suggest the proper course to pursue when +in doubt and will stimulate the power of self-control. It will lead to the +stopping of "nerve leaks" and to the maintenance of harmonious relations +with one's fellows. It will cause one to recoil from the use of alcohol +and other nerve poisons, as from a deadly serpent, seeing the end in the +beginning, and will be the means eventually of leading the body into its +greatest accomplishments. + +*Summary.*--The nervous system, on account of its delicate structure, is +liable to injury through wrong methods of using it and also through the +introduction of drugs, or poisons, into the body. There are also found in +our methods of living and systems of education conditions that tend to +waste the nervous energy. To protect the nervous system from all these +threatened dangers requires, among other things, the power of +self-control. This enables the individual to direct his life according to +his highest ideals and to free himself from habits known to be injurious. +Children must have their nervous systems safeguarded by parents and +teachers. Especially must they be kept from becoming enslaved to some +drug, such as alcohol or the nicotine of tobacco. + +*Exercises.*--1. In what respect is the hygiene of the nervous system the +hygiene of the entire body? + +2. Of what value in the hygiene of the nervous system is the power of +self-control? How is the habit of self-control formed? + +3. Name several forms of activity that waste the nervous energy. + +4. Name several influences that react unfavorably on the nervous systems +of children. + +5. How may too much reading prove injurious to the nervous system? + +6. What forms of physical exercise are beneficial to the brain worker? + +7. Why is the use of alcohol even in small quantities to be regarded as a +dangerous practice? + +8. Name several causes of nervousness. + +9. What are the unanswerable arguments for preventing the use of tobacco +by the young? + +10. Why do cigarettes have a more harmful effect upon the body than other +forms of tobacco? + +11. Enumerate conditions in the schoolroom that dissipate the nervous +energy of pupils; that economize it. + + + + +CHAPTER XX - PRODUCTION OF SENSATIONS + + +Our study of the nervous system has shown that impulses arising at the +surface of the body are able, through connecting neurons, to bring about +various activities. Moving along definite pathways, they induce motion in +the muscles, and in the glands the secretion of liquids. It is now our +purpose to consider the effect produced by afferent impulses upon the +brain and, through the brain, upon the mind.(114) This effect is +manifested in a variety of similar forms, known as + +*The Sensations.*--Sensations constitute the lowest forms of mental +activity. Roughly speaking, they are the states of mind experienced as the +_direct_ result of impulses reaching the brain. In a sense, just as +impulses passing to the muscles cause motion, impulses passing to the +brain cause sensations. The feeling which results from the hand's touching +a table is a sensation and so also is the pain which is caused by an +injury to the body. The mental action in each case is due to impulses +passing to the brain. Care must be exercised by the beginner, however, not +to confuse sensations with the nervous impulses, on the one hand, or with +_secondary_ mental effects, such as emotion or imagination, on the other. +Sensations are properly regarded as the first conscious effects of the +afferent impulses and as the _beginning stage_ in the series of mental +processes that may take place on account of them. + +In some way, not understood, the mind associates the sensation with the +part of the body from which the impulses come. Pain, for example, is not +felt at the brain where the sensation is produced, but at the place where +the injury occurs. This association, by the mind, of the sensations with +different parts of the body, is known as "localizing the sensation." + +*Sensation Stimuli.*--While the sensations are dependent upon the afferent +impulses, the afferent impulses are in turn dependent upon causes outside +of the nervous system. If these are removed, the sensations cease and they +do not start up again unless the exciting influences are again applied. +Any agency, such as heat or pressure, which, by acting on the neurons of +the body, is able to produce a sensation, may be called a _sensation +stimulus_. It has perhaps already been observed that the stimuli that lead +to voluntary action, as well as those that produce reflex action of the +muscles, cause sensations at the same time. From this we may conclude that +sensation stimuli are the same in character as those that excite motion. +On the other hand, it should be noted that sensations are constantly +resulting from stimuli that are of too mild a nature to cause motion. + +*Classes of Sensations.*--Perhaps as many as twenty distinct sensations, +such as pain, hunger, touch, etc., are recognized. If these are studied +with reference to their origin, it will be seen that some of them result +from the action of definite forms of stimuli upon the neurons terminating +in sense organs; while the others, as a rule, arise from the action of +indefinite stimuli upon neurons in parts of the body that do not possess +sense organs. The members of the first class--and these include the +sensations of touch, temperature, taste, smell, hearing, and sight--are +known as the _special_ sensations. The others, including the sensations of +pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of +disease, are known as _organic_, or general, sensations. These two classes +of sensations differ in their purpose in the body as well as in the manner +of their origin. + +*Purposes of Sensations.*--Any given sensation is related to the stimulus +which excites it as an _effect_ to a _cause_. It starts up or stops, +increases in intensity or diminishes, according to the action of the +exciting stimulus. As the stimuli are outside of the nervous system, and +in the majority of cases outside of the body, the sensations indicate to +the mind what is taking place either in the body itself or in its +surroundings. They supply, in other words, the means through which the +mind acquires information. By means of the special sensations, a knowledge +of the physical surroundings of the body is gained, and through the +organic sensations the needs of the body and the state of the various +organs are indicated. In general, sensations are made to serve two great +purposes in the body, as follows: + +1. They provide the necessary conditions for intelligent and purposeful +action on the part of the body. + +2. They supply the basis for the higher mental activities, as perception, +memory, thought, imagination, and emotion. + +Intelligent action is impossible without a knowledge both of the bodily +organs and of the body's surroundings. Protection and the regulation of +the work of an organ necessitate a knowledge of its condition, while the +adapting and adjusting of the body to its surroundings require a knowledge +of what those surroundings are. The dependence of all the higher forms of +mental activity upon sensations is recognized by psychologists and is +easily demonstrated by a study of the manner in which we acquire +knowledge. "Without sensation there can be no thought." + +*Steps in the Production of Sensations.*--The steps in the production of +sensations are not essentially different from those in the production of +reflex action. First of all, external stimuli act upon the fiber +terminations in the sense organs, or elsewhere, starting impulses in the +neurons. These pass into the central nervous system and there excite +neurons which in turn discharge impulses into the cerebrum. The result is +to arouse an activity of the mind--a sensation. The steps in the production +of any _special_ sensation naturally involve the following parts: + +1. A sense organ where the terminations of the neurons are acted upon by +the stimulus. + +2. A chain of neurons which connect the sense organ with the brain. + +3. The part of the cerebrum which produces the sensation. + +*Sense Organs.*--The sense organs are not parts of the afferent neurons, +but are structures of various kinds, in which the neurons terminate. Their +function is to enable the sensation stimuli to start the impulses. By +directing, concentrating, or controlling the stimuli, the sense organs +enable them to act to the best advantage upon the neurons. When it is +recognized that such widely different forces as light waves, sound waves, +heat, pressure, and odors are enabled by them to stimulate neurons, the +importance of these organs becomes apparent. As would naturally be +inferred, the construction of any sense organ has particular reference to +the nature of the stimulus which it is to receive. This is most apparent +in the sense organs of sight and hearing. + +*Simple Forms of Sense Organs.*--The simplest form of a sense organ (if +such it may be called) is one found among the various tissues. It consists +of the terminal branches of nerve fibers which spread over a small area of +cells, as a network or plexus. Such endings are numerous in the skin and +muscles. + +Next in order of complexity are the so-called _end-bulbs_. These consist +of rounded, or elongated, connective tissue capsules, within which the +nerve fibers terminate. On the inside the fibers lose their sheaths and +divide into branches, which wind through the capsule. End-bulbs are +abundant in the lining membrane of the eye, and are found also in the skin +of the lips and in the tissues around the joints. + +Slightly more complex than the end-bulbs are the _touch corpuscles_. These +are elongated bulb-like bodies, having a length of about one +three-hundredth of an inch, and occupying the papillae of the skin (Fig. +144). They are composed mainly of connective tissue. Each corpuscle +receives the termination of one or more nerve fibers. These, on entering, +lose the medullary sheath and separate into a number of branches that +penetrate the corpuscle in different directions. + + [Fig. 144] + + + Fig. 144--*A touch corpuscle* highly magnified. (See text.) + + +The largest of the simple forms of sense organs are bodies visible to the +naked eye and called, from their discoverer Pacini, the _Pacinian +corpuscles_. They lie along the course of nerves in many parts of the +body, and have the general form of grains of wheat. (See Practical Work.) +The Pacinian corpuscles are composed of connective tissue arranged in +separate layers around a narrow central cavity called the core (Fig. 145). +Within the core is the termination of a large nerve fiber. These +corpuscles are found in the connective tissue beneath the skin, along +tendons, around joints, and among the organs of the abdominal cavity. + + [Fig. 145] + + + Fig. 145--*Pacinian corpuscle*, magnified. _A._ Medullated nerve fiber. + _B._ Axis cylinder terminating in small bulb at _C._ _D._ Concentric + layers of connective tissue. _E._ Inner bulb. + + +The simple forms of sense organs have a more or less general distribution +over the body, and are concerned in the production of at least three +special sensations. These are _touch, temperature_, and the _muscular +sensation_. + +*Touch*, or feeling, is perhaps the simplest of the sensations. The sense +organs employed are the touch corpuscles, and the external stimulus is +some form of pressure or impact. Pressure applied to the skin, by acting +on the fiber terminations in the corpuscles, starts the impulses that give +rise to the sensation. The touch corpuscles render the fiber terminations +so sensitive that the slightest pressure is able to arouse sensations of +touch. It is found that _a change of pressure_, rather than pressure that +is constant, is the active stimulus. That all parts of the skin are not +equally sensitive to pressure, and that the mind does not interpret +equally well the sensations from different parts, are facts easily +demonstrated by experiment. (See Practical Work.) + +*The Temperature Sensation.*--Temperature sensations, like those of touch, +are limited almost entirely to the skin. They are of two kinds, and are +designated as _heat_ sensations and as _cold_ sensations. Whether the +sense organs for temperature are different from those of touch is not +known. It is known, however, that the same corpuscles do not respond alike +to heat, cold, and pressure. + +_A Change of Temperature_, rather than any specific degree of heat or +cold, is the active temperature stimulus. The sensation of warmth is +obtained when the temperature of the skin is being raised, and of cold +when it is being lowered. This explains why in going into a hallway from a +heated room one receives a sensation of cold, while in coming into the +same hallway from the outside air he receives a sensation of warmth. It is +for the same reason that we are able to distinguish only the relative, not +the actual, temperature of bodies. + +*Muscular Sensations.*--These are sensations produced by impulses arising +at the muscles. Such impulses originate at the fiber terminations which +are found in both the muscles and their tendons. By muscular sensations +one is conscious of the location of a contracting muscle and of the degree +of its tension. They also make it possible to judge of the weight of +objects. + + [Fig. 146] + + + Fig. 146--*Sense organs of taste.* _A._ Map of upper surface of tongue, + showing on the left the different kinds of papillae, and on the right the + areas of taste (after Hall). Area sensitive to bitter (----); to acid + (....); to salt (--.--.--.--); to sweet (--------). _B._ Section through a + papilla. _n._ Small nerve connecting with taste buds at _d. e._ + Epithelium. _C._ Single taste bud magnified. _n._ Nerve, the fibers of +which terminate between the spindle-shaped cells _a. e._ Epithelial cells. + + +*The Sensation of Taste.*--The sense organs of taste are found chiefly in +the mucous membrane covering the upper surface of the tongue. Scattered +over this surface are a number of rounded elevations, or large papillae (A, +Fig. 146). Toward the back of the tongue two rows of these, larger than +the others, converge to meet at an angle, where is located a papilla of +exceptional size. Surrounding each papilla is a narrow depression, within +which are found the sense organs of taste (B, Fig. 146). These are called, +from their shape, _taste buds_, and each bud contains a central cavity +which communicates with the surface by a small opening--_the gustatory +pore_. Within this cavity are many slender, spindle-shaped cells which +terminate in hair-like projections at the end nearest the pore, but in +short fibers at the other end. Nerve fibers enter at the inner ends of the +buds and spread out between the cells (_C_, Fig. 146). These fibers pass +to the brain as parts of two pairs of nerves--those from the front of the +tongue joining the trigeminal nerve, and those from the back of the +tongue, the glossopharyngeal nerve. + +The gustatary, or _taste stimulus_, is some chemical or physical condition +of substances which is manifested only when they are in a liquid state. +For this reason _only liquid substances can be tasted_. Solids to be +tasted must first be dissolved. + +The different taste sensations are described as bitter, sweet, sour, and +saline, and in the order named are recognized as the tastes of quinine, +sugar, vinegar, and salt. As to how these different tastes are produced, +little is known. Flavors such as vanilla and lemon, and the flavors of +meats and fruits, are really smelled and not tasted. Taste serves two main +purposes: it is an aid in the selection of food and it is a means of +stimulating the digestive glands (page 161). + + [Fig. 147] + + +Fig. 147--*Sense organ of smell.* _A._ Distribution of nerves in outer wall + of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves. + 3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing + connection of neurons concerned in smell. + + +*The Sensation of Smell.*--The sense organs of smell are found in the +mucous membrane lining the upper divisions of the nasal cavities. Here are +found two kinds of cells in great abundance--column-shaped epithelial cells +and the cells which are recognized as the sense organs of smell. These +olfactory cells are spindle-shaped, having at one end a slender, +thread-like projection which reaches the surface, and at the other end a +fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory +cells resemble closely the cell-bodies of neurons, and are thought to be +such. The divisions of the olfactory nerve pass through many small +openings in the ethmoid bone to connect with the olfactory bulbs, which in +turn connect with the cerebrum (A, Fig. 147). + +*The Olfactory Stimulus.*--Only substances in the gaseous state can be +smelled. From this it is inferred that the stimulus is supplied by gas +particles. Solids and liquids are smelled because of the gas particles +which separate from them. The substance which is smelled must be kept +moving through the nostrils and made to come in direct contact with the +olfactory cells. There is practically no limit to the number of distinct +odors that may be recognized. + +*Value of Smell.*--Although the sense of smell is not so acute in man as in +some of the lower animals, it is, nevertheless, a most important and +useful gift. It is the only sense that responds to matter in the gaseous +state, and is, for this reason, the only natural means of detecting +harmful constituents of the atmosphere. In this connection it has been +likened to a sentinel standing guard over the air passages. Many gases +are, however, without odor, and for this reason cannot be detected by the +nostrils. It is of especial importance that gases which are likely to +become mixed with the air supply to the body have odor, even though the +odor be disagreeable. The bad odors of illuminating gas and of various +compounds of the chemical laboratory, since they serve as danger signals +to put one exposed to them on his guard, are of great protective value. + +*Sight and Hearing.*--The sense organs of sight and hearing are highly +complicated structures, and will be considered in the chapters following. + +*Summary.*--Sensations are certain activities of the mind that result from +excitations within the body or at its surface. These cause the neurons to +discharge impulses which on reaching the cerebrum cause the sensations. +Sensations are necessary for intelligent and purposeful action and for +acquiring all kinds of knowledge. To enable the stimuli to act to the best +advantage in starting the impulses, special devices, called sense organs, +are employed. These receive the terminations of the neurons, and by their +special structure enable the most delicate stimuli to start impulses. The +simpler forms of sense organs are those of touch, temperature, taste, and +smell. + +*Exercises.*--1. Compare sensations and reflex actions with reference to +their nature and cause. Give steps in the production of each. + +2. Give examples of sensation stimuli. State the purpose of sense organs. + +3. How do general sensations differ from special sensations? + +4. Of what value is pain in the protection of the body? + +5. Show that sensations lead to the higher forms of mental activity, such +as emotion and imagination. + +6. Of what value to the body is the "localizing of the sensation"? + +7. What kinds of sense organs are found in the skin? State the purpose of +each. + +8. Through what sense avenues is one made aware of solids, of liquids, and +of gases? + +9. Of what special protective value is the sense of smell? + + + +PRACTICAL WORK + + +*To demonstrate the Pacinian Corpuscles.*--Spread out the mesentery from +the intestine of a cat and hold it between the eye and the light: Pacinian +corpuscles will appear as small translucent bodies having the general form +of grains of wheat. Secure a portion of the mesentery over a circular +opening in a thin piece of cork and examine it with a microscope of low +power. Follow the course of the nerve fiber to the nerve from which it +branches. + +*To show Relative Sensitiveness of Different Parts of the Skin.*--Holding a +bristle between the fingers, bring the end in contact with the skin, +noting the amount of pressure necessary to cause a sensation of touch. +Test the lips, tongue, tips of fingers, and palm and back of hand, trying +different sizes of bristles. Has the degree of sensitiveness any relation +to the thickness of the cuticle? + +*To show Perceptive Differences of Different Portions of the Skin.*--Place +the points of a pair of dividers on the back of the hand of one who looks +in the opposite direction. Is one point felt or two? Repeat several times, +changing the distance between the points until it is fully determined how +near the two points must be placed in order to be felt as one. In like +manner test other parts of the body, as the tips of the fingers and the +back of the neck. Compare results obtained at different places. + +*To locate Warm and Cold Sensation Spots.*--Slowly and evenly draw a +blunt-pointed piece of metal over the back of the neck. If it be of the +same temperature as the skin, only touch sensations will be experienced. +If it be a little colder (the temperature of the room) sensations of cold +will be felt at certain spots. If slightly warmer than the body, heat +sensation spots will be found on other parts of the skin. If the heat and +cold sensation spots be marked and tested from day to day they will be +found to remain constant as to position. Inference. + + + + +CHAPTER XXI - THE LARYNX AND THE EAR + + +Man is a social being. His inclinations are not to live alone, but to be a +part of that great human organization known as society. For men to work +together, to be mutually helpful one to another, requires the ability to +exchange ideas and this in turn requires some means of communication.(115) +One means of communication is found in certain movements of the +atmosphere, known as _sound waves_. In the exchange of ideas by this means +there are employed two of the most interesting divisions of the body--the +larynx and the ear. The first is an instrument for the production of sound +waves; the second is the sense organ which enables the sound waves to act +as stimuli to the nervous system. + +*Nature of Sound Waves.*--If some sonorous body, as a bell, be struck, it +is given a quivering, or vibratory, motion. This is not confined to the +bell, but is imparted to the air and other substances with which the bell +comes in contact. These take up the movements and pass them to objects +more remote, and they in turn give them to others, until a very +considerable distance is reached. Such progressive vibrations are known as +waves, and, since they act as stimuli to the organs of hearing, they are +called _sound waves_. Sound waves _always originate in vibrating +bodies_.(116) They are transmitted chiefly _by the air_, which, because of +its lightness, elasticity, and abundance, readily takes up the vibrations +and spreads them in all directions (Fig. 148). + +While these vibratory movements of the atmosphere are correctly classified +as waves, they bear little resemblance to the waves on water. Instead of +being made of crests and troughs, as are the water waves, the sound waves +consist of alternating successions of slightly condensed and rarefied +layers of air. Then, while the general movement of the water waves is that +of ever widening circles _over a surface_, the sound waves spread as +enlarging spherical shells _through_ the air. In sound waves, as in all +other waves, however, it is only the form of the wave that moves forward. +The individual particles of air that make up the wave simply vibrate back +and forth. + + [Fig. 148] + + + Fig. 148--Diagram illustrating the spreading of sound waves through air. + + +*How Sound Waves act as Stimuli.*--Any sound wave represents a small but +definite amount of energy, this being a part of the original force that +acted on the vibrating body to set it in motion. The hammer, for instance, +in striking a bell imparts to it a measurable quantity of energy, which +the bell in turn imparts to the air. This energy is in the sound waves and +is communicated to the bodies against which they strike.(117) Though the +force exerted by most sound waves is, indeed, very slight, it is +sufficient to enable them to act as stimuli to the nervous system. + +*How Sounds Differ.*--Three distinct effects are produced by sound waves +upon the nerves of hearing, and through them upon the mind. These are +known as _pitch, intensity_, and _quality_, and they are dependent upon +the vibrations of the sound-producing bodies. + +_Pitch_, which has reference to the height, or degree of sharpness, of +tones, is determined by the rapidity of the vibrations of the vibrating +body. The more rapid the vibrations, the higher the pitch, the number of +vibrations doubling for each musical interval known as the octave. + +_Intensity_ is the energy, or force, of the sound waves. This is +recognized by the strength of the sensation and is expressed by the term +_loudness_. Intensity is governed mainly by the width of the vibrations of +the vibrating body, and the width depends upon the force applied to the +body to make it vibrate. + +_Quality_ is that peculiarity of sound that enables tones from different +instruments to sound differently, although they may have the same pitch +and intensity. Quality depends upon the fact that most tones are complex +in nature and result from the blending together of simple tones of +different pitch. + +*Reenforcement of Sound Waves.*--The sound vibrations from small bodies are +not infrequently reenforced by surrounding conditions so that their +outgoing waves reach farther and are more effective than waves from larger +bodies. This is true of the sound waves produced by most musical +instruments and also those produced by the human larynx. Such +reenforcement is effected in two general ways--by sounding boards and by +inclosed columns of air. Stringed instruments--violin, guitar, piano, +etc.--employ sounding boards, while wind instruments, as the flute, pipe +organ, and the various kinds of horns, employ air columns for reenforcing +their vibrations. In the use of the sounding board, the vibrations are +communicated to a larger surface, and in the use of the air column the +vibrations are communicated to the inclosed air. (See Practical Work.) + +*Value of Sound Waves to the Body.*--From a physiological standpoint, the +value of sound waves is not easily overestimated. In addition to the use +made of them in the communication of ideas, they serve the purpose of +protecting the body, and in the sphere of music provide one of the most +elevating forms of entertainment. Sounds from different animals, as well +as from inanimate objects, may also be the means of supplying needed +information. The existence of two kinds of sound instruments in the +body--the one for the production, the other for the detection, of sound--is +certainly suggestive of the ability of the body to adjust itself to, and +to make use of, its physical environment. Both the larynx and the ear are +constructed with special reference to the nature and properties of sound +waves. + + + +THE LARYNX + + +*The Sound-producing Mechanism of the Body* consists of the following +parts: + +1. Delicately arranged bodies that are easily set in vibration. + +2. An arrangement for supplying the necessary force for making these +bodies vibrate. + +3. Contrivances for modifying the vibrating parts so as to produce changes +in pitch and intensity. + +4. Parts that reenforce the vibrations. + +5. Organs by means of which the sounds are converted into the forms of +speech. + +The central organ in this complex mechanism is + +*The Larynx.*--The larynx forms a part of the air passages, being a short +tube at the upper end of the trachea. Mucous membrane lines the inside of +it and muscles cover most of the outer surface. The framework is made of +cartilage. At the top it is partly encircled by a small bone (the hyoid), +and its opening into the pharynx is guarded by a flexible lid, called the +_epiglottis_. The cartilage in its walls is in eight separate pieces, but +the greater portion of the structure is formed of two pieces only. These +are known as the _thyroid cartilage_ and the _cricoid cartilage_ (Fig. +149). Both can be felt in the throat--the thyroid as the projection known +as "Adam's apple," and the cricoid as a broad ring just below. + + [Fig. 149] + + + Fig. 149--The larynx.--_A._ Outside view. _B._ Vertical section through + larynx, showing inside. 1. Thyroid cartilage. 2. Cricoid cartilage. 3. +Trachea. 4. Hyoid bone. 5. Epiglottis. 6. Vocal cord. 7. False vocal cord. + 8. Lining of mucous membrane. + + +The _thyroid cartilage_ consists of two V-shaped pieces, one on either +side of the larynx, meeting at their points in front, and each terminating +at the back in an upward and a downward projection. Between the back +portions of the thyroid is a space equal to about one third of the +circumference of the larynx. This is occupied by the greater portion of +the _cricoid cartilage_. This cartilage has the general shape of a signet +ring and is so placed that the part corresponding to the signet fits into +the thyroid space, while the ring portion encircles the larynx just below +the thyroid. Muscles and connective tissue pass from the thyroid to the +cricoid cartilage at all places, save one on each side, where the downward +projections of the thyroid form hinge joints with the cricoid. These +joints permit of motion of either cartilage upon the other. + +At the summit of the cricoid cartilage, on each side, is a small piece of +triangular shape, called the _arytenoid cartilage_. Each arytenoid is +movable on the cricoid and is connected with one end of a vocal cord. + + [Fig. 150] + + + Fig. 150--*Vocal cords* as seen from above. _A._ In producing sound, _B._ + During quiet breathing. + + +*The Vocal Cords* are formed by two narrow strips of tissue which, +connecting with the thyroid cartilage in front and the arytenoid +cartilages behind, lie in folds of the mucous membrane. They have the +general appearance of ridge-like projections from the sides of the larynx, +but at their edges they are sharp and smooth. The open space between the +cords is called the _glottis_. When sound is not being produced, the +glottis is open and has a triangular form, due to the spreading apart of +the arytenoid cartilages and the attached cords. But when sound is being +produced, the glottis is almost completely closed by the cords. Above the +vocal cords, and resembling them in appearance, are two other folds of +membrane, called the _false vocal cords_ (B, Fig. 149). The false cords do +not produce sound, but they aid in the closing of the glottis. + +*How the Voice is Produced.*--The voice is produced through the vibrations +of the vocal cords. A special set of muscles draws the arytenoid +cartilages toward each other, thereby bringing their edges very near and +parallel to each other in the passage. At the same time other muscles act +on the thyroid and cricoid cartilages to separate them at the top and give +the cords the necessary tension. With the glottis now almost closed, +blasts of air from the lungs strike the sharp edges of the cords and set +them in vibration (Fig. 150). The vocal cords do not vibrate as strings, +like the strings of a violin, but somewhat as reeds, similar to the reeds +of a French harp or reed organ. + +The location of the vocal cords in the air passages enables the lungs and +the muscles of respiration to aid in the production of the voice. It is +their function to supply the necessary force for setting the cords in +vibration. The upper air passages (mouth, nostrils, and pharynx) supply +resonance chambers for reenforcing the vibrations from the vocal cords, +thereby greatly increasing their intensity. In ordinary breathing the +vocal cords are in a relaxed condition against the sides of the larynx and +are not acted upon by the air as it enters or leaves the lungs. + +*Pitch and Intensity of the Voice.*--Changes in the pitch of the voice are +caused mainly by variations in the tension of the cords, due to the +movements of the thyroid and cricoid cartilages upon each other.(118) In +the production of tones of very high pitch, the vibrating portions of the +cords are thought to be actually shortened by their margins being drawn +into contact at the back. This raises the pitch in the same manner as does +the shortening of the vibrating portion of a violin string. + +The _intensity_, or loudness, of the voice is governed by the force with +which the air is expelled from the lungs. The vibrations of the cords, +however, are greatly reenforced by the peculiar structure of the upper air +passages, as stated above. + +*Production of Speech.*--The sounds that form our speech or language are +produced by modifying the vibrations from the vocal cords. This is +accomplished by "mouthing" the sounds from the larynx. The distinct +sounds, or words, are usually complex in nature, being made up of two or +more elementary sounds. These are classed either as _vowels_ or +_consonants_ and are represented by the different letters of the alphabet. +The vowel sounds are made with the mouth open and are more nearly the pure +vibrations of the vocal cords. The consonants are modifications of the +vocal cord vibrations produced by the tongue, teeth, lips, and throat. + +*Words and their Significance.*--In the development of language certain +ideas have become associated with certain sounds so that the hearing of +these sounds suggests the ideas. Our words, therefore, consist of so many +sound signals, each capable of arousing a definite idea in the mind. To +talk is to express ideas through these signals, and to listen is to assume +an attitude of mind such that the signals may be interpreted. In learning +a language, both the sounds of the words and their associated ideas are +mastered, this being necessary to their practical use in exchanging ideas. +From spoken language man has advanced to written language, so that the +sight of the written or printed word also arouses in the mind the +associated idea. + + + +THE EAR + + +*The Ear* is the sense organ which enables sound waves to so act upon +afferent neurons as to excite impulses in them. The effect upon the mind +which these impulses produce is known as the _sensation of hearing_. In +the performance of its function the ear receives and transmits sound waves +and also concentrates them upon a suitable exposure of nerve cells. It +includes three parts--the _external ear_, the _middle ear_, and the +_internal ear_. + +*External Ear.*--The external ear consists of the part on the outside of +the head called the _pinna_, or auricle, and the tube leading into the +middle ear, called the _auditory canal_ (Fig. 151). The pinna by its +peculiar shape aids to some extent the entrance of sound waves into the +auditory canal.(119) It consists chiefly of cartilage. The auditory canal +is a little more than an inch in length and one fourth of an inch in +diameter, and is closed at its inner end by a thin, but important +membrane, called + +*The Membrana Tympani.*--This membrane consists of three thin layers. The +outer layer is continuous with the lining of the auditory canal; the inner +is a part of the lining of the middle ear; and the middle is a fine layer +of connective tissue. Being thin and delicately poised, the membrana +tympani is easily made to vibrate by the sound waves that enter the +auditory canal. In this way it serves as a receiver of sound waves from +the air. It also protects + + [Fig. 151] + + + Fig. 151--*Diagram of section through the ear*, showing relations of its + various parts. (See text.) + + +*The Middle Ear.*--The middle ear, or tympanum,(120) consists of an +irregular cavity in the temporal bone which is lined with mucous membrane +and filled with air. It is connected with the pharynx by a slender canal +called the _Eustachian tube_. Extending across the middle ear and +connecting with the membrana tympani on one side, and with a membrane +closing a small passage to the internal ear on the other, is a tiny bridge +formed of three small bones. These bones, named in their order from the +membrana tympani, are the _malleus_, the _incus_, and the _stapes_ (Fig. +151). Where the malleus joins the membrane is a small muscle whose +contraction has the effect of tightening the membrane. The Eustachian tube +admits air freely to the middle ear, providing in this way for an equality +of atmospheric pressure on the two sides of the drum membrane. The bridge +of bones and the air in the middle ear receive vibrations from the +membrana tympani and communicate them to the membrane of the internal ear. + +*Purposes of the Middle Ear. *--The middle ear serves two important +purposes. In the first place, it makes it possible for sound waves to set +the membrana tympani in vibration. This membrane could not be made to +vibrate by the more delicate of the sound waves if it were stretched over +a bone, or over some of the softer tissues, or over a liquid. Its +vibration is made possible by the presence of air on _both_ sides, and +this condition is supplied, on the inner side, by the middle ear. The +Eustachian tube, by providing for an _equality_ of pressure on the two +sides of the membrane, also aids in this purpose. + +In the second place, the middle ear provides a means for _concentrating +the force of the sound waves_ as they pass from the membrana tympani to +the internal ear. This concentration is effected in the following manner: + +1. The bridge of bones, being pivoted at one point to the walls of the +middle ear, forms a lever in which the malleus is the long arm, and the +incus and stapes the short arm, their ratio being about that of three to +two. This causes the incus to move through a shorter distance, but with +greater force than the end of the malleus. + +2. The area of the membrana tympani is about twenty times as great as the +membrane of the internal ear which is acted upon by the stapes. The force +from the larger surface is, therefore, concentrated by the bridge of bones +upon the smaller surface. By the combination of these two devices, the +waves striking upon the membrane of the internal ear are rendered some +thirty times more effective than are the same waves entering the auditory +canal. + +*The Internal Ear*, or labyrinth, occupies a series of irregular channels +in the petrous process of the temporal bone.(121) It is very complicated +in structure, and at the same time is very small. Its greatest length is +not more than three fourths of an inch and its greatest diameter not more +than one half of an inch. It is filled with a liquid which at one place is +called the _perilymph_, and at another place the _endolymph_. It is a +double organ, being made up of an outer portion which lies next to the +bone, and which surrounds an inner portion of the same general form. The +outer portion is surrounded by a membrane which serves as periosteum to +the bone and, at the same time, holds the liquid belonging to this part, +called the perilymph. The inner portion, called the _membranous +labyrinth_, consists essentially of a closed membranous sac, which is +filled with the endolymph. The auditory nerve terminates in this portion +of the internal ear. Three distinct divisions of the labyrinth have been +made out, known as the _vestibule_, the _semicircular canals_, and the +_cochlea_ (Fig. 152). + + [Fig. 152] + + +Fig. 152--*General form, of internal ear.* The illustration represents the + structures of the internal ear surrounded by a thin layer of bone. 1. + Vestibule. 2. Cochlea. 3. Semicircular canals. 4. Fenestra ovalis. 5. + Fenestra rotunda. + + +*The Vestibule* forms the central portion of the internal ear and is +somewhat oval in shape. It is in communication with the middle ear through +a small opening in the bone, called the _fenestra ovalis_, at which place +it is separated from the middle ear only by a thin membrane. Sound waves +enter the liquids of the internal ear at this point, the foot of the +stapes being attached to the membrane. Six other openings lead off from +the vestibule at different places. One of these enters the cochlea. The +other five open into + +*The Semicircular Canals.*--These canals, three in number, pass through the +bone in three different planes. One extends in a horizontal direction and +the other two vertically, but each plane is at right angles to the other +two. Both ends of each canal connect with the vestibule, though two of +them join by a common opening. The inner membranous labyrinth is +continuous through each canal, and is held in position by small strips of +connective tissue. + +The purpose of the semicircular canals is not understood. It is known, +however, that they are not used in hearing. On the other hand, there is +evidence to the effect that they act as equilibrium sense organs, exciting +sensations necessary for balancing the body. Their removal or injury, +while having no effect upon the hearing, does interfere with the ability +to keep the body in an upright position. + + [Fig. 153] + + + Fig. 153--Diagram showing the divisions of cochlear canal. + + +*The Cochlea* is the part of the internal ear directly concerned in +hearing. It consists of a coiled tube which makes two and one half turns +around a central axis and bears a close resemblance to a snail shell +(Figs. 151 and 152). It differs in plan from a snail shell, however, in +that its interior space is divided into three distinct channels, or +canals. These lie side by side and are named, from their relations to +other parts, the _scala vestibula_, the _scala tympani_, and the _scala +media_. Any vertical section of the cochlea shows all three of these +channels (Fig. 153). + +*The Scala Vestibula and the Scala Tympani* appear in cross section as the +larger of the canals. The former, so named from its connection with the +vestibule, occupies the upper position in all parts of the coil. The +latter lies below at all places, and is separated from the channels above +partly by a margin of bone and partly by a membrane. It receives its name +from its termination at the tympanum, or middle ear, from which it is +separated only by a thin membrane.(122) Both the scala vestibula and the +scala tympani belong to the outer portion of the internal ear and are, for +this reason, filled with the perilymph. At their upper ends they +communicate with each other by a small opening, making by this means one +continuous canal through the cochlea. This canal passes from the vestibule +to the tympanum and, in so doing, goes entirely around + +*The Scala Media.*--This division of the cochlea lies parallel to and +between the other two divisions. It is above the scala tympani and below +the scala vestibula, and is separated from each by a membrane. The scala +media belongs to the membranous portion of the internal ear and is, +therefore, filled with the endolymph. It receives the terminations of +fibers from the auditory nerve and may be regarded as the true sense organ +of hearing. The nerve fibers terminate upon the membrane known as the +_basilar membrane_, which separates it from the scala tympani. This +membrane extends the length of the cochlear canals, and is stretched +between a projecting shelf of bone on one side and the outer wall of the +cochlea on the other. It is covered with a layer of epithelial cells, some +of which have small, hair-like projections and are known as the _hair +cells_. Above the membrane, and resting partly upon it, are two rows of +rod-like bodies, called the _rods of Corti_. These, by leaning toward each +other, form a kind of tunnel beneath. They are exceedingly numerous, +numbering more than 6000, and form a continuous series along the margin of +the membrane. + + [Fig. 154] + + + Fig. 154--*Diagram* illustrating passage of sound waves through the ear. + + +*How We Hear.*--The sound waves which originate in vibrating bodies are +transmitted by the air to the external ear. Passing through the auditory +canal, the waves strike against the membrana tympani, setting it into +vibration. By the bridge of bones and the air within the middle ear the +vibrations are carried to and concentrated upon the liquid in the internal +ear (Fig. 154). From here the vibrations pass through the channels of the +cochlea and set into vibration the contents of the scala media and +different portions of the basilar membrane. This serves as a stimulus to +the fibers of the auditory nerve, causing them to transmit impulses which, +on passing to the brain, produce the sensation of hearing. + +Much of the peculiar structure of the cochlea is not understood. Its +minute size and its location in the temporal bone make its study extremely +difficult. The connection of the scala vestibula with the scala tympani, +and this with the middle ear, is necessary for the passage of vibrations +through the internal ear. Its liquids, being practically incompressible +and surrounded on all sides by bones, could not otherwise yield to the +movements of the stapes. (See Practical Work.) The rods of Corti are +thought to act as dampers on the basilar membrane, to prevent the +continuance of vibrations when once they are started. + +*Detection of Pitch.*--The method of detecting tones of different pitch is +not understood. Several theories have been advanced with reference to its +explanation, one of the most interesting being that proposed by Helmholtz. +This theory is based on our knowledge of sympathetic vibrations. The +basilar membrane, while continuous throughout, may be regarded as made up +of many separate cords of different lengths stretched side by side. A tone +of a given pitch will set into vibration only certain of these cords, +while tones of different pitch will set others into vibration. + +Another theory is that the basilar membrane responds to all kinds of +vibrations and the analysis of sound takes place in the brain. + +A third view is that the filaments from the hair cells, rather than the +basilar membrane, respond to the vibrations and in turn stimulate the +terminations of the nerve fibers. + + [Fig. 155] + + + Fig. 155--*Diagram* showing how wax may plug the auditory canal and cause + deafness. + + +*Hygiene of the Ear.*--The ear, being a delicate organ, is frequently +injured by careless or rough treatment. The removal of the ear wax by the +insertion of pointed instruments has been found to interfere with the +natural method of discharge and to irritate the membrane. It should never +be practiced. It is unnecessary in the healthy ear thus to cleanse the +auditory canal, as the wax is passed by a natural process to where it is +easily removed by a damp cloth. If the natural process is obstructed, +clean warm water and a soft linen cloth may be employed in cleansing the +canal, without likelihood of injury. Clean warm water may also be +introduced into the auditory canal as a harmless remedy in relieving +inflammation of the auditory canal and of the middle ear. Children's ears +are easily injured, and it goes without saying that they should never be +pulled nor boxed. + +It frequently happens that a mass of wax collects in the auditory canal +and closes the passage so completely as to cause deafness (Fig. 155). This +may come about without pain and so gradually that one does not think of +seeking medical aid. Such masses are easily removed by the physician, the +hearing being then restored. Both for painful disturbances of the ear and +for the gradual loss of hearing, the physician should be consulted. + +*The Hearing of School Children.*--School children not infrequently have +defective hearing and for this reason are slow to learn. The hearing is +easily tested with a watch, the normal ear being able to hear the watch +tick at a distance of at least two feet. Pupils with defective hearing +should, of course, have medical attention, and in the classroom should be +seated where they can hear to the best advantage. + +*Summary.*--Sound waves constitute the external stimuli for the sensation +of hearing. They consist of progressive vibratory movements of the air +that originate in vibrating bodies. Through the larynx and the ear, sound +waves are utilized by the body in different ways, but chiefly as a means +of communication. The larynx produces sound waves which are reenforced and +modified by the air passages. The ear supplies suitable conditions for the +action of sound waves upon nerve cells. Both the ear and the larynx are +constructed with special reference to the nature and properties of sound +waves, and they illustrate the body's ability to adjust itself to, and to +make use of, its physical environment. + +*Exercises.*--1. For what different purposes are sound waves employed in +the body? + +2. How do sound waves originate? How are they transmitted? How do they +differ from the waves on water? + +3. How are sound waves able to act as nerve stimuli? + +4. Describe two methods of reenforcing sound waves. Which method is +employed in the body? + +5. Name all the parts of the body that are directly or indirectly +concerned in the production of sound. + +6. Describe the larynx. + +7. Describe the condition of the vocal cords in speaking and in ordinary +breathing. + +8. How are sounds differing in pitch and intensity produced by the larynx? + +9. How is the sound produced by the vocal cords changed into speech? + +10. What parts of the ear are concerned in transmitting sound waves? + +11. Give the purposes of the middle ear. + +12. Trace a sound wave from a bell to the basilar membrane, and trace the +impulse that it causes from there to the brain. + +13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the +semicircular canals. + +14. Give directions for the proper care of the ear. + + + +PRACTICAL WORK + + +*To illustrate the Origin of Sound.*--1. Strike a bell an easy blow and +hold some light substance, as a pith ball attached to a thread, against +the side, noting the result. 2. Sound a tuning fork by striking it against +the table. Test it for vibrations as above, or by letting the vibrating +prongs touch the surface of water. 3. Pluck a string of a guitar or +violin, and find proof that it is vibrating while giving out sound. + +*To show the Transmission of Sound.*--1. Vibrate a tuning fork and press +the stem against a table or desk. The vibrations which are reenforced in +this way will be heard in all parts of the room. Now press one end of a +wooden rod, as a broom handle, against the table, and bring the stem of +the vibrating fork against the other end. The vibrations now move down the +stick to the table, from whence they are communicated to the air. Observe +that the sound waves, to reach the ear, must pass through the rod, the +table, and the air. 2. Fasten the tuning fork to a flat piece of cork by +pressing the stem into a small hole in the center. Vibrate the fork and +let the cork rest on the surface of water in a half-filled tumbler on the +table. The sound will, as before, pass to the table and then to the air. +Observe that in this case the vibrations are transmitted by a liquid, a +solid, and by the air. Compare this action with the transmission of sound +waves by different portions of the ear. + +*To show Effects of Sound Waves.*--1. Place two large tuning forks of the +same pitch, and mounted on thin boxes for reenforcing their vibrations, +near each other on a table. Vibrate one of the forks for a moment and then +stop it by means of the hand. Observe that the other fork has been set in +vibration. (This experiment does not work with forks of different pitch.) +2. While holding a thin piece of paper against a comb with the open lips, +produce musical tones with the vocal cords. These will set the paper in +vibration, producing the so-called "comb music." 3. Examine the disk in a +telephone which is set in vibration by the voice. Observe that it is a +thin disk and, like the membrane of the ear, has air on both sides of it. + +*To show the Reenforcement of Sound.*--1. Vibrate a tuning fork in the air, +noting the feebleness of the tone produced. Then hold the stem against a +door or the top of a table, noting the difference. 2. Hold a vibrating +tuning fork over a tall jar, or bottle, and gradually add water. If the +vessel is sufficiently tall, a depth will be reached where the air in the +vessel reenforces the sound from the fork. 3. Hold a vibrating fork over +the mouth of a small fruit jar, partly covered with a piece of cardboard. +By varying the size of the opening, a position will be found where the +sound is reenforced. If not successful at first, try bottles and jars of +different sizes. + +*To illustrate the Manner of Vibration of the Liquid in the Internal +Ear.*--Tie a piece of dental rubber over the end of a glass or wooden tube +about half an inch in diameter and six inches in length. Fill the tube +entirely full of water and, without spilling, tie a piece of thin rubber +tightly over the other end. Holding the tube horizontally, press the +rubber in at one end and note that it is pushed out at the other end. Make +an imitation of a vibration with the finger against the rubber at one end +of the tube and note the effect at the other end. To what do the tube and +the rubber on the ends of the tube correspond in the internal ear? + + [Fig. 156] + + + Fig. 156--*Simple apparatus* for demonstrating the larynx. + + +*To show the Plan of the Larynx.*--Cut from stiff paper four pieces of +different shapes as indicated in Fig. 156. (The piece to the left should +have a length of about six inches, the others proportionally large.) The +largest represents the thyroid cartilage, the next in size the cricoid, +and the two smallest the arytenoid cartilages. By means of pins, or +threads, connect these with each other according to the description of the +larynx on page 253. With this simple model the movements of the different +cartilages and their effect upon the vocal cords may be illustrated. + +*To show the Relation of the Movements of the Vocal Organs to the +Production of Different Sounds.*--1. Lightly grasp the larynx with the +fingers while talking. Observe the changes, both in the position and shape +of the larynx, in the production of sounds of different pitch. 2. Observe +the difference in the action of the muscles of respiration in the +production of loud and faint sounds. 3. Pronounce slowly the vowels, A, E, +I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape +of the mouth, the position of the tongue, and the action of the lips in +each case. + +*To demonstrate the Ear.*--Examine a dissectible model of the ear, locating +and naming the different parts. Trace as far as possible the path of the +sound waves and find the termination of the auditory nerve. Note also the +relative size of the parts, and calculate the number of times the model is +larger than the natural ear. _Suggestion_: The greatest diameter of the +internal ear is about three fourths of an inch. + +In an extended course it is a profitable exercise to dissect the ear of a +sheep or calf, observing the auditory canal, middle ear, bridge of bones, +and the tympanic membrane with attached malleus and tensor tympanic +muscle. Pass a probe from the nasal pharynx through the Eustachian tube +into the middle ear. With bone forceps or a fine saw, split open the +petrous portion of the temporal bone and observe the cochlea and the +semicircular canals. By a careful dissection other parts of interest may +also be shown. + + + + +CHAPTER XXII - THE EYE + + +Sight is considered the most important of the sensations. It is the chief +means of bringing the body into proper relations with its surroundings +and, even more than the sensation of hearing, is an avenue for the +reception of ideas. The sense organs for the production of sight are the +eyes; the external stimulus is + +*Light.*--Light, like sound, consists of certain vibrating movements, or +waves. They differ from sound waves, however, in form, velocity, and in +method of origin and transmission. Light waves are able to pass through a +vacuum, thus showing that they are not dependent upon air for their +transmission. They are supposed to be transmitted by what the physicist +calls ether--a highly elastic and exceedingly thin substance which fills +all space and penetrates all matter. As a rule, light waves originate in +bodies that are highly heated, being started by the vibrations of the +minute particles of matter. + +Light is influenced in its movements by various conditions. In a substance +of uniform density it moves with an unchanging velocity and in a straight +line. If it enters a less dense, or rarer, substance, its velocity +increases; if one more dense, its velocity diminishes; and if it enters +either the rarer or denser substance in any direction other than +perpendicularly, it is bent out of its course, or _refracted_. If it +strikes against a body lying in its course, it may be thrown off +(_reflected_), or it may enter the body and either be passed on through +(_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is +transformed into heat. + +*Kinds of Reflection.*--Waves of light striking against the smooth surface +of a mirror are thrown off in definite directions, depending on the angle +at which they strike. (Illustrate by holding a mirror in the direct rays +of the sun.) But light waves that strike rough surfaces are reflected in +practically all directions and apparently without reference to the angle +at which they strike. (Illustrate by placing a piece of white paper in the +direct rays of the sun. It matters not from what direction it is viewed, +waves of light strike the eye.) This kind of reflection is called +_diffusion_, and it serves the important purpose of making objects +visible. The light waves passing out in all directions from objects which +have received light from the sun, or some other luminous body, enable them +to be seen. + + [Fig. 157] + + + Fig. 157--*Diagram illustrating passage of light waves.*On the right the + light is transmitted by the glass, reflected by the mirror, refracted by +the prism, and absorbed by the black cloth. On the left the light from the + candle forms an image by passing through a small hole in a cardboard and + falling upon a screen. + + +*Formation of Images.*--Another principle necessary to seeing is that of +refraction. _Refraction_ means the bending, or turning, of light from a +straight course. One of the most interesting effects of refraction is the +formation of images of objects, such as may be accomplished by light from +them passing in a certain manner through convex lenses. If, for example, a +convex lens be moved back and forth between a candle and a screen in a +dimly lighted room, a position will be found where a picture of the candle +falls upon the screen. This picture, called the _image_, results from the +refraction of the candle light in passing through the lens. + + [Fig. 158] + + + Fig. 158--*Diagram illustrating formation of images.* On the right the +image is formed by a double convex lens; on the left by the lenses of the + eye. The candle flame represents a luminous, or light-giving, body; but + light passes from the large arrow by reflection. (See text.) + + +In order to form an image, the light waves spreading out from the object +must be brought together, or focused. Focusing means literally the +bringing of light to a point, but it is evident in the formation of an +image that all the waves are not brought to a single point. If they were, +there would be no image. In the example of the candle given above, the +explanation is as follows: + +The light from the candle comes from a great number of separate and +distinct points in the candle flame. The lens, by its peculiar shape, +bends the waves coming from any single point so that they are brought to a +corresponding point on the screen. Furthermore, the points of focused +light are made to occupy the same relative positions on the screen as the +points from which they emanate in the candle flame (Fig. 158). This is why +the area of light on the screen has the same form as the candle, or makes +an image of it. The same explanation applies if, instead of the luminous +candle, a body that simply reflects light, as a book, is used. + +*The Problem of Seeing.*--What we call _seeing_ is vastly more than the +stimulation of the brain through the action of light upon afferent +neurons. It is the _perceiving _of all the different things that make up +our surroundings. If one looks toward the clear sky, he receives a +_sensation of light_, but sees no object. He may also get a sensation of +light with the eyelids closed, if he turn the eyes toward the window or +some bright light. But how different when the light from various objects +enters the eyes. There is apparently no consciousness of light, but +instead a consciousness of the size, form, color, and position of the +objects. _Seeing is perceiving objects._ Stimulation by the light waves is +only the means toward this end. The chief problem in the study of sight is +that of determining _how light waves enable us to become conscious of +objects._ + +*Sense Organs of Sight.*--The sense organs of sight consist mainly of the +two eyeballs. Each of these is located in a cavity of the skull bones, +called the _orbit_, where it is held in position by suitable tissues and +turned in different directions by a special set of muscles. A cup-shaped +receptacle is provided within the orbit, by layers of fat, and a smooth +surface is supplied by a double membrane that lies between the fat and the +eyeball. In front the eyeballs are provided with movable coverings, called +the _eyelids_. These are composed of dense layers of connective tissue, +covered on the outside by the skin and lined within by a sensitive +membrane, called the _conjunctiva_. At the base of the lids the +conjunctiva passes to the eyeball and forms a firmly attached covering +over its front surface. This membrane prevents the passage of foreign +materials back of the eyeball, and by its sensitiveness stimulates effort +for the removal of irritating substances from beneath the lids. The +eyelashes and the eyebrows are also a means of protecting the eyeballs. + +*The Eyeball*, or globe of the eye, is a device for _focusing_ light upon +a sensitized nervous surface which it incloses and protects. In shape it +is nearly spherical, being about an inch in diameter from right to left +and nine tenths of an inch both in its vertical diameter and from front to +back. It has the appearance of having been formed by the union of two +spherical segments of different size. The smaller segment, which forms +about one sixth of the whole, is set upon the larger and forms the +projecting transparent portion in front. The walls of the eyeballs are +made up of three separate layers, or coats--an _outer coat_, a _middle +coat_, and an _inner coat_ (Fig. 159). + + [Fig. 159] + + + Fig. 159--*Diagram of the eyeball in position.* 1. Yellow spot. 2. Blind +spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens. + 7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris + containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13. +Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle +of eyelid. 16. Bone. _A._ Posterior chamber containing the vitreous humor. + _B._ Anterior chamber containing the aqueous humor. + + +*The Outer Coat* surrounds the entire globe of the eye and consists of two +parts--the sclerotic coat and the cornea. The _sclerotic coat_ covers the +greater portion of the larger spherical segment and is recognized in front +as "the white of the eye." It is composed mainly of fibrous connective +tissue and is dense, opaque, and tough. It preserves the form of the +eyeball and protects the portions within. It is pierced at the back by a +small opening which admits the optic nerve, and in front it becomes +changed into the peculiar tissue that makes up the cornea. + +The _cornea_ forms the transparent covering over the lesser spherical +segment of the eyeball, shading into the sclerotic coat at its edges. It +has a complex structure, consisting in the main of a transparent form of +connective tissue. It serves the purpose of admitting light into the +eyeball. + +*The Middle Coat* consists of three connected portions--the _choroid coat_, +the _ciliary processes_, and the _iris_. These surround the larger +spherical segment. All three parts are rich in blood vessels, containing +the blood supply to the greater portion of the eyeball. + +The _choroid coat_ lies immediately beneath the sclerotic coat at all +places except a small margin toward the front of the eyeball. It is +composed chiefly of blood vessels and a delicate form of connective tissue +that holds them in place. It contains numerous pigment cells which give it +a dark appearance and serve to absorb surplus light. Near where the +sclerotic coat joins the cornea, the choroid coat separates from the outer +wall and, by folding, forms many slight projections into the interior +space. These are known as the _ciliary processes_. The effect of these +folds is to collect a large number of capillaries into a small space and +to give this part of the eyeball an extra supply of blood. Between the +ciliary processes and the sclerotic coat is a small muscle, containing +both circular and longitudinal fibers, called the _ciliary muscle_. + +The _iris_ is a continuation of the choroid coat across the front of the +eyeball. It forms a dividing curtain between the two spherical segments +and gives the color to the eye. At its center is a circular opening, +called the _pupil_, which admits light to the back of the eyeball. By +varying the size of the pupil, the iris is able to regulate the amount of +light which passes through and it employs for this purpose two sets of +muscular fibers. One set of fibers forms a thin band which encircles the +pupil and serves as a sphincter to diminish the opening. Opposing this are +radiating fibers which are attached between the inner and outer margins of +the iris. By their contraction the size of the opening is increased. Both +sets of fibers act reflexively and are stimulated by variations in the +light falling upon the retina. + + [Fig. 160] + + + Fig. 160--*Diagram showing main nervous elements in the retina.* Light +waves stimulate the rods and cones at back surface of the retina, starting +impulses which excite the ganglion cells at the front surface. Fibers from + the ganglion cells pass into the optic nerve. + + +*The Inner Coat, or Retina.*--This is a delicate membrane containing the +expanded termination of the optic nerve. It rests upon the choroid coat +and spreads over about two thirds of the back surface of the eyeball. +Although not more than one fiftieth of an inch in thickness, it presents a +very complex structure, essentially nervous, and is made up of several +distinct layers. Of chief importance in the outer layer are the cells +which are acted upon directly by the light and are named, from their +shape, the _rods_ and _cones_. In contact with these, but occupying a +separate layer, are the ends of small afferent nerve cells. These in turn +communicate with nerve cells in a third layer, known as the ganglion +cells, that send their fibers into the optic nerve (Fig. 160). + +In the center of the retina is a slight oval depression having a faint +yellowish color, and called, on that account, the _yellow spot_. This is +the part of the retina which is most sensitive to light. Directly over the +place of entrance of the optic nerve is a small area from which the rods +and cones are absent and which, therefore, is not sensitive to light. This +is called the _blind spot_. (See Practical Work.) + +*The Crystalline Lens.*--Immediately back of the iris and touching it is a +transparent, rounded body, called the crystalline lens. This is about one +fourth of an inch thick and one third of an inch through its long +diameter, and is more curved on the back than on the front surface. It is +inclosed in a thin sheath, called the _membranous capsule_, which connects +with a divided sheath from the sides of the eyeball, called the +_suspensory ligament_ (Fig. 159). Both the lens and the capsule are highly +elastic. + +*Chambers and "Humors" of the Eyeball.*--The crystalline lens together with +the suspensory ligament and the ciliary processes form a partition across +the eyeball. This divides the eye space into two separate compartments, +which are filled with the so-called "humors" of the eye. The front cavity +of the eyeball, which is again divided in part by the iris, is filled with +the _aqueous_ humor. This is a clear, lymph-like liquid which contains an +occasional white corpuscle. It has a feeble motion and is slowly added to +and withdrawn from the eye. It is supplied mainly by the blood vessels in +the ciliary processes and finds a place of exit through a small lymph duct +at the edge of the cornea (Fig. 159). + +The back portion of the eyeball is filled with a soft, transparent, +jelly-like substance, called the _vitreous_ humor. It is in contact with +the surface of the retina at the back and with the attachments of the lens +in front, being surrounded by a thin covering of its own, called the +_hyaloid membrane_. The aqueous and vitreous humors aid in keeping the +eyeball in shape and also in focusing. + +*How we see Objects.*--To see an object at least four things must happen: + +1. Light must pass from the object into the eye. Objects cannot be seen +where there is no light or where, for some reason, it is kept from +entering the eye. + +2. The light from the object must be focused (made to form an image) on +the retina. In forming the image, an area of the retina is stimulated +which corresponds to _the form of the object_. + +3. Impulses must pass from the retina to the brain, stimulating it to +produce the sensations. + +4. The sensations must be so interpreted by the mind as to give an +impression of the object. + +*Focusing Power of the Eyeball.*--The eyeball is essentially a device for +focusing light. All of its transparent portions are directly concerned in +this work, and the portions that are not transparent serve to protect and +operate these parts and hold them in place. Of chief importance are the +crystalline lens and the cornea. Both of these are lenses. The cornea with +its inclosed liquid is a plano-convex lens, while the crystalline lens is +double convex.(123) Because of the great difference in density between the +air on the outside and the aqueous humor within, the cornea is the more +powerful of the two. The crystalline lens, however, performs a special +work in focusing which is of great importance. The iris also aids in +focusing since it, through the pupil, regulates the amount of light +entering the back chamber of the eyeball and causes it to fall in the +center of the crystalline lens, the part which focuses most accurately. + + [Fig. 161] + + + Fig. 161--*Diagram showing changes in shape of crystalline lens* to adapt + it to near and distant vision. + + +*Accommodation.*--A difficulty in focusing arises from the fact that the +degree of divergence of the light waves entering the eye from different +objects, varies according to their distance. Since the waves from any +given point on an object pass out in straight lines in all directions, the +waves that enter the eye from distant objects are at a different angle +from those that enter from near objects. In reality waves from distant +objects are practically parallel, while those from very near objects +diverge to a considerable degree. To adjust the eye to different distances +requires some change in the focusing parts that corresponds to the +differences in the divergence of the light. This change, called +_accommodation_, occurs in the crystalline lens.(124) In the process of +accommodation, changes occur in the shape of the crystalline lens, as +follows: + +1. In looking from a distant to a near object, the lens becomes more +convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary +because the greater divergence of the light from the near objects requires +a greater converging power on the part of the lens.(125) + +2. In looking from near to distant objects, the lens becomes flatter and +thinner (Fig. 161). This change is necessary because the less divergent +waves from the distant objects require less converging power on the part +of the lens. + +The method employed in changing the shape of the lens is difficult to +determine and different theories have been advanced to account for it. The +following, proposed by Helmholtz, is the theory most generally accepted: + +The lens is held in place back of the pupil by the suspensory ligament. +This is attached at its inner margin to the membranous capsule, and at its +outer margin to the sides of the eyeball, and entirely surrounds the lens. +It is drawn perfectly tight so that the sides of the eyeball exert a +continuous tension, or pull, on the membranous capsule, which, in its +turn, exerts pressure on the sides of the lens, tending to flatten it. +This arrangement brings the elastic force of the eyeball into opposition +to the elastic force of the lens. The ciliary muscle plays between these +opposing forces in the following manner: + +_To thicken the lens_, the ciliary muscle contracts, pulling forward the +suspensory ligament and releasing its tension on the membranous capsule. +This enables the lens to thicken on account of its own elastic force. _To +flatten the lens_, the ciliary muscle relaxes, the elastic force of the +eyeball resumes its tension on the suspensory ligament, and the membranous +capsule resumes its pressure on the sides of the lens. This pressure, +overcoming the elastic force of the lens, flattens it. + +*Movements of the Eyeballs.*--In order that the light may enter the +eyeballs to the best advantage, they must be moved in various directions. +These movements are brought about through the action of six small muscles +attached to each eyeball. Four of these, named, from their positions, the +superior, inferior, internal, and external recti muscles, are attached at +one end to the sides of the eyeball and at the other end to the back of +the orbit (Fig. 162). These, in the order named, turn the eyes upward, +downward, inward, and outward. The other two, the superior and inferior +oblique muscles, aid in certain movements of the recti muscles and, in +addition, serve to rotate the eyes slightly. The movements of the eyeballs +are similar to those of ball and socket joints. + + [Fig. 162] + + + Fig. 162--*Exterior muscles of eyeball.* + + +*Binocular Vision.*--In addition to directing the eyeballs so that light +may enter them to the best advantage from different objects, the muscles +also enable two eyes to be used as one. Whenever the eyes are directed +toward the same object, an image of this object is formed on the retina of +each. Double vision is prevented only by having the images fall on +corresponding places in the two eyes. This is accomplished by the muscles. +In each act of seeing, it becomes the task of the superior and inferior +recti muscles to keep the eyes in the same plane, and of the external and +internal recti muscles to give just the right amount of convergence. If +slight pressure is exerted against one of the eyes, the action of the +muscles is interfered with and, as a consequence, one sees double. The +advantages of two eyes over one in seeing lie in the greater distinctness +and broader range of vision and in the greater correctness of judgments of +distance. + +*Visual Sensations.*--The visual sensations include those of _color_ and +those of a _general sensibility to light_. Proof of the existence of these +types of sensation is found in color blindness, a defect which renders the +individual unable to distinguish certain colors when he is still able to +see objects. Color sensations are the results of light waves of different +lengths acting on the retina. While the method by which waves of one +length produce one kind of sensation and those of another length a +different sensation is not understood, the cones appear to be the portions +of the retina acted on to produce the color. On the other hand, the rods +are sensitive to all wave lengths and give general sensibility to light. + +*Visual Perceptions.*--"Seeing" is very largely the mental interpretation +of the primary sensations and the conditions under which they occur. For +example, our ability to see objects in their natural positions when their +images are inverted on the retina is explained by the fact that we are not +conscious of the retinal image, but of the mind's interpretation of it +through experience. Experience has also taught us to locate objects in the +direction toward which it is necessary to turn the eyes in order to see +them. In other words, we see objects in the direction from which the light +enters the eyes. That the object is not always in that direction is shown +by the image in the mirror. The apparent size and form of objects are +inferences, and they are based in part upon the size and form of the area +of the retina stimulated. We judge of distance by the effort required to +converge the eyes upon the objects, by the amount of divergence of the +waves entering the pupil, and also by the apparent size of the object. + +*The Lachrymal Apparatus.*--Seeing requires that the light penetrate to the +retina. For this reason all the structures in front of the retina are +transparent. One of these structures, the cornea, on account of its +exposure to the air, is liable to become dry, like the skin, and to lose +its transparency. To preserve the transparency of the cornea, and also to +lubricate the eyelids and aid in the removal of foreign bodies, a +secretion, called _tears_, is constantly supplied. + + [Fig. 163] + + + Fig. 163--*Diagram of irrigating system of the eye.* After wetting the + eyeball the tears may also moisten the air entering the lungs. + + +The lachrymal, or tear, glands are situated at the upper and outer margins +of the orbits. They have the general structure of the salivary glands and +discharge their liquid by small ducts beneath the upper lids. From here +the tears spread over the surfaces of the eyeballs and find their way in +each eye to two small canals whose openings may be seen on the edges of +the lids near the inner corner (Fig. 163). These canals unite to form the +_nasal duct_, which conveys the tears to the nasal cavity on the same side +of the nose. When by evaporation the eyeball becomes too dry, the lids +close reflexively and spread a fresh layer of tears over the surface. Any +excess is passed into the nostrils, where it aids in moistening the air +entering the lungs. + + + +HYGIENE OF THE EYE + + +*Defects in Focusing.*--The delicacy and complexity of the sense organs of +sight render them liable to a number of imperfections, or defects, the +most frequent and important being those of focusing. Such defects not only +result in the imperfect vision of objects, but they throw an extra strain +upon the nervous system and may render the process of seeing exceedingly +painful. + +A normal eye is able, when relaxed, to focus light accurately from objects +which are twenty feet or more away and to accommodate itself to objects as +near as five inches. An eye is said to be _myopic_, or _short-sighted_, +when it is unable to focus light waves from distant objects, but can only +distinguish the objects which are near at hand. In such an eye the ball is +too long for the converging power of the lenses, and the image is formed +in front of the retina (_C_, Fig. 164). + + [Fig. 164] + + + Fig. 164--*Diagrams illustrating long-sightedness and short-sightedness*, + and method of remedying these defects by lenses. _A._ Normal eye. _B._ + Long-sighted eye. _C._ Short-sighted eye. + + +A _long-sighted_, or _hypermetropic_, eye is one which can focus light +from distant objects, but not from near objects. In such an eye the ball +is too short for the converging power of the lenses and the image tends to +form back of the retina (_B_, Fig. 164). These defects in focusing are +remedied by wearing glasses with lenses so shaped as to counteract them. +Short-sightedness is corrected by concave lenses and long-sightedness by +convex lenses, as shown in diagrams above. + +_Astigmatism_ is another defect in the focusing power of the eye. In +astigmatism the parts of the eye fail to form the image in the same plane, +so that all portions of the object do not appear equally distinct. Certain +parts of it are indistinct, or blurred. The cause is found in some +difference in curvature of the surfaces of the cornea or crystalline lens. +It is corrected by lenses so ground as to correct the particular defects +present in a given eye. + +Whenever defects in focusing are present, particularly in astigmatism, +extra work is thrown on the ciliary muscle as well as the muscles that +move the eyeballs. The result is frequently to induce a condition, known +as _muscle weakness_, which renders it difficult to use the eyes. Even +after the defect in focusing has been remedied, the muscles recover slowly +and must be used with care. For this reason glasses should be fitted by a +competent oculist(126) as soon as a defect is known to exist. When one is +unduly nervous, or suffers from headache, the eyes should be examined for +defects in focusing (page 326). + +*Eye Strain and Disease.*--The extra work thrown upon the nervous system +through seeing with defective eyes, especially in reading and other close +work, is now recognized as an important cause of disease. Through the tax +made upon the nervous system by the eyes, there may be left an +insufficient amount of nervous energy for the proper running of the vital +processes. As a result there is a decline of the health. Ample proof that +eye strain interferes with the vital processes and causes ill health, is +found in the improvements that result when, by means of glasses, this is +relieved. + +*The Eyes of School Children.*--School children often suffer from defects +of vision which render close work burdensome, and cause headache, general +nervousness, and disease. Furthermore, the visual defects may be unknown +both to themselves and to their parents. Pupils showing indications of +eye-strain should be examined by an oculist, and fitted with glasses +should defects be discovered.(127) The precaution, adopted by many +schools, of having the eyes of all children examined by a competent +physician employed for the purpose, is most excellent and worthy of +imitation. + +*Reading Glasses.*--Many people whose eyes are weak, because slightly +defective, find great relief in the use of special glasses for reading and +other close work. By using such glasses they may postpone the time when +they are compelled to wear glasses constantly. It is in the close work +that the extra strain comes upon the eyes, and if this is relieved, one +can much better withstand the work of distant vision. The reading glasses +should be fitted by a competent oculist, and used only for the purpose for +which they are intended. + +*General Precautions in the Use of the Eyes.*--If proper care is exercised +in the use of the eyes, many of their common ailments and defects may be +avoided. Any one, whether his eyes are weak or strong, will do well to +observe the following precautions: + +1. Never read in light that is very intense or very dim. 2. When the eyes +hurt from reading, stop using them. 3. Never hold a book so that the +smooth page reflects light into the eyes. The best way is to sit or stand +so that the light passes over the shoulder to the book. 4. Never study by +a lamp that is not shaded. 5. Practice cleanliness in the care of the +eyes. Avoid rubbing the eyes with the fingers unless sure the fingers are +clean. + +If the eyes are weak, use them less and avoid, if possible, reading by +artificial light. Weak eyes are sometimes benefited by bathing them in +warm water, or with water containing enough salt to make them smart +slightly. Boracic acid dissolved in water (40 grains to 4 ounces of +distilled water) is also highly recommended as a wash for weak eyes. + + [Fig. 165] + + + Fig. 165--*Method of procedure in lifting the eyelid* (Pyle). + + +*Removal of Foreign Bodies from the Eyes.*--Foreign bodies embedded in the +eyeball should be removed by the oculist or physician. Small particles of +dust or cinder may be removed without the aid of the physician, by +exercising proper care. First let the tears, if possible, wash the +offending substance to the corner of the eye, or edge of the lid, where it +can be removed with a soft cloth. If it sticks to the ball or the under +surface of the lid, it will be necessary to find where it is located, and +then dislodge it from its position. Begin by examining the lower lid. Pull +it down sufficiently to expose the inner surface, and, if the foreign +substance be there, wipe it off with the hem of a clean handkerchief. If +it is not under the lower lid, it will be necessary to fold back the upper +lid. "The patient is told to look down, the edge of the lid and the lashes +are seized with the forefinger and thumb of the right hand (Fig. 165), and +the lid is drawn at first downward and forward away from the globe; then +upward and backward over the point of the thumb or forefinger of the left +hand, which is held stationary on the lid, and acts as a fulcrum."(128) +The foreign body is now removed in the same manner as from the lower lid. +A large lens may be used to good advantage in finding the irritating +substance. + +*Strong Chemicals in the Eyes.*--Students in the laboratory frequently, +through accident, get strong chemicals, as acids and bases, in the eyes. +The first thing to do in such cases is quickly and thoroughly to _flood +the eyes with water_. Any of the chemical which remains may then be +counteracted by the proper reagent, care being taken to use a very dilute +solution. To counteract an acid, use sodium bicarbonate (cooking soda), +and for bases use a very dilute solution of acetic acid (vinegar). To +guard against getting the counteractive agent too strong for the inflamed +eye, it should first be tried on an eye that has not been injured. + +*Summary.*--The nervous impulses that cause the sensation of sight are +started by light waves falling upon a sensitized nervous surface, called +the retina. By means of refractive agents, forming a part of the eyeball +in front of the retina, light from different objects is focused and made +to form images of the objects upon the surface. In this way the light is +made to stimulate a portion of the retina corresponding to the form of the +object. This, _the image method of stimulation_, enables the mind to +recognize objects and to locate them in their various positions. While the +greater portion of the eyeball is concerned in the focusing of light, the +crystalline lens, operated by the ciliary muscle, serves as the special +instrument of accommodation. Muscles attached to the eyeballs turn them in +different directions, and so adjust them with reference to each other that +double vision is avoided. + +*Exercises.*--1. Under what conditions are light waves reflected, +refracted, and absorbed? + +2. Why does the body not need a light-producing apparatus, corresponding +to the larynx in the production of sound? + +3. How is the light from a candle made to form an image? + +4. What different things must happen in order that one may see an object? + +5. Make a sectional drawing of the eyeball, locating and naming all the +parts. + +6. Of what parts are the outer, middle, and inner coats of the eyeball +made up? + +7. What portions of the eyeball reflect light? What absorb light? What +transmit light? What refract light? + +8. Show how the iris, the crystalline lens, the retina, the ciliary +muscle, and the cornea aid in seeing. + +9. Trace a wave of light from a visible object to the retina. + +10. Why does not the inverted image on the retina cause us to see objects +upside down? + +11. What change occurs in the shape of the crystalline lens when we look +from distant to near objects? From near to distant objects? Why are these +changes necessary? How are they brought about? + +12. How does the method of adjustment, or accommodation, of the eyeball +differ from that of a telescope or a photographer's camera? + +13. With two eyes how are we kept from seeing double? + +14. What different purposes are served by the tears. Trace them from the +lachrymal glands to the nostrils. + +15. Show how the proper lenses remedy short- and long-sightedness. + +16. Describe the conjunctiva and give its functions. Why should it be so +sensitive? + +17. How may eye strain cause disease in parts of the body remote from the +eyes? + +18. How does "image stimulation" differ from light stimulation in general? + + + +PRACTICAL WORK + + +*To illustrate Simple Properties of Light.*--1. Heat an iron or platinum +wire in a clear gas flame. Observe that when a high temperature is reached +it gives out light or becomes luminous. + +2. Cover one hand with a white and the other with a black piece of cloth, +and hold both for a short time in the direct rays of the sun. Note and +account for the difference in temperature which is felt. + +3. Stand a book or a block of wood by the side of an empty pan in the +sunlight, so that the end of the shadow falls on the bottom of the pan. +Mark the place where the shadow terminates and fill the pan with water. +Account for the shadow's becoming shorter. + +4. Place a coin in the center of an empty pan and let the members of the +class stand where the coin is barely out of sight over the edges of the +pan. Fill the pan with water and account for the coin's coming into view. +Show by a drawing how light, in passing from the water into the air, is so +bent as to enter the eye. + +5. With a convex lens, in a darkened room, focus the light from a candle +flame so that it falls on a white screen and forms an image of the candle. +Observe that the image is inverted. In a well-lighted room focus the light +from a window upon a white screen. Show that, as the distance from the +window to the screen is changed, the position of the lens must also be +changed. (Accommodation.) + +6. Hold a piece of cardboard, about eight inches square and having a +smooth, round hole an eighth of an inch in diameter in the center, in +front of a lighted candle in a darkened room. Back of the opening place a +muslin or paper screen (Fig. 157). Observe that a dim image is formed. +Account for the fact that it is inverted. Hold a lens between the +cardboard and the screen so that the light passes through it also. The +image should now appear smaller and more distinct. + + [Fig. 166] + + + Fig. 166--*Diagram* for proving presence of the blind spot. + + +*To prove the Presence of the Blind Spot.*--Close the left eye and with the +right gaze steadily at the spot on the left side of this page (Fig. 166). +Then starting with the book a foot or more from the face, move it slowly +toward the eye. A place will be found where the spot on the right entirely +disappears. On bringing it nearer, however, it is again seen. As the book +is moved forward or backward, the position of the image of this spot +changes on the retina. When the spot cannot be seen, it is because the +image falls on the blind spot. + +*Dissection of the Eyeball.*--Procure from the butcher two or three +eyeballs obtained from cattle. After separating the fat, connective +tissue, and muscle, place them in a shallow vessel and cover with water. +Insert the blade of a pair of sharp scissors at the junction of the +sclerotic rotic coat with the cornea and cut from this point nearly around +the entire circumference of the eyeball, passing near the optic nerve. +Spread open in the water and identify the different parts from the +description in the text. Open the second eyeball in water by cutting away +the cornea. Examine the parts in front of the lens. + + [Fig. 167] + + + Fig. 167--*Model* for demonstrating the eyeball. + + +*To illustrate Accommodation.*--Paste together the ends of a strip of stiff +writing paper (two by five inches) making a ring a little less than three +inches in diameter. This is to represent the crystalline lens. Now paste a +piece of thin paper (two by seven inches) upon a second strip of the same +size, leaving an open place in the middle for the insertion of the paper +lens. A flexible piece of cardboard (three by twelve inches) is now bent +into the form of a half circle and to its ends are fastened the strips of +paper containing the ring. Make a small hole in each of the four corners +of the bent cardboard. Through these holes pass two loops of thread, or +fine string, in opposite directions, letting the ends hang loose from the +cardboard. + +When everything is in position, the tension from the cardboard flattens +the paper lens, while pulling the strings releases this tension and +permits the lens to become more rounded. With this simple device the +changes in the curvature of the lens for near and distant vision are +easily shown. + + + + +CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL + + + "To cure was the voice of the Past: to prevent is the divine + whispering of To-day." + + +As stated in the introduction to our study, the fundamental law of hygiene +is the law of harmony: _Habits of living must harmonize with the plan of +the body._ Having acquainted ourselves with the plan of the body, we may +now review briefly those conditions that help or hinder its various +activities. The hygiene already presented in connection with the study of +the various organs may be condensed into general rules, or laws, as +follows: + +1. Of exercise: Exercise daily the important groups of muscles. + +2. Of form: Preserve the natural form of the body. + +3. Of energy: Observe regular periods of rest and exercise and avoid +exhaustion. + +4. Of nutriment: Eat moderately of a well-cooked and well-balanced diet +and drink freely of pure water. + +5. Of respiration: Breathe freely and deeply of pure air and spend a part +of each day out of doors. + +6. Of nervous poise: Suppress wasteful and useless forms of nervous +activity, avoid nervous strain, and practice cheerfulness. + +7. Of cleanliness: Keep the body and its immediate surroundings clean. + +8. Of restraint: Abstain from the unnecessary use of drugs as well as from +the practice of any form of activity known to be harmful to the body. + +9. Of elimination: Observe all the conditions that favor the regular +discharge of waste materials from the body. + +Obedience to these laws is of vast importance in the proper management of +the body. They should, indeed, be so thoroughly impressed upon the mind as +to become fixed habits. There are, however, other conditions that relate +to this problem, and it is to these that we now turn. These conditions +have reference more specifically to + +*The Prevention of Disease.*--While the average length of life is not far +from thirty-five years, the length of time which the average individual is +capable of living is, according to some of the lowest estimates, not less +than seventy years. This difference is due to disease. People do not, as a +rule, die on account of the wearing out of the body as seen in extreme old +age, but on account of the various ills to which flesh is heir. It is true +that many people meet death by accident and not a few are killed in wars, +but these numbers are small in comparison with those that die of bodily +disorders. The prevention of disease is the greatest of all human +problems. Though the fighting of disease is left largely to the physician, +much is to be gained through a more general knowledge of its causes and +the methods of its prevention. + +*Causes of Disease.*--Disease, which is some _derangement of the vital +functions_, may be due to a variety of causes. Some of these causes, such +as hereditary defects, are remote and beyond the control of the +individual. Others are the result of negligence in the observance of +well-recognized hygienic laws. Others still are of the nature of +influences, such as climate, the house in which one lives, or one's method +of gaining a livelihood, that produce changes in the body, imperceptible +at the time, but, in the long run, laying the foundations of disease. And +last, and most potent, are the minute living organisms, called microbes or +germs, that find their way into the body. Although there are two general +kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_ +(one-celled animals), most of our germ diseases are caused by bacteria. + +*Effects of Germs.*--While there are many kinds of germs that have no ill +effect upon the body and others that are thought to aid it in its work, +there are many well-known varieties that produce effects decidedly +harmful. They gain an entrance through the lungs, food canal, or skin, +and, living upon the fluids and tissues, multiply with great rapidity +until they permeate the entire body. Not only do they destroy the +protoplasm, but they form waste products, called _toxins_, which act as +poisons. Diseases caused by germs are known as infectious, or contagious, +diseases.(129) The list is a long one and includes smallpox, measles, +diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe, +malaria, yellow fever, and others of common occurrence. In addition to the +diseases that are well pronounced, it is probable that germs are +responsible also for certain bodily ailments of a milder character.(130) + +*Avoidance of Germ Diseases.*--The problem of preventing diseases caused by +germs is an exceedingly difficult one and no solution for all diseases has +yet been found. One's chances of avoiding such diseases, however, may be +greatly enhanced: + +1. By strengthening the body through hygienic living so that it offers +greater resistance to the invasions of germs. + +2. By living as far as possible under conditions that are unfavorable to +germ life. + +3. By understanding the agencies through which disease germs are spread +from person to person. + +*Conditions Favorable and Unfavorable for Germs.*--Conditions favorable for +germ life are supplied by animal and vegetable matter, moisture, and a +moderate degree of warmth. Hence disease germs may be kept alive in damp +cellars and places of filth. Even living rooms that are poorly lighted or +ventilated may harbor them. Water may, if it contain a small per cent of +organic matter, support such dangerous germs as those of typhoid fever. +Fresh air, sunlight, dryness, cleanliness, and a high temperature, on the +other hand, are destructive of germs. The germs in impure water, as +already noted (page 165), are destroyed by boiling. + +*How Germs are Spread.*--Some of the more common methods by which the germs +of disease are spread, and by so doing find new victims, are as follows: + +1. _By Means of Foods._--Foods, on account of the locality in which they +are produced or the method of gathering or of handling-them, may become +contaminated with germs, which are then transported with the foods to the +consumer. + +2. _By Means of Dust._--Material containing germs, _e.g._, discharges from +the throat and lungs, will on drying form dust. This is lifted with other +fine particles by the air and may be carried quite a distance. The dust +from public halls and other places where people congregate is the kind +most likely to contain disease germs. Dust should be breathed as little as +possible and only through the nostrils. Where one is compelled, as in +sweeping, to breathe dust-laden air for some time, he should inhale +through a moistened sponge, or cloth, tied in front of the nostrils. + +3. _By Means of Domestic Pets and Different Kinds of Household +Vermin._--Germs sticking to the bodies of small animals are carried about +and may be easily communicated to people. By this means, rats, mice, +bedbugs, etc., where such exist, are frequently the means of spreading +disease; and particularly dangerous, on this account, is the common house +fly. Feeding as it does on filth of all kinds, it is easy for it to +transfer the bacteria that may stick to its body to the food which is +supplied to the table. The proper screening of houses and the destruction +of material in which flies may develop, such as the refuse from stables, +are necessary precautions. + +Germs are spread also by the clothing of people, by railroad and steamship +lines, by the mails, and by the natural elements. In fact, any kind of +carrier, in or upon which germs can live, may serve as a means of +spreading those of certain kinds. + +*Public Sanitation.*--The general conditions under which germs may thrive +and some of the means by which they are scattered, emphasize the practical +value of measures which have for their purpose the making of one's +surroundings more wholesome and hygienic. Such measures may be directed +both toward one's immediate surroundings--the home--and toward the +neighborhood, town, or city in which one lives. The hygienic conditions of +primary importance in every city or town are as follows: + +1. An adequate public supply of pure water. + +2. An efficient system of underground pipes for the removal of sewage. + +3. An efficient system for removing from the streets and alleys everything +of the nature of waste. + +4. Prevention, by enforcement of ordinances, of spitting upon sidewalks +and the floors of public halls and conveyances. + +5. A hospital or sanitarium in which people can be cared for when sick +with infectious diseases. + +In the larger cities other hygienic measures demand attention, such as +provisions for parks and playgrounds, the proper housing of the poor of +the city, and the suppression of the smoke and dust nuisances. Crowded +together as people are in the cities, the welfare of each individual +depends in a large measure upon the welfare of all. Hence the problems of +public sanitation are matters in which all are vitally concerned. + +*Sanitary Conditions of the Home.*--The home, being the feeding and resting +place for the entire family, is the most important factor in one's +physical, as well as moral, environment. For this reason there is no place +where careful attention to hygienic requirements will yield better +results. Much of the danger from germs may be prevented by instituting and +maintaining proper sanitary conditions in and about the home. + +One of the first requisites of the home is a suitable location for the +house. The house should be built upon ground that is well drained, and if +natural drainage be lacking, artificial drainage must be supplied. It +should not be situated nearer than a quarter of a mile to any marsh or +swamp and, if so near as that, it ought to be on the side from which the +wind usually blows. A stone foundation should be provided, and at least +eighteen inches of ventilated air space should be left between the ground +and the floor. Ample provisions must be made for pure air and sunlight in +all the rooms. The cellar, if one is desired, needs to be constructed with +special care. It should be perfectly dry and provided with windows for +light and ventilation. Adequate means must also be provided, by sewage +pipes and other methods, for the disposal of all waste. Where drainage +pipes are provided, care must be taken to prevent the entrance of sewer +gas into the house and also the passage of material from these pipes into +the water supply. The placing and connecting of sewer pipes should, of +course, be under the direction of a plumber. + +*The Water Supply.*--Since water readily takes up and holds the impurities +with which it comes in contact, it should be exposed as little as possible +in the process of collecting. Where cistern water is used, care must be +taken to prevent filth from the roof (Fig. 168), water pipes, or soil from +getting into the reservoir. Water should be collected from the roof only +after it has rained long enough for the roof and pipes to have been +thoroughly cleaned. The cistern should have no leaks (Fig. 169), and the +top should be tightly closed to prevent the entrance of small animals and +rubbish. + + [Fig. 168] + + + Fig. 168--*Contamination of cistern water* by birds nesting in the gutter + trough. + + +Shallow wells are to be condemned, as a rule, because of the likelihood of +surface drainage (Fig. 169), and water from springs should, for the same +reason, be used with caution. Deep wells that are kept clean usually may +be relied on to furnish water free from organic impurities, but such water +often holds in solution so much of mineral impurities as to render it +unfit for drinking. The presence in water of any considerable quantity of +the compounds of iron or calcium makes it objectionable for regular use. + + [Fig. 169] + + + Fig. 169--*Sources of contamination of cistern and well water.* + Illustration shows liability of contamination from surface drainage and + from entrance of filth at top. + + +*Hygienic Housekeeping.*--However carefully a house has been constructed +from a sanitary standpoint, the constant care of an intelligent +housekeeper is required to keep it a healthful place in which to live. +Daily cleaning and airing of all living rooms are necessary, while such +places as the kitchen, the cellar, and the closets need extra +thoughtfulness and, at times, hard work. Moreover, the problem is not all +indoors. The immediate premises must be kept clean and sightly, and all +decaying vegetable and animal matter should be removed. Home sanitation +consists, not of one, but of many, problems, all more or less complex. +None of these can be slighted or turned over to a novice. + +*Destruction of Infectious Material.*--At times the housekeeping has to be +directed especially toward hygienic requirements, such an occasion being +the sickness of one of the inmates with some contagious disease. Unless +special precautions are taken, the disease will spread to other members of +the household and may reach people in the neighborhood. Not only must +great care be exercised that nothing used in connection with the sick +shall serve as a carrier of disease, but germs passing from the patient +should, as far as possible, be actually destroyed. All discharges from the +body likely to contain bacteria, should be burned or treated with +disinfectants and buried deeply at a remote distance from the water supply +to the house. + +After recovery all clothing, bedding, and furniture used in connection +with the sick should be disinfected or burned. The room also in which the +sick was cared for should be thoroughly disinfected and cleaned; in some +instances the woodwork ought to be repainted and the walls repapered or +calcimined. The purpose is, of course, to destroy all germs and prevent, +by this means, a recurrence of the disease. + +*Fumigation.*--To destroy germs in the air or adhering to the walls of +rooms, furniture, clothing, etc., fumigation is employed. This is +accomplished by saturating the air of rooms with some vapor or gas which +will destroy the germs. Fumigation is quite generally employed in the +general cleaning after the patient leaves his room. This, to be effective, +must be thorough. Formaldehyde is considered the best disinfectant for +this purpose, and it should be evaporated with heat in the proportion of +one half pint of the 40 per cent solution to 1000 cu. ft. of space. Since +formaldehyde is inflammable and easily boils over, it has to be evaporated +with care. It should be boiled in a tall vessel (a tin or copper vessel +which holds about four times the quantity to be evaporated) over a quick +fire, the room being tightly closed (openings around windows and doors +plugged with cotton or cloth). After three or four hours the room may be +opened and thoroughly aired. Since formaldehyde is most disagreeable to +breathe, one should not attempt to occupy the room until it is free from +the gas. This will require a day or more of thorough ventilation. + +*Facts Relating to the Spread of Certain Diseases.*--The problem of +preventing disease in general often resolves itself into the problem of +preventing the spread of some particular disease. It is then of vital +importance to know the special method by which the germs of this disease +leave the body of the patient and are conveyed to the bodies of others. +Some of these methods are novel in the extreme, and are not at all in +accord with prevailing notions. Particularly is this true of that disease +known as + +*Malaria, or Malarial Fever.*--This disease, so common in warm climates and +also prevalent to a large extent in the temperate zones, is due to animal +germs (protozoa), which attack and destroy the red corpuscles of the +blood. These germs, it is found, pass from malarial patients to others +through the agency of a variety of mosquitoes known as _Anopheles_. In +sucking the blood of a malarial patient, the mosquito first infects her +own body.(131) In the body of the mosquito the germs undergo an essential +stage of their development, after which they are injected beneath the skin +of whomsoever the mosquito feeds upon. For the spreading of malaria, then, +two conditions are necessary: first, there must be people who have the +disease; and second, there must be in the neighborhood the special variety +of mosquito that spreads the disease. If either condition be lacking, the +disease is not spread. The malarial mosquito (_Anopheles_) may be +distinguished from the harmless variety (_Culex_) by the position which it +assumes in resting, as shown in Fig. 170. + + [Fig. 170] + + + Fig. 170--*Mosquitoes* in resting position. (From Howard's _Mosquitoes_.) + On left the malarial mosquito (_Anopheles_); on the right the harmless + mosquito (_Culex_). + + +*Remedies against Mosquitoes.*--The natural method of preventing the spread +of malaria is, of course, the destruction of mosquitoes. This is +accomplished by draining pools of water where they are likely to breed, +and by covering pools of water that cannot be drained with crude petroleum +or kerosene. The kerosene, by destroying the larvae, prevents the +development of the young. In communities where such measures have been +diligently carried out, the mosquito pest has been practically eliminated. +Other methods are also under investigation, such as the stocking of +shallow bodies of water with varieties of fish that feed upon the mosquito +larvae. + + [Fig. 171] + + + Fig. 171--*Stegomyia*, the yellow-fever mosquito (after Howard). + + +*Yellow Fever.*--This scourge of the tropics is, like malaria, caused by +animal germs. It is also propagated in the same manner as malaria, but by +a different variety of mosquito (_Stegomyia_, Fig. 171). The stamping out +of yellow fever in Havana, the Panama Canal Zone, and other places, +through the destruction of this variety of mosquito, affords ample proof +of the correctness of the "mosquito theory." + + [Fig. 172] + + + Fig. 172--*Consumption germs* from the spit of one having the disease. + Highly magnified and stained. (Huber's _Consumption and Civilization_.) + + +*Consumption*, or tuberculosis of the lungs, spoken of as the "white +plague," was among the first diseases shown to be due to bacteria. +Consumption is now recognized as an infectious disease, though not so +readily communicated as some other diseases. Several methods are +recognized by which the germs are passed from the sick to the well, the +most important being as follows: + +1. By personal contact of the sick with the well, especially in kissing. + +2. By the sputum, or spit, which, if allowed to dry, is blown about as +dust and breathed into the lungs(132) (Fig. 172). + +3. By means of objects (drinking cups, tableware, etc.) that have been +handled by consumptives. + +4. By infectious material associated with houses or rooms in which +consumptives have lived. + +These methods of spreading consumption suggest the necessity for the +greatest care, on the part of both the patient and those having him in +charge.(133) The material coughed up from the lungs and throat should be +collected on cloths or paper handkerchiefs and afterwards burned. The +house where a consumptive has lived should be disinfected, repapered or +calcimined, and thoroughly cleaned before it is again occupied. The inside +woodwork should also be repainted. The approaches to the house where the +patient may have expectorated should be disinfected and cleaned. Since the +germs are able to live in the soil, fresh lime or wood ashes should be +spread around the doorsteps and along the walks. + +*Typhoid Fever*, one of our most dangerous diseases, is caused by germs +(bacteria) that enter the body through the food canal. They attack certain +glands in the walls of the small intestine, where they produce toxins that +pass with the germs to all parts of the body. Typhoid fever germs spread +from those having the disease to others, chiefly through the discharges +from the bowels and the kidneys. The germs contained in these, if not +destroyed by disinfectants, find their way into the soil, or into sewage, +where they may be picked up by water and widely distributed. Finding +suitable places, such as those containing decaying material, the germs may +rapidly increase in number, and from these sources find their way into the +bodies of new victims. They are likely, on account of manures, to get on +vegetables; on account of uncleanly methods of milking, to get into the +milk supply; and from sewerage outlets, to get into the oysters that grow +in bays and harbors near seaboard cities; but they are most frequently +introduced into the body through the drinking of impure water. + +*Diphtheria*, also known as "membranous croup," is caused by germs that +attack the membranes of the throat. This most dangerous of children's +diseases is spread chiefly by discharges from the mouth and throat. These +should be collected on cloths and burned, or rendered harmless with +disinfectants. The disease may be spread also by objects brought into +contact with the mouth, such as cups, toys, pencils, etc. Children are +known to have diphtheria germs in the mouth for some time after recovering +from the disease, and should, for this reason, be kept away from other +children until pronounced safe by the physician. + +The _antitoxin method_ of treating diphtheria has robbed this disease of +much of its terror, yet it not infrequently happens that the physician is +called too late to administer this remedy to the best advantage. Since +certain cases of diphtheria are likely to be mistaken for croup, the +parent frequently does not realize the serious condition of the child. A +croupy cough _that lasts through the day_, or a sore throat which shows +small white patches, are indications of diphtheria. + +*Scarlet Fever, Measles, Chicken Pox, and Smallpox*, on account of the +eruptions of the skin which attend them, are classed as eruptive diseases. +As the eruptions heal, scales separate from the skin, and these are +supposed to be the chief means of spreading the germs. Attention must be +given to the destruction of these scales by burning or thoroughly +disinfecting all objects, such as clothing, bedding, etc., that may serve +as carriers of them. Those having eruptive diseases should be confined to +their rooms as long as the scales continue to separate from the body. + +*Vaccination.*--The method of preventing smallpox known as vaccination, +which has been practiced since its discovery in 1796 by Jenner, has always +proved effective. In some instances the sore arm causes considerable +inconvenience, but this generally results from neglect to cleanse the arm +thoroughly before applying the virus, or from contact of the sore with the +clothing later. The virus should be applied by a physician and the wound +should be protected after the operation. If discomfort is felt when it +"takes," medical advice should be sought. + +*Isolation*, or quarantining, is a most important method of combating +contagious diseases. By removing the sick from the well many outbreaks of +disease are quickly checked. Isolation of individual patients, and +sometimes of infected neighborhoods, is absolutely necessary; and while +this works a hardship to the few, it is frequently the only safeguard of +the many. The community, on the other hand, should make ample provision +for the care of the afflicted in the way of hospitals, or sanitaria, and +if it is deemed necessary to remove people from their homes, they should +not be subjected to unnecessary hardship. + +Where one is sick from some contagious disease in the home and there is +liability of communicating it to the other members of the family, _room +isolation_ should be practiced. Infection cannot spread through solid +walls, and where the doors, and the cracks around the doors, are kept +completely closed and the usual precautions are observed by those +attending the patient, the other inmates of the house can be protected +from the disease. + +*The Physician and His Work.*--In combating disease the services of the +physician are a prime necessity. The special knowledge which he has at his +command enables the conflict to be carried on according to scientific +requirements and vastly increases the chances for recovery. He should be +called early and his directions should be carefully followed. Everything, +however, must not be left to the physician, for recovery depends as much +upon proper nursing and feeding as upon the drugs that are administered. +Of great importance is _the saving of the energy of the patient_, and to +accomplish this visitors should, as a rule, be excluded from the sick +room. + +*Precautions in Recovery from Disease.*--Many diseases, if severe, not only +leave the body in a weakened condition, but may, through the toxins which +the germs deposit, cause untold harm if the patient leaves his bed or +resumes his usual activities too soon. Especially is this true of typhoid +fever,(134) diphtheria, scarlet fever, and measles. Rheumatism and +affections of the heart, lungs, kidneys, and other bodily organs +frequently follow these diseases, as the result of slight exposure or +exertion before the body has sufficiently recovered from the effects of +the toxins. To guard against such results, certain physicians require +their patients to keep their beds for a week, or longer, after apparent +recovery from diseases like typhoid fever, diphtheria, and scarlet fever. + +*Relation of Vocation to Disease.*--With a few exceptions, the pursuit of +one's vocation, or calling in life, does not supply either the quantity or +the kind of activity that is most in harmony with the plan of the body. +Especially is this true of work that requires most of the time to be spent +indoors, or which exercises but a small portion of the body. The effect of +such vocations, if not counteracted, is to weaken certain organs, thereby +disturbing the functional equilibrium of the body--a result that may be +brought about either by the overwork of particular organs or by lack of +exercise of others. Herein lies the explanation of the observed fact that +people of the same calling in life have similar diseases. + +*A Special Problem for the Brain Worker.*--Farthest removed from those +forms of activity which harmonize with the plan of the body, and which +therefore are most hygienic, is that class of workers known as the +professional class, or the "brain workers." This class includes not only +the members of the learned professions--law, medicine, and the ministry--but +a vast army of business men, engineers, teachers, stenographers, office +clerks, etc., a class that is ever increasing as our civilization +advances. It is this class in particular that must give attention to those +conditions that indirectly, but profoundly, influence the bodily +well-being and must seek to obviate if possible such weaknesses as the +occupation induces. + +*The Remedy* lies in two directions--that of spending sufficient time away +from one's work to allow the body to recover its normal condition, and +that of counteracting the effect of the work by special exercise or other +means. In many cases the first symptoms of weakness indicate a suitable +remedy. Thus exhaustion from overwork suggests rest and recreation. The +diverting of too much blood from other parts of the body to the brain +suggests some form of exercise which will equalize the circulation. If +feebleness of the digestive organs is being induced, some natural method +of increasing the blood supply to these organs is to be looked for. And +effects arising from lack of fresh air and sunlight are counteracted by +spending more time out of doors. + +*Exercise as a Counteractive Agent.*--In counteracting tendencies to +disease and in the maintenance of the functional equilibrium of the body, +no agent has yet been discovered of greater importance than physical +exercise, when applied systematically and persistently. This may consist +of exercises that call into play all the muscles of the body, or which are +concentrated upon special parts. When general tonic effects are desired, +the exercise should be well distributed; but when counteractive or +remedial effects are wanted, it must be applied chiefly to the parts that +are weak or that have not been called into action by the regular work. +Unfortunately, health is sometimes confused with physical strength and +exercise is directed toward the stronger parts of the body with the effect +of making them still stronger. Not only is health not to be measured by +the pounds that one can lift or by some gymnastic feat that one can +perform, but the possession of great muscular power may, if the heart and +other vital organs be not proportionally strong, prove a menace to the +health. This being true, one having his health primarily in view will use +physical exercise, in part at least, as a means of building up organs that +are weak. Since the body, like a chain, can be no stronger than its +weakest part, this is clearly the logical method of fortifying it against +disease. + +*Value of Work.*--Although there may exist in one's vocation certain +tendencies to disease, it must not be inferred that work in itself is +detrimental to health. Health demands activity, and those forms of +activity that provide a regular and systematic outlet for one's surplus +energy and compel the formation of correct habits of eating, sleeping, and +recreating best serve the purpose. Work furnishes activity of this kind +and serves also as a safeguard against the unhealthful and immoral habits +contracted so often from idleness. Even physical exercise which has for +its purpose the reenforcement of the body against disease may frequently +consist of useful work without diminishing its hygienic effects. + +*The Mental Attitude.*--While a proper thoughtfulness and care for the body +is both desirable and necessary, it is also true that over-anxiety about, +or an unnatural attention to, the needs of the body reacts unfavorably +upon the nervous system. Observance of the laws of health, therefore, +should be natural and without special effort--a matter of habit. The +attention should never be turned with anxiety upon any organ or process, +but the mental attitude should at all times be that of _confidence in the +power of the body organization to do its work_. Fear and morbidity, which +are disturbing and paralyzing factors, should be supplanted by courage, +cheerfulness, and hopefulness. + +Let it be borne in mind that hygienic living requires nothing more than +the application of the same intelligence and practical common sense to the +care of the body that the skillful mechanic applies to an efficient, but +delicate, machine. And, just as in the case of the machine, care of the +body keeps its efficiency at the maximum and lengthens the period that it +may be used. This end and aim of hygienic living is best attained by +cultivating that attitude of mind toward the body that avoids interference +in the vital processes and permits the natural appetites, sensations, and +desires to indicate very largely the body's needs. + +*Attitude toward Habit-forming Drugs.*--Among the different substances +introduced into the body, either as foods or as medicines, are a number +which have the effect of developing an artificial appetite or craving +which leads to their continued use. Since the effect of such substances is +usually harmful and since they tend to engraft themselves upon communities +as social customs, they present a twofold relation to the general problem +of keeping well. The individual may be injured through the personal use +which he makes of them, or he may be injured through the effect which they +have upon relatives or friends or upon society at large. Since our social +environment is a factor in health little less important than our physical +environment, the conditions that make for their continuance should be more +generally understood. + +*How Social Agencies perpetuate the Use of Habit-forming Drugs.*--When the +use of some habit-forming drug has risen to the importance of a general +custom, a number of conditions arise which tend to continue its use, even +though the fact may be quite generally known that the substance does harm. +In the first place, those who have formed the habit suffer inconvenience +and distress when deprived of its use. In the second place, a number of +people will have become interested in the production and sale of the +substance, and these will lose financially if it is discontinued. In the +third place, those of the rising generation will, from imitation or +persuasion, be constantly acquiring the habit before they are sufficiently +mature to decide what is best for them. Thus may the use of a substance +most harmful, such as the opium of the Chinese, be indefinitely +continued--a species of slavery from which the individual finds it hard to +escape. + +Such is human nature and such are the forces and influences of human +society, that the freeing of a people from the bondage of some +habit-forming drug cannot be accomplished without strenuous and persistent +effort. Education, persuasion, the good example of abstainers, and legal +restrictions must be pitted against the forces that make for its +continuance. Such a struggle is now in progress in all civilized countries +relative to the use of alcoholic beverages.(135) + +*How the Use of Alcohol became a Social Custom.*--The general use of +alcohol as a beverage may be accounted for by three facts. Alcohol is a +habit-forming drug; it has a stimulating effect which many have found +agreeable; and being a product of the fermentation of fruit juices and +other liquids containing sugar, it is easily obtained. Through the +operation of these causes the human family became habituated very early to +the use of alcohol. The "wine" of primitive man, however, did little harm +as compared with the alcoholic liquors of modern times. It was a weak +solution and on account of the crude methods of manufacture and storage +could only be produced in limited quantities. Perhaps the worst effect of +its early use was the establishment of a general belief in its power to +benefit, since this laid the foundation for excess in its use when the +developments of a later period made it possible. + +During the eleventh century the method of making alcoholic drinks from +starch-producing substances, such as wheat, barley, and potatoes, became +quite generally known, and also the method of concentrating them by +distillation. This knowledge made possible the manufacture of alcoholic +drinks in large quantities and in considerable variety. Alcoholic +indulgence was now no longer the pastime of the few, but the privilege of +all. Its evil effects followed as a matter of course; and as these became +more and more apparent, there began the struggle to restrict the +consumption of alcohol which has continued with varying success to the +present time. + +*Counts against Alcohol.*--The statements found in different parts of this +book relative to the effects of alcohol upon the body may here be +summarized as follows:-- + +1. Alcohol has an injurious effect upon the white corpuscles of the blood +and lessens the power of the body to resist attacks of disease (pages 35, +98). + +2. Alcohol injures the heart and the blood vessels (page 56). + +3. Alcohol causes diseases of the liver and kidneys and interferes with +the discharge of waste through these organs (pages 210, 212). + +4. Alcohol interferes seriously with the regulation of the body +temperature (page 271). + +5. Alcohol is one of the worst enemies to the nervous system (pages 326, +332-334. 336, 337). + +6. Through its effect upon the nervous system and through its interference +with the production of bodily energy (page 195), alcohol greatly +diminishes the efficiency of the individual. + +7. The taking of alcohol in amounts that apparently do not harm the +tissues is, nevertheless, liable to produce a habit which leads to its use +in amounts that are decidedly harmful. + +*Alcohol and the Social Environment.*--Our social environment includes the +people with whom we are directly or indirectly associated. The presence in +any community of those who are immoral, inefficient, or defective, places +a burden upon those who are mentally and physically capable and renders +them liable to results which are the outgrowth of weakness or viciousness. +The fact that alcohol causes pauperism, crime, and general inefficiency, +thereby rendering the social environment less conducive to what is best in +life, is plainly evident. To realize how alcohol harms the individual +through its effects upon society in general, one has only to take into +account his dependence upon society for intellectual and moral stimuli, +for industrial and economic opportunity, for protection, and for general +conditions that make for health and happiness. As we strive to improve our +physical environment, so should we also strive for the betterment of +social conditions. + +*Industrial Use of Alcohol.*--Interesting and instructive in this +connection is the fact that alcohol is, after all, a substance capable of +rendering great service to humanity. The injury which it causes is the +result of its misuse. Though unfit for introduction into the human body, +except in the most guarded manner, it is adapted to a great variety of +uses outside of the body. A combustible substance which is readily +convertible into a gas, it may be substituted for gasoline in the cooking +of food, lighting of dwellings, and the running of machinery. As a solvent +for gums, resins, essential oils, etc., it is used in the preparation of +varnishes, extracts, perfumes, medicines, and numerous other substances of +everyday use. Through its chemical interactions, it is used in the +manufacture of ether, chloroform, explosives, collodion, celluloid, +dyestuffs, and artificial silk. In fact, alcohol is stated by one +authority to be, next to water, the most valuable liquid known.(136) + +Opposed to an extensive use of alcohol for industrial purposes is the +guard which the government must keep over its manufacture on account of +its use in beverages. Though alcohol may be profitably manufactured and +sold at thirty cents per gallon, the government revenue stamp of $2.08 per +gallon practically prohibits its use for many purposes. A step toward a +wider application to industrial purposes has been taken by the law +permitting the sale of so-called "denatured"(137) alcohol without the tax +for revenue. This law has proved beneficial to some extent, though the +practical solution of the problem is still remote. + +*Nicotine and Social Custom.*--The influences which brought about a general +use of tobacco are similar to, though not identical with, those that +engrafted alcohol upon society. The drug nicotine is a habit-forming +substance and the plant producing it is easily cultivated.(138) Its +immediate effect upon the user is generally agreeable, acting as a +stimulant to some, but having a soothing effect upon the nerves of others. +Moreover, a strong deterring factor in its use is lacking, since its +harmful effects are not readily discernible and by many are avoided +through moderation in its use. + +As with alcohol, tobacco is conveniently used to promote sociability among +men, a fact which has much to do with its very general use. If it could be +limited to social purposes, it would likely do little harm, but the habit, +once started, is continued without reference to sociability--a matter of +selfish indulgence. In fact, one effect of tobacco is to cause the user to +become less sensitive to the rights of others, this being evidenced by +smokers who do not hesitate to make rooms and public halls almost +unbearable to those unaccustomed to tobacco. + +*Counts against Nicotine.*--The physiological objections to the use of +tobacco, as already stated (pages 56, 92, 326, 333, 336), are the +following:-- + +1. The use of tobacco before one reaches maturity stunts the growth. The +boy who uses it cannot develop into so strong and capable a man as he +would by leaving it alone. + +2. Tobacco injures the heart. + +3. Tobacco injures the air passages, especially when inhalation is +practiced. + +4. Tobacco injures the nervous system and by this means interferes in a +general way with the bodily processes. For the same reason it interferes +with mental and moral development, the cigarette being a chief cause of +criminal tendencies in boys. + +5. In some cases tobacco injures the vision. + +6. The tobacco habit is expensive and is productive of no good results. + +*Tobacco and the Rising Generation.*--The problem of limiting the use of +tobacco to the point where it would do slight harm, in comparison to what +it now does, would be solved if those under twenty years of age could be +kept from using it. But few would then acquire the habit, and those who +did would not be so seriously injured. In our own country it lies within +the province of the home and the school to bring about this result. The +fact that parents use tobacco is no reason why the boys should also +indulge. The decided difference in effects upon the young and upon the +mature makes this point very clear. Laws protecting boys from the evil +effects of tobacco, not only cigarettes, but other forms as well, are both +just and necessary. + +*Social Custom and the Caffeine Habit.*--By suitable processes a white, +crystalline solid, easily soluble in water, can be separated from the +leaves of tea, and from the berry of the coffee plant. This is the drug +caffeine, the substance which gives to tea and coffee their stimulating +properties, but not their agreeable flavors. Less injurious, on the whole, +than either alcohol or tobacco, caffeine has come into general use in much +the same way as these substances. In a sense, however, caffeine is more +deceptive than either alcohol or nicotine, because the usual mode of +preparing tea and coffee gives them the appearance of real foods. The +housewife who would feel condemned in purchasing caffeine put up as a drug +somehow feels justified when she extracts it from plant products in the +regular preparation of the meal. + +*Counts against Caffeine.*--People of vigorous constitutions and of active +outdoor habits are injured but slightly, if at all, by either tea or +coffee when these are used in moderation. As already stated (pages 56, +167, 326, 329), they do harm when used to excess and, in special cases, in +very small amounts, in one of the following ways:-- + +1. By stimulating the nervous system, thereby causing nervousness and +insomnia and interfering with vital organs. + +2. By introducing a waste which forms uric acid into the body, thereby +throwing an extra burden upon the organs of elimination. + +In this connection it may also be stated that there appears to be little, +if any, real advantage to the healthy body from the use of either tea or +coffee, beyond that of temporary stimulation and the gratification of an +appetite artificially acquired. Hence the large sums of money expended for +these substances in this country yield no adequate returns. + +*Caffeine Restrictions Necessary.*--Though with many the cup of tea or +coffee at breakfast does no harm, but gives an added pleasure to the meal, +there is no question but that the use of caffeine beverages should be +greatly curtailed. Children should not be permitted to drink either tea or +coffee. Brain workers and indoor dwellers generally should use these +substances very sparingly, and people having a tendency to indigestion, +nervousness, constipation, rheumatism, or diseases of the heart, kidneys, +or liver frequently find it best to omit them altogether. + +*Caffeine and "Soft" Drinks.*--Recently the practice has sprung up of using +caffeine as a constituent of certain drinks supplied at the soda-water +fountains. Such drinks usually purport to be made from the kola nut, which +contains caffeine, or to consist of extracts from the plants which yield +cocoa and chocolate, when in reality they consist of artificial mixtures +to which caffeine has been added. Those using these beverages are +stimulated as they would be by tea or coffee and soon acquire the habit +which makes them regular customers. Chief harm comes to the children who +frequent the soda fountains and to those who, on account of constitutional +tendencies, should avoid caffeine in all of its forms. It is generally +understood that the so-called "soft" drinks are harmless. If this +reputation is to be maintained, those containing caffeine must be +excluded. + +*Danger from Certain Medicinal Agents.*--Among the most valuable drugs used +by the physician in the treatment of disease are several, such as +morphine, chloral, and cocaine, which possess the habit-forming +characteristic. Sad indeed are the cases in which some pernicious drug +habit has been formed through the reckless administration of such +medicines. Even the taking of such a drug as quinine as a "tonic" tends to +develop a dependence upon stimulation which is equivalent to a habit. In +the same list come also the drugs that are taken to relieve a frequently +recurring indisposition, such as headache. The so-called headache powders +are most harmful in their effects upon the nervous system and should be +carefully avoided.(139) + +*Stimulants in Health Unnecessary.*--Stimulants have been aptly styled "the +whips of the nervous system." The healthy nervous system, however, like +the well-disposed and well-fed horse, needs no whip, but is irritated and +harmed through its use. Even in periods of weakness and depression, +stimulants are usually not called for, but a more perfect provision for +hygienic needs. Rest, relaxation, sleep, proper food, and avoidance of +irritation, not stimulants, are the great restorers of the nervous system. +A surplus of nervous energy gained through natural means is more conducive +to health and effective work than any result that can possibly be secured +through drugs. Then withal comes the satisfaction of knowing that one has +the expression of his real self in the way in which he feels and in what +he accomplishes--not a "whipped-up" condition that must be paid for by +weakness or suffering later on. + +*Summary.*--To solve the problem of keeping well, one must live the life +which is in closest harmony with the plan of the body. Such a life, +because of differences in physical organization, as well as differences in +environment and occupation, cannot be the same for all. All, however, may +observe the conditions under which the body can be used without injuring +it and the special hygienic laws relative to the care of different organs. +Causes of disease, whether they be in one's environment, vocation, in his +use of foods or drugs, or in his mode of recreation, must either be +avoided or counteracted. + +While the problem is beset with such difficulties as lack of sufficient +knowledge, inherited weakness, and time and opportunity for doing what is +known to be best for the body, yet study and work that have for their aim +the preservation or improvement of the health are always worth while. +_Health is its own reward._ The expression of the poet, + + "Each morn to feel a fresh delight to wake to life, + To rise with bounding pulse to meet whate'er of work, of care, of +strife, + day brings to me," + +suggests the _joy_ of being well. But the ultimate realization of one's +aims and ambitions in life and the actual prolongation of one's period of +usefulness are _higher and more enduring rewards_. + +*Exercises.*--1. Summarize the different laws of hygiene. Upon what one +fundamental law are these based? + +2. State the important differences between a condition of health and one +of disease. + +3. In what general ways may disease originate in the body? + +4. Describe a model sanitary home. With what special hygienic problems has +the housekeeper to deal? + +5. Describe a method of collecting a wholesome supply of cistern water. +State possible objections to well and spring water. + +6. What means may be employed in preventing the spread of contagious +diseases? + +7. By what means are malaria, typhoid fever, diphtheria, and tuberculosis +spread from one individual to another? + +8. Why are extra precautions necessary in the recovery from certain +diseases, as typhoid fever, diphtheria, and scarlet fever? + +9. How may one's vocation become a cause of disease? What conditions in +the life of a student may, if uncounteracted, lead to poor health? + +10. Of what special value are the parks and pleasure grounds in a city to +the health of its inhabitants? + +11. Discuss the hygienic value of work. + +12. What conditions lead to the continuance of habit-forming substances +after their use has become general? + +13. How is it possible for one not using alcohol to be injured by this +substance? + +14. Discuss the effect of alcoholic abuse upon social environment. + +15. Summarize the rewards of hygienic living. + + + +SUMMARY OF PART II + + +For the maintenance of life the needs of the cells must be supplied and +_the body as a whole must be brought into proper relations with its +surroundings_. The last-named condition requires that the body be moved +from place to place; that its parts be controlled and cooerdinated; and +that it be adjusted in its various activities to external physical +conditions. To accomplish these results there are employed: + +1. The skeleton, or bony framework, which preserves the form of the body +and supplies a number of mechanical devices, or machines, for causing a +variety of special movements. + +2. The muscular system, which supplies the energy necessary for executing +the movements of the body. + +3. The nervous system, which (_a_) controls and cooerdinates the various +activities and (_b_) provides for the _intelligent_ adjustment of the body +to its environment. (Review Summary of Part I, page 215, and consult Fig. +92, page 214.) + + + + + +APPENDIX + + +*Equipment.*--Nearly all of the apparatus and materials called for in this +book may be found in the physical, chemical, and biological laboratories +of the average high school. There should be ready, however, for frequent +and convenient use, the following: One or more compound microscopes with +two-thirds and one-fifth inch objectives; a set of prepared and mounted +slides of the various tissues of the body; a set of dissecting +instruments, including bone forceps; a mounted human skeleton and a +manikin or a set of physiological charts; a set of simple chemical +apparatus including bottles, flasks, test tubes, and evaporating dishes; +and a Bunsen burner or some other means of supplying heat. + +The few chemicals required may be obtained from a drug store or from the +chemical laboratory. Access to a work bench having a set of carpenter's +tools will enable one to prepare many simple pieces of apparatus as they +are needed. + +*Physiological Charts* are easily prepared by teachers or pupils by +carefully enlarging the more important illustrations found in text-books +or by working out original sketches and diagrams. These, if drawn on heavy +Manila paper, may be hung on the wall as needed and preserved +indefinitely. By the use of colors, necessary contrasts are drawn and +emphasis placed on parts as desired. The author has for a number of years +used such home-made charts in his teaching and has found them quite +satisfactory. His plan has been to draw on heavy Manila paper, cut in +sizes of two by three feet, the general outline in pencil and then to mark +over this with the desired colors. There is of course an opportunity for +producing results that are artistic as well as practical, and if one has +time and artistic skill, better results can be obtained. Many of the cuts +in this book are excellently suited to enlargement and, if properly +executed, will provide a good set for general class purposes. + +*Models.*--The use of prepared models of the different bodily organs is +strongly urged. These may be so used in elementary courses as to obviate +much of the dissections upon lower animals. Although the actual tissues +cannot be so well portrayed, the general form and construction of organs +are much better shown. Models well adapted to class or laboratory work are +easily obtained through supply houses. Illustrations of several of these +are shown in connection with the "Practical Work." + + + + + +INDEX + + +Abdomen; dissection of, 169. + +Abdominal cavity, 7, 138, 152. + +Absorption, 173-186. + Defined, 18, 173. + +Accommodation, 379. + To illustrate, 391. + +Acid reactions, 171. + +Acquired reflexes, 314. + +Adipose tissue, 5, 178. + +Afferent neurons, 296. + +Air, 76. + Changes it undergoes in lungs, 101. + Complemental, 89, 103. + Reserve, 89, 103. + Residual, 89, 103. + Tidal, 88, 103. + +Air passages, 80. + +Albuminoids, 119. + Purpose served by, 121. + +Alcohol, + A cause of crime, 333. + Effects on circulation, 55, 56. + Effects on digestion, 167. + Effects on energy supply, 195. + Effects on respiratory organs, 98. + Effects on social environment, 413. + Effect on temperature regulation, 271. + Effects on waste elimination, 212. + General considerations, 412-415. + +Alimentary canal, coats of, 138. + +Alimentary muscles, work of, 159. + +Alkaline reactions, 171. + +Alveoli, 82. + +Amylopsin, 155, 156. + +Anatomy, defined, 1. + +Animal heat, 192. + +Anopheles, 401. + +Antiseptic ointment, 275. + +Antitoxin, 405. + +Appetite, natural, 163. + +Aqueous humor, 377. + +Arachnoid, 299. + +Arteries, 47. + Bronchial, 84. + Functions of, 51. + Pulmonary, 84. + Renal, 202. + To illustrate elasticity of, 62. + Why elastic, 48. + +Articulations, 230-232. + Kinds of, 230. + +Assimilation, 18, 182. + +Astigmatism, 384. + +Atlas, 223. + +Atoms, defined, 105. + +Attraction sphere, 15. + +Auditory canal, 358. + +Auricles, 42. + +Axis, 223. + +Axis cylinder, 284. + +Axon, 283. + Form and length of, 284. + Function of, 306. + Structure of, 284. + + + +Bacteria, 394. + +Ball-and-socket joint, 231. + +Basement membrane, 197. + +Basilar membrane, 363. + +Bathing, 272, 274. + +Biceps muscle, action of, 263. + +Bicuspids, 143. + +Bile, 154, 155. + +Binocular vision, 381. + +Blind spot, 377. + To prove presence of, 390. + +Blood, 24-39. + Changes in, 34. + Checking flow from wounds, 58. + Coagulation of, 31. + Experiments with, 37-39. + Flow of, how regulated, 50. + Functions of, 33. + Hygiene of, 34-36. + Physical properties of, 24. + Quantity of, 33. + Supply to lungs, 82. + Velocity of, 54. + Where found, 24. + +Blood platelets, 25. + +Blood pressure, 52, 70. + +Blood pressure and velocity, 52. + +Blood vessels, to strengthen, 57. + +Body, organization of, 19. + +Bone groups, 223-229. + +Bones, 216-242. + Adaptation of, 228. + Composition, 217. + Gross structure of, 218. + Minute structure of, 219. + Observation on gross structure, 241. + Properties of, 217. + Table of, 229. + To show composition of, 241. + To show minute structure of, 242. + +Bowels, rules for care of, 166. + +Brachial plexus, 302. + +Brain, 280, 288-291. + Disturbed circulation, 327. + Protection of, 299. + +Brain workers, 408. + +Breathing, _see_ Respiration. + Causes of shallow, 92. + Illustrated, 87. + To prevent shallow, 92. + +Breathing exercises, 93. + +Bronchus, 80. + +Bulb, 291. + + + +Caecum, 151, 158. + +Calcium carbonate, 122. + +Calcium phosphate, 122. + +Calorie, defined, 126. + +Cane sugar, 120. + +Canines, 143. + +Capillaries, 50, 64, 249. + Blood pressure at, 70. + Functions of, 51. + Work of, 174. + +Carbohydrates, 119, 125. + Purpose served by, 121. + Storage of, 177. + Tests for, 135. + +Carbon, 134. + +Carbon dioxide, + Final disposition of, 111. + Preparation, 115. + Pressure, 110. + Properties, 110, 115. + +Cardiac cycle, 46. + +Cardiac orifice, 147. + +Carpals, 227. + +Carpus, 228. + +Cell body, 283. + Functions of, 305. + +Cell-division, 16. + +Cell nucleus, 14. + +Cell reproduction, 16. + +Cell structure, 14. + +Cell surroundings, 17. + +Cell wall, 15. + +Cells, 13-23. + Bone, how nourished, 220. + Ciliated epithelial, 81. + Food supply to, 180. + General work of, 17. + Importance of, 15. + Passage of materials to, 183. + Relation to nutrient fluid, 20. + Specialized, 197. + Special work of, 18. + Striated muscle, 244. + +Cerebellum, 290. + Functions of, 317. + +Cerebral functions, localization of, 318. + +Cerebral hemispheres, 289. + +Cerebral peduncles, 290. + +Cerebrum, 288. + Functions of, 317. + +Chlorine, 135. + +Cholesterine, 155. + +Chordae tendineae, 43. + +Choroid coat, 375. + +Chyme, 150. + +Cigarettes, 333. + +Cilia, 81. + To observe, 101. + +Ciliary muscle, 375. + +Ciliary processes, 375. + +Circulation of blood, 40-64. + Causes of, 54. + Discovery of, by Harvey, 40. + Divisions of, 51, 52. + Effects of exercise upon, 63. + Effects of gravity upon, 64. + In a frog's foot, 64. + Organs of, 40-54. + Routes to, 174. + +Coagulation, + Causes of, 31. + Purpose of, 32. + Time required for, 33. + +Cochlea, 362. + +Coffee, + Effects on complexion, 274. + Effects on digestion, 167. + Effects on heart, 56. + +Colds, 193. + Serious nature of, 94. + To cure, 94. + +Colon, parts of, 158. + +Complexion, care of, 273. + +Compound, defined, 104. + +Conduction pathways, 286. + +Conductivity, 304. + +Condyloid joint, 232. + +Conjunctiva, 373. + +Consumption, _see_ Tuberculosis. + +Control of arteries, 319. + +Convolutions, 289. + +Cooerdination, defined, 279. + +Cornea, 375. + +Corpora quadrigemina, 290. + +Corpora striata, 289. + +Corpus callosum, 289, 293. + +Cortex, 288, 294. + +Coughing, 81. + +Cranial cavity, 7, 225. + +Cranial nerves, 296. + +Crura cerebri, 290. + +Crystalline lens, 380. + +Culex, 402. + +Cytoplasm, 15. + + + +Defects in focusing, 383. + +Deformities of skeleton, 233-236. + Correction of, 236. + Prevention of, 235. + +Deglutition, 145. + Steps in, 146. + +Dendrites, 283, 306. + +Dentine, 143. + +Dermis, 264. + +Dextrose, 30, 120, 150. + +Diaphragm, 88. + To illustrate action of, 102. + +Diastole, 46. + +Diaxonic neuron, 283. + +Diet, one-sided, 124. + +Diffusion, 371. + +Digestion, 130-172. + Hygiene of, 160. + Nature of, 130. + Not a simple process, 131. + Of fat, 156. + Purpose of, 177. + Stomach, 148. + +Digestive fluids, 132. + +Digestive organs, 160. + Table of, 138. + +Digestive processes, 130, 141. + Illustrated, 137. + +Diphtheria, 94, 405. + Care after, 211. + +Disaccharides, 120. + +Disease, 392-412. + Causes of, 393. + Eruptive, 405. + Precautions in recovery from, 407. + Prevention of, 393. + +Dislocations, 239. + +Dorsal-root ganglia, 295. + +Drill, "setting up," 237. + +Drugs, effects of, 35, 55, 129, 332. + +Duodenum, 151. + +Dura, 299. + + + +Ear, 358. + Hygiene of, 365. + To demonstrate, 369. + +Ear drum, 359. + +Efferent neurons, 296. + +Element, defined, 104. + +Elevators of the ribs, 87. + +Emetics, 151. + +Emotional states, effects of, 330. + +End bulbs, 342. + +Endocardium, 42. + +Endolymph, 361. + +End-plate, 244. + +End-to-end connections, 286. + +Energy, 107, 186-196. + Bodily control of, 192. + From sun to cells, 191. + How plants store sun's, 189. + Increasing one's bodily, 194. + In food and oxygen, 190. + Kinds of, 186. + Methods of storing, 187, 188. + Transformation of, in muscle, 248, 249. + +Enzymes, 132, 155. + Of the tissues, 184. + +Epidermis, 264, 266. + +Epiglottis, 80, 354. + +Epithelium, 139. + +Eruptive diseases, 405. + +Esophagus, 146. + +Eustachian tube, 359. + +Excessive reading, 331. + +Excitant impulse, 305. + +Excretion, 197-213. + Defined, 18. + Necessity for, 201. + +Exercise, 256, 257, 328, 409. + General rules for, 259. + Results of, 257. + +Exhaustion, nervous, 211. + Results of, 195. + +External ear, 358. + +External stimuli, action of, 307. + +Eye, 370-391. + +Eyeball, 373. + Chambers of, 377. + Focusing power of, 378. + Movements of, 381. + +Eyelids, 373. + +Eyes, + Care of, 386. + Removal of foreign bodies from, 387, + Strong chemicals in, 388. + +Eye strain, 211. + And disease, 385. + + + +Fat, 30, 149, 162. + Digestion of, 156. + Emulsification of, 157. + Purpose served by, 121. + Route taken by, 175. + Tests for, 137. + Where stored, 178. + +Fatty acid, 156. + +Fenestra ovalis, 361. + +Fenestra rotunda, 363. + +Ferments, _see_ Enzymes. + +Fibrin, 31. + +Fibrin ferment, 32. + +Fibrinogen, 30, 31. + +Fissures, 289. + +Food, 117-137. + Advantages of coarse, 167. + Classes of, 118, 119. + Composition of, 124. + Dangers from impure, 165. + Defined, 117. + Elements supplied by, 134. + Excess of proteid, 208. + Frequency of taking, 165. + Materials, table of, 126, 126. + Nitrogenous, 119. + Order of taking, 161. + Preparation of, 164. + Purity of, 128. + Quantity of, 164. + Simple, 118. + Variety, 128. + With reference to digestive changes, 132. + +Foot lever, diagram of, 253. + +Foot-pound, 196. + +Foot-wear, hygienic, 238. + +Fractures, treatment of, 239. + +Fumigation, 400. + +Furniture, school, 236. + + + +Gall bladder, 154. + +Ganglia, 281. + Dorsal-root, 295. + Sympathetic, 298. + +Gastric glands, 147. + +Gastric juice, to illustrate action of, 172. + +Gelatine, 218. + +Germ diseases, avoidance of, 394. + +Germs, 29, 394, 395. + How spread, 395. + +Glands, 197-213. + Digestive, 140. + Ductless, 208. + Excretory, work of, 201. + Gastric, 147. + Kinds of, 197, 198. + Lymphatic, 68, 208. + Perspiratory, 206. + Salivary, 144. + Structure of, 197. + Thymus, 208. + Thyroid, 208. + +Gliding joint, 232. + +Glottis, 355. + +Glycogen, 120, 177. + +Grape sugar, tests for, 120, 136. + +Gross anatomy, defined, 1. + +Gullet, 146. + +Gustatory pore, 345. + +Gustatory stimulus, 345. + + + +Habits, 315, 334. + +Hair, 267. + Care of, 276. + +Hair cells, 363. + +Hair follicle, 267. + +Haversian canals, 219. + +Hearing, defective, 366. + +Heart, 41. + Care of, 55. + Connection with arteries and veins, 45. + Difference in parts of, 44. + How it does its work, 45. + Observations on, 60, 61, 62. + Sounds of the, 47. + Valves of, 43. + +Heart muscle, structure of, 247. + +Heat and cold, effects of, 330. + +Hemoglobin, 26. + +Hepatic artery, 154. + +Hepatic veins, 154. + +Hindbrain, 290. + +Hinge joint, 231. + +Histology, defined, 1. + +Humerus, 227. + +Hyaloid membrane, 378. + +Hydrochloric acid, 149, 150. + +Hydrogen, 134. + +Hygiene, + Defined, 2. + General aim of, 2. + General laws of, 2, 392. + Of digestion, 160. + Of skeleton, 238. + Relation of physiology and anatomy to, 3. + +Hygienic housekeeping, 399. + +Hypoglossal nerves, 298. + + + +Ileo-caecal valve, 151. + +Ileum, 151. + +Images, + Diagram illustrating, 372. + Formation of, 371. + +Incisors, 143. + +Incus, 359. + +Infectious diseases, 394. + +Infundibula, 80, 84. + +Inhibitory impulse, 305. + +Insomnia, 329. + +Inspiratory force, 70. + +Intercellular material, production of, 13, 18. + +Internal ear, 360. + +Intestinal juice, 152, 157. + +Iris, 375. + +Iron, 135. + +Irritability, 6, 243, 304. + +Isolation, 406. + + + +Jejunum, 151. + +Joints, 230-232, 242. + + + +Kidneys, 201. + Blood supply to, 204. + Cortex of, 204. + Inflammation of, 211. + Pelvis of, 202. + Structure, 202. + Symptoms of diseased, 211. + Work of, 205. + +Knee jerk reflex, 322. + + + +Lachrymal glands, 383. + +Lacteals, work of, 174. + +Lacunae, 220. + +Laminae, 220. + +Large intestine, 157. + Division of, 158. + Work of, 159. + +Larynx, 80, 353-357. + To show plan of, 368. + +Lever, 251. + Application to the body, 251. + Classes of, in body, 251. + Producing motion, diagram of, 252. + To show action of, 252. + +Leucocytes, 27. + +Levulose, 120, 150. + +Life, maintenance of, 20. + +Light, 370, 371. + Simple properties, illustrated, 389. + +Light waves, diagram illustrating passage of, 370. + +Lime water, to prepare, 101. + +Liver, 52, 152-155, 178. + Protection of, 210. + Work of, 206. + +Lockjaw, 276. + +Longsightedness, 384. + +Lung capacity, diagram illustrating, 89. + +Lung diseases, out-door cure for, 98. + +Lungs, 77-103. + Capacity of, 88. + Changes air undergoes in, 101. + Excretory work of, 207. + Interchange of gases in, 88. + Observations of, 100. + Supply of blood to, 82. + To estimate capacity of, 103. + Weakest portions of, 92. + +Lymph, 65-75. + Composition, 66. + Movements at the cells, 71. + Origin of, 65. + Physical properties, 66. + Where it enters the blood, 70. + +Lymph movements, causes of, 69. + +Lymph spaces, 66. + +Lymph vessels, 66. + Variable pressure on the walls of, 70. + + + +Magnesium, 135. + +Malarial fever, 401. + +Malleus, 359. + +Malpighian capsules, 203. + +Maltose, 120. + +Massage, 259. + +Mastication, + Muscles of, 144. + Slow, 145. + Thorough, 160. + To show importance of, 171. + +Matrix, 267. + +Measles, 94. + Care after, 211. + +Median fissures, 289. + +Medulla oblongata, 291. + +Medullary sheath, 284. + +Membrana tympani, 358. + +Membrane, + Active, 173. + Basement, 197. + Basilar, 363. + +Membranous capsule, 377. + +Membranous labyrinth, 361. + +Mesentery, 152. + +Metacarpals, 227. + +Midbrain, 289. + +Middle ear, 359. + Purposes of, 360. + +Milk sugar, 120. + +Mineral salts, 30. + Uses, 121. + +Moderate drinkers, 333. + +Molars, 143. + +Molecules, defined, 105. + +Mon-axonic neuron, diagram of, 282. + +Mono-saccharides, 120. + +Mosquitoes, 401-403. + Remedies against, 402. + +Mouth, 141. + +Movable joints, + Kinds of, 231. + Structure of, 230. + +Mucous membrane, 80, 264. + +Mucus, 139. + +Muscle organ, 245. + +Muscles, 243-263. + Alimentary, 189. + Important, 254-256. + Intercostal, 87. + Of mastication, 144. + Properties of, 243. + +Muscular force, plan of using, 249. + +Muscular sensations, 344. + +Muscular stimulus, 248. + +Muscular stimulus and contraction, to illustrate, 261. + +Muscular tissue, kinds of, 243, 244. + + + +Nails, 267. + Care of, 276. + +Nasal duct, 383. + +Neck exercise, 328. + +Nerve cells, 281, 282. + +Nerve fibers, 282, 293, 294. + +Nerve path, diagram of, 286. + +Nerve pathways, to demonstrate, 322. + +Nerves, 281. + +Nerve skeleton, 280. + Diagram of, 281. + +Nerve stimuli, 306. + +Nerve trunks, 281. + +Nervous activity, wasteful forms of, 325. + +Nervous control of, + Body temperature, 320. + Circulation of blood, 318. + Respiration, 320. + +Nervous energy, economizing of, 315. + +Nervous impulse, 248, 305. + +Nervousness, 326. + +Nervous system, 279-337. + Diagram of, 287. + Dissection of, 302. + Divisions of, 287. + Hygiene of, 324-337. + Nature of, 287. + Physiology of, 304-323. + Work of, 280. + +Neural arch, 224. + +Neurilemma, 284. + +Neurons, 281, 282. + Arrangement of, 284, 293. + Diagram, illustrating, 285. + Properties of, 304. + +Nicotine, + Effects of, 333. + Relation of age to effects, 333. + +Nitrogen, 134. + +Non-striated cells, to show, 261. + +Non-striated muscles, + Purpose of, 246. + Structure of, 246. + Work of, 247. + +Normal temperature, 269. + +Nosebleed, 58. + +Nucleoplasm, 14. + +Nutrients (_see_ Foods), + Composition of, 135. + Relative quantity needed, 123. + Routes taken by, 175. + Tests for, 136. + +Nutriment, storage of, 177-180. + + + +Olfactory stimulus, 347. + +Opsonins, 34. + +Optic thalami, 289. + +Orbit, 373. + +Organ, defined, 7. + +Organism, defined, 19. + +Organization, defined, 10. + +Osmosis, 72. + At the cells, 72. + To illustrate, 75. + +Ossein, 218. + +Overstudy, 211. + +Oxidation, defined, 106. + +Oxygen, 104-117. + Combined, 105, 113. + Free, 105, 113. + How it unites, 105. + Main uses of, 108. + Movement a necessity, 106, 108, 115. + Movement in body, 106, 108, 115. + Nature of, 104. + Passage of, from cells, 110. + Passage of, through blood, 109. + Passage of, toward cells, 109. + Preparation of, 113. + Pressure, 109. + Properties of, 113. + Purpose of, in the body, 106. + +Oxyhemoglobin, 27. + + + +Pacinian corpuscles, 342, 343. + To demonstrate, 348. + +Pancreas, 155. + +Pancreatic juice, 155. + +Papillae, 266. + +Patent medicines, 166. + +Pelvic girdle, 226. + +Pepsin, 149. + +Peptones, 149, 176. + +Pericardium, 41. + +Perilymph, 361. + +Perimysium, 245. + +Periosteum, 218. + +Peritoneum, 180. + +Perspiration, 207. + +Pharynx, 145. + Openings into, 145, 146. + +Phosphorus, 135. + +Phrenic nerve, 302. + +Physiological salt solution, 38. + +Physiology, defined, 2. + +Pia, 299. + +Pigment granules, 266. + +Pinna, 358. + +Pitch, detection of, 365. + +Pivot joint, 232. + +Plasma, 25, 29. + +Pleura, 84. + +Plexus, 281. + +Pneumonia, 94. + +Pons, 290. + +Pons Varolii, 290. + +Portal vein, 154. + +Primitive sheath, 284. + +Proteids, 161. + Circulating, 179. + Kinds of, 118. + Purposes of, 119. + Supplied by, 125. + Tests for, 135, 136. + Tissue, 179. + +Proteoses, 149, 176. + +Protoplasm, 14. + +Protozoa, 394. + +Ptyalin, 145. + +Public sanitation, 396. + +Pulp cavity, 143. + +Pupil, 375. + +Pure food law, 128. + +Pus, 28, 29. + +Pyloric orifice, 147. + +Pyramids, 202. + + + +Quarantine, 406. + + + +Radius, 227. + +Reaction time, to determine, 323. + +Reading glasses, 386. + +Receptacle of the chyle, 68, 170. + +Rectum, 158. + +Red corpuscles, 25. + Disappearance of, 27. + Function of, 26. + Origin of, 27. + To examine, 38. + To prepare models of, 39. + +Red marrow, 219. + +Reenforcement of sound, 352, 356, 368. + +Reflection, kinds of, 371. + +Reflex action, 308. + Diagram illustrating, 310. + In circulation of blood, 311. + In digestion, 310. + Purposes of, 311. + +Reflex action and mind, 308. + +Reflex action pathway, 309. + +Refraction, 371. + +Rennin, 149. + +Respiration, 76-103. + Artificial, 97. + Internal, 89. + Lung, 76. + +Retina, 376. + +Retinitis, 333. + +Rheumatism, + Effects on the heart, 56. + Sequel to other diseases, 407. + +Right lymphatic duct, 67. + +Rods and cones, 377. + +Rods of Corti, 364. + + + +Sacrum, 224. + +Saliva, 145. + Composition of, 145. + Uses of, 145. + To show action on starch, 171. + +Salivary glands, 144. + Kinds of, 144. + Reflex action of, 323. + +Sanitation, defined, 2. + +Sarcolemma, 244. + +Sarcoplasm, 244. + +Scala media, 363. + +Scala tympani, 363. + +Scala vestibula, 363. + +Scarlet fever, care after, 211. + +Sciatic nerve, 302. + +Sclerotic coat, 374. + +Secondary reflex action, 314. + +Secretions, 197. + Kinds of, 200. + +Secretory process, nature of, 199. + +Seeing, problem of, 372. + +Self-control, 326, 334. + Habit of, 325. + +Semicircular canals, 362. + +Semilunar valves, 44. + +Sensations, 338-349. + Classes of, 339. + Production of, 338, 349. + Purposes of, 340. + Special, 340. + +Sensations (_continued_). + Steps in production of, 341. + +Sensation stimuli, 339. + +Sense organs, simple forms of, 341, 342 + +Serous coat, 140, 148. + +Serous membrane, 264. + +Serum albumin, 30. + +Serum globulin, 30. + +Shortsightedness, 384. + +Shoulder girdle, 226. + +Sight, organs of, 373. + +Sigmoid flexure, 158. + +Simple life, 410. + +Skeleton, 216-243. + How deformed, 234. + Hygiene of, 233. + Plan of, 221. + Purpose of, 221. + +Skin, 264-277. + As regulator of temperature, 270. + Experiments on, 349. + Functions of, 267, 268. + Observations on skin, 278. + +Skin wounds, treatment of, 275. + +Skull, 225. + +Sleep, 329. + +Small intestine, 151. + Mucous membrane of, 151. + Muscular coat of, 152. + As organ of absorption, 173. + Parts of, 151. + Serous coat of, 152. + Work of, 157. + +Smell, + Sensation of, 346. + Value of, 347. + +Sneezing, 81. + +Sodium, 135. + +Sodium carbonate, 155. + +Sodium chloride, 122. + +Soft palate, 141. + +Solution, 131. + Kinds of, 73. + +Solution theory, 156. + +Solvents, 131. + +Sound, + To illustrate origin of, 367. + To show transmission of, 367. + +Sound waves, + As stimuli, 331. + Nature of, 350. + Reenforcement of, 352. + To show effects of, 368. + Value of, 353. + +Speech, production of, 357. + +Spinal column, 223-225. + Hygiene of, 233. + +Spinal cord, 280. + Protection of, 299. + +Spinal nerves, 295. + Double nature of, 295. + +Spitting, 403. + +Spleen, 208. + +Sprains, 239, 240. + +Stapes, 359. + +Starch, 162. + Action of, on saliva, 171. + Animal, 120. + Tests for, 136. + +Steapsin, 155, 156. + +Stegomyia, 403. + +Sternum, 225. + +Stomach, 147. + Mucous membrane of, 147. + Muscular action of, 150. + Muscular coat, 148. + +Serous coat, 148. + +Storage of nutriment, 177-179. + +"Strenuous life," 410. + +Striated fibers, to show, 261. + +Striated muscles, to show, 261. + +Stroma, 25. + +Sugars, kinds, 120. + +Sulphur, 135. + +Supra-renal bodies, 208. + +Suspensory ligament, 377. + +Sutures, 230. + +Sympathetic ganglia and nerves, 298. + Work of, 316. + +Synovial fluid, 231. + +Synovial membrane, 231. + +System, defined, 20. + +Systole, 46. + + + +Taste buds, 345. + +Tea, + Effects on digestion, 167. + Effects on heart, 56. + +Tears, 383. + +Teeth, 142. + Care of 163. + Kinds of, 143. + +Temperature, + Body, 207. + Corpuscles, 271, 345. + Sensation, 343. + +Tendon of Achilles, 256. + +Tendons, 246. + +Tests for foods, 136, 137. + +Tetanus, 262, 275. + +Thoracic cavity, 7, 85, 100, 102. + +Thoracic duct, 67, 170. + +Thorax, 85. + Bones of, 225. + +Tissue enzymes, 182. + +Tissues, 4. + Complex nature of, 13. + Defined, 20. + General purposes of, 5. + Kinds of, 5, 6. + Observations on, 12. + Properties of, 6. + +Tobacco, effect on heart, 56. + +"Tobacco heart," 56, 333. + +Tongue, 143. + +Tonic bath, 273. + +Touch, 343. + +Touch corpuscles, 342. + +Toxins, 394. + +Trachea, 80. + +Trypsin, 155, 156. + +Tuberculosis, 90, 92, 94, 98. + How communicated, 403. + Outdoor treatment, 98. + To prevent, 404. + +Tympanum, 359. + +Typhoid fever, 404, 407. + + + +Ulna, 227. + +Urea, 110, 205, 207, 210. + +Ureters, 170. + +Uriniferous tubules, 203. + + + +Vaccination, 406. + +Valves, + Advantages of, in veins, 49, 63. + Mitral, 43. + Position of, in veins, 63. + Purposes of, 49, 63. + Tricuspid, 43. + +Veins, 47. + Functions of, 51. + Renal, 202. + +Ventilation, 94. + Rules for, 95, 96. + +Ventricles, 42. + To illustrate action of, 62. + +Vermiform appendix, 158. + +Vertebrae, 223-225. + Interlocking of, 225. + Joining of, 224. + Kinds, 223. + +Vestibule, 361. + +Villi, 152. + Parts of, 173, 174. + +Visual perceptions, 382. + +Visual sensations, 382. + +Vitreous humor, 378. + +Vocal cords, 355. + +Voice, 353-357. + How produced, 356. + Pitch and intensity, 356. + +Voluntary action, 311, 312. + +Voluntary action pathways, 312. + +Vomiting, 151, 152. + + + +Waste material, passage from body, 210. + +Wastes, 30. + +Water, + Importance of, 123. + Supply of, 398. + Value of, 210. + +Water-vapor, 208. + +White corpuscles, 27, 28. + Functions of, 29. + To examine, 39. + +Work, + Hygienic value of, 328, 409. + +Worry, 211. + + + +Yellow fever, 403. + +Yellow marrow, 218. + +Yellow spot, 377. + + + + + + +FOOTNOTES + + + 1 The body is affected by what it does (exercise, work, sleep), by + things taken into it (food, air, drugs), and by things outside of it + (the house in which one lives, climate, etc.). That phase of hygiene + which has for its object the making of the surroundings of the body + healthful is known as _sanitation_. + + 2 When classified according to their essential structure, the tissues + fall into four main groups: epithelial and glandular tissue, + muscular tissue, nervous tissue, and connective tissue. According to + this system the osseous, cartilaginous, and adipose tissues are + classed as varieties of connective tissue. See page 18. + + 3 The properties of substances are the qualities or characteristics + (color, weight, etc.) by means of which they are recognized. + + 4 Certain of these cells also form deposits of fat, giving rise to the + adipose, or fatty, tissue. + + 5 Any organized structure, such as the body, whose parts are pervaded + by a common life, is known as an _organism_. The term "organism" is + frequently applied to the body. + + 6 In birds, reptiles, amphibians, and fishes the red corpuscles have + nuclei (Fig. 9). + + 7 The micron is the unit of microscopical measurements. It is equal to + 1/1000 of a millimeter and is indicated by the symbol {~GREEK SMALL LETTER MU~}. + + 8 The peculiar shape of the red corpuscle has no doubt some relation + to its work. Its circular form is of advantage in getting through + the small blood vessels, while its extreme thinness brings all of + its contents very near the surface--a condition which aids the + hemoglobin in taking up oxygen. If the corpuscles were spherical in + shape, some of the hemoglobin could not, on account of the distance + from the surface, so readily unite with the oxygen. + + 9 The coloring matter of the bile consists of compounds formed by the + breaking down of the hemoglobin; the spleen contains many large + cells that seem to have the power first of "engulfing" and later of + decomposing red corpuscles. A further evidence that the spleen aids + in the removal of worn-out corpuscles is found in the fact that + during diseases that cause a destruction of the red corpuscles, such + as the different forms of malaria, the spleen becomes enlarged. + + 10 An infected part of the body, such as a boil or abscess, should + never be bruised or squeezed until the time of opening. Pressure + tends to break down the wall of white corpuscles and to spread the + infection. Pus from a sore contains germs and should not, on this + account, come in contact with any part of the skin. (See treatment + of skin wounds, Chapter XVI.) + + 11 Coagulation is not confined to the blood. The white of an egg + coagulates when heated and when acted upon by certain chemicals, and + the clabbering of milk also is a coagulation. + + 12 If the blood be stirred or "whipped" while it is coagulating, the + clot may be broken up and the fibrin separated as fast as it forms. + The blood which then remains consists of serum and corpuscles and + will not coagulate. It is known as "defibrinated" blood. + + 13 Certain substances, called _opsonins_, have recently been shown to + exist in the plasma, that aid the white corpuscles in their work of + destroying germs. The opsonins appear to act in such a manner as to + weaken the germs and make them more susceptible to the attacks of + the white corpuscles. + + 14 Some of the changes in the blood are very closely related to our + everyday habits and inclinations. For example, a lack of nourishment + in the blood causes hunger and this leads to the taking of food. If + the fluids of the body become too dense, a feeling of thirst is + aroused which prompts one to drink water. + + 15 Metchnikoff, _The New Hygiene_. + + 16 A physiological salt solution is prepared by dissolving .6 of a gram + of common salt in 100 cc. of distilled water or pure cistern water. + This solution, having the same density as the plasma of the blood, + does not act injuriously upon the corpuscles. + + 17 The term "circulation" literally means moving in a circle. While the + blood does not move through the body in a circle, the term is + justified by the fact that the blood flows out continually from a + single point, the heart, and to this point is continually returning. + + 18 The heart at first glance seems to bear little resemblance to the + pumps in common use. When it is remembered, however, that any + contrivance which moves a fluid by varying the size of a cavity is a + pump, it is seen that not only the heart, but the chest in breathing + and also the mouth in sucking a liquid through a tube, are pumps in + principle. The ordinary syringe bulb illustrates the class of pumps + to which the heart belongs. (See Practical Work.) + + 19 The contraction of the heart is known as the _systole_ and its + relaxation as the _diastole_. The systole plus the diastole forms + the so-called "cardiac cycle" (Fig. 18). This consists of (1) the + contraction of the auricles, (2) the contraction of the ventricles, + and (3) the period of rest. The heart systole includes the + contraction of both the auricles and the ventricles. + + 20 Martin, _The Human Body_. + + 21 The pressure maintained by the left ventricle has been estimated to + be nearly three and one half pounds to the square inch--a pressure + sufficient to sustain a column of water eight feet high. The + pressure maintained by the right ventricle is about one third as + great. In maintaining this pressure the heart does a work equal to + about one two-hundredth of a horse power. + + 22 The location of the heart in the thoracic cavity causes movements of + the chest walls to draw blood into the right auricle for the same + reason that they "draw" air into the lungs. + + 23 Active exercise through short intervals, followed by periods of + rest, such as the exercise furnished by climbing stairs, or by short + runs, is considered the best means of strengthening the heart. + + 24 Nosebleed in connection with any kind of severe sickness should + receive prompt attention, since a considerable loss of blood when + the body is already weak may seriously delay recovery. + + 25 Newton, _Practical Hygiene_. + + 26 On account of its position in the body, the lymph is not easily + collected for examination. Still, nearly every one will recall some + experience that has enabled him to see lymph. The liquid in a water + blister is lymph, and so also is the liquid which oozes from the + skin when it is scraped or slightly scratched. Swelling in any part + of the body is due to the accumulation of lymph at that place. + + 27 In certain small animals of the lowest types a single liquid, + serving as a medium of exchange between the cells and the body + surface, supplies all the needs of the organism. In larger animals, + however, where materials have to be moved from one part of the cell + group to another, a portion of the nutrient fluid is used for + purposes of transportation. This is confined in channels where it is + set in motion by suitable organs. The portion which remains outside + of the channels then transfers material between the cells, on the + one hand, and the moving liquid, on the other. + + 28 Surgeons in opening veins near the thoracic cavity have to be on + their guard to prevent air from being sucked into them, thereby + causing death. + + 29 Oxygen forms about 21 per cent of the atmosphere, nitrogen about 78 + per cent, carbon dioxide about .03 per cent, and the recently + discovered element argon about 1 per cent. The oxygen is in a + _free_, or uncombined, condition--the form in which it can be used in + the body. + + 30 The peculiar work devolving upon the organs of respiration + necessitates a special plan of construction--one adapted to the + properties of the atmosphere. Being concerned in the movement of + air, a gaseous substance, they will naturally have a structure + different from the organs of circulation which move a liquid (the + blood). All the organs of the body are adapted by their structure to + the work which they perform. + + 31 In ordinary inspirations the force that causes the air to move + through the passages is scarcely an ounce to the square inch, while + in forced inspirations it does not exceed half a pound. On this + account the closing of any of the air passages by pressure, or by + the presence of foreign substances, would keep the air from reaching + some part of the lungs. + + 32 Coughing, which is a forceful expulsion of air, has for its purpose + the ejection of foreign substances from the throat and lungs. + Sneezing, on the other hand, has for its purpose the cleansing of + the nostrils. In coughing, the air is expelled through the mouth, + while in sneezing it is expelled through the nostrils. + + 33 The amount of dust suspended in what we ordinarily think of as pure + air is shown when a beam of direct sunlight enters an otherwise + darkened room. + + 34 Some children find it difficult to breathe through the nostrils on + account of growths (called adenoids) in the upper pharynx. Such + children should have medical attention. The removal of these growths + not only improves the method of breathing, but in many instances + causes a marked improvement in the general health and personal + appearance. + + 35 The weakest portions of the lungs appear to be the tiny lobes at the + top. As they occupy the part of the thorax most difficult to expand, + air penetrates them much less freely than it does the lobes below. + In most cases of consumption (some authorities give as high as + eighty per cent), the upper lobes are the first to be affected. Flat + chests and round shoulders, by increasing this natural difficulty in + breathing, have long been recognized as causes which predispose to + consumption. + + 36 The following exercise, from Dudley A. Sargent's _Health, Strength, + and Power_, will be found most beneficial: "Stand with the feet + together, face downward, arms extended downward, and backs of the + hands touching. Raise the hands, arms, and elbows, keeping the backs + of the hands together until they pass the chest and face. Then + continue the movement upward, until the hands separate above the + head with the face turned upward, when they should be brought + downward and outward in a large circle to the starting point. Begin + to inhale as the arms are raised and take in as much air as possible + by the time the hands are above the head, then allow the breath to + go out slowly as the arms descend." + + 37 Colds may frequently be broken up at their beginning by taking a + prolonged _hot_ bath and going to bed. After getting a start, + however, they run a course of a few days, a week, or longer, + depending upon the natural vigor of the individual and the care + which he gives his body during the time. In throwing off a cold, the + following suggestions will be found helpful: + + 1. Dress warmly (without overdoing it) and avoid getting chilled. 2. + Diminish the usual amount of work and increase the period for sleep. + If very weak, stay in bed. Save the energy for throwing off the + cold. 3. If able to be about, spend considerable time in light + exercise out of doors, but avoid getting chilled. 4. Keep the bowels + active, taking a cathartic if necessary. 5. To relieve pain in the + chest, apply a mustard plaster or a flannel cloth moistened with + some irritating substance, such as turpentine or a mixture of equal + parts of kerosene and lard. Keep up a mild irritation until the pain + is relieved, but avoid blistering. + + 38 Not only do the lungs remove oxygen from the air and add carbon + dioxide to it, but they separate from the body considerable moisture + and, according to some authorities, a small amount of an impurity + referred to as "animal matter." Odors also arise from the skin, + teeth, and clothing which, if not dangerous to the health, are + offensive to the nostrils. If on going into a room such odors are + detected, the ventilation is not sufficient. This is said to be a + reliable test. + + 39 E.A. Schaffer, "Artificial Respiration in its Physiologic Aspects," + _The Journal of the American Medical Association_, September, 1908. + + 40 Testing the prone-posture method by suitable apparatus, Professor + Schaffer has found it capable of introducing more air per minute + into the lungs than any of the other methods of artificial + respiration, and more even than is introduced by ordinary breathing. + + 41 Osier, _The Principles and Practice of Medicine_. + + 42 Huber, _Consumption and Civilization_. + + 43 To prepare limewater some small lumps of _fresh_ lime (either + slacked or unslacked) are added to a large bottle of water and + thoroughly shaken. This is put aside until the lime all settles to + the bottom and the water above is perfectly clear. This is now ready + for use and may be poured off as needed. When the supply is + exhausted add more water and shake again. + + 44 An _element_ is a single kind of matter. Those substances are + classed as elements which cannot be separated into different kinds + of matter. Two or more elements combined in definite proportions by + weight form a _compound_. The elements are few in number, only about + eighty being known. Compounds, on the other hand, are exceedingly + numerous. + + 45 The term _energy_, as used here, has the same general meaning as the + word _power_. See Chapter XII. + + 46 The oxygen pressure of the atmosphere is that portion of the total + atmospheric pressure which is due to the weight of the oxygen. Since + oxygen comprises about one fifth of the atmosphere, the pressure + which it exerts is about one fifth of the total atmospheric + pressure, or, at the sea level, about three pounds to the square + inch (15 x 1/5 = 3). This is the oxygen pressure of the atmosphere. + The low oxygen pressure in the tissues is due to its scarcity, and + this scarcity is due to its entering into combination at the cells. + + 47 See footnote on oxygen pressure, page 109. + + 48 The impression prevails to some extent that carbon dioxide, on + account of its weight, settles out of the atmosphere, collecting in + old wells and at the floor in crowded rooms. Any such settling of + the carbon dioxide is prevented by the rapid motion of its + molecules. This motion not only prevents a separation of carbon + dioxide and air after they are mixed, but causes them to mix rapidly + when they are separated, if they still have surface contact. The + carbon dioxide found in old wells is formed there by decaying + vegetable or animal matter. In rooms it is no more abundant at the + floor than in other parts. + + 49 On account of the formation of carbon dioxide in places containing + decaying material, the descent into an old well or other opening + into the earth is often a hazardous undertaking. Before making such + a descent the air should always be tested by lowering a lighted + lantern or candle. Artificial respiration is the only means of + restoring one who has been overcome by this gas (page 97). + + 50 While awaiting oxidation at the cells, the carbohydrates and fats + are stored up by the body, the carbohydrates as glycogen and the + fats as some form of fat. In this sense they are sometimes looked + upon as serving to build up certain of the tissues. + + 51 The following table shows the main elements in the body and their + relation to the different nutrients: + + [Nutrient Table] + + + 52 The recently advanced theory that the molecules of the mineral + salts, by dissolving in water, separate into smaller divisions, part + of which are charged with positive electricity and part with + negative electricity, has suggested several possible uses for sodium + chloride and other mineral salts in the body. The sodium chloride in + the tissues is in such concentration as to be practically all + separated into its sodium and chlorine particles, or ions. It has + recently been shown that the sodium ions are necessary for the + contraction of the muscles, including the muscles of the heart. + There is also reason for believing that the different ions may enter + into temporary combination with food particles, and in this way + assist in the processes of nutrition. + + 53 Chittenden, _The Nutrition of Man_. + + 54 Compiled from different sources, but mainly from Atwater's _Foods: + Nutritive Value and Cost_, published by the U.S. Department of + Agriculture. + + 55 The calorie is the adopted heat unit. As used in this table it may + be defined as the quantity of heat required to raise 1 kilogram (2.2 + pounds) of water, 1 degree centigrade. The calories also show the + relative amount of energy supplied by the different foods. + + 56 While alcohol cannot be classed as a food, it is believed by some + authorities to contain _food value_ and, in the hands of the + physician, to be a substance capable of rendering an actual service + in the treatment of certain diseases. It might, for example, be used + where one's power of digestion is greatly impaired, since alcohol + requires no digestion. But upon this point there is a decided + difference of opinion. Certain it is that no one should attempt to + use alcohol as food or medicine except under the advice and + direction of his physician. + + 57 A layer of connective tissue between the mucous membrane and the + muscular coat is usually referred to as the _submucous_ coat. This + contains numerous blood vessels and nerves and binds the muscular + coat to the mucous membrane. + + 58 The saliva may continue to act for a considerable time after the + food enters the stomach. "Careful examination of the contents of the + fundus (large end of the stomach) by Cannon and Day has shown that + no inconsiderable amount of salivary digestion occurs in the + stomach."--FISCHER, _The Physiology of Alimentation_. + + 59 Perhaps the simplest method of inducing vomiting is that of + thrusting a finger down the throat. To make this method effective + the finger should be held in the throat until the vomiting begins. + An emetic, such as a glass of lukewarm salt water containing a + teaspoonful of mustard, should also be taken, and, in the case of + having swallowed poison, the vomiting should be repeated several + times. It may even be advantageous to drink water and then vomit it + up in order to wash out the stomach. + + 60 Hammerstein, _Text-book of Physiological Chemistry._ + + 61 Amylopsin is absent from the pancreatic juice of infants, a + condition which shows that milk and not starch is their natural + food. + + 62 The fact that butter is more easily digested than other fatty + substances is probably due to its consisting largely of a kind of + fat which, on splitting, forms a fatty acid (butyric) which is + soluble in water. + + 63 Fischer, _Physiology of Alimentation._ + + 64 Beginning the meal with a little soup, as is frequently done, may be + of slight advantage in stimulating the digestive glands. To serve + this purpose, however, and not interfere with the meal proper, it + should contain little greasy or starchy material and should be taken + in small amount. + + 65 Dr. William Beaumont, an American surgeon of the last century, made + a series of observations upon a human stomach (that of Alexis St. + Martin) having an artificial opening, the result of a gunshot wound. + Much of our knowledge of the digestion of different foods was + obtained through these observations. In spite of the protests of his + physician, St. Martin would occasionally indulge in strong drink and + always with the same result--the lining of the stomach became much + inflamed and very sensitive, and the natural processes of digestion + were temporarily suspended. + + 66 The lacteals (from the Latin _lacteus_, milky) are so called on + account of their appearance, which is white, or milk-like, due to + the fat droplets. + + 67 Peptones and proteoses, when injected directly into the blood, are + found to act as poisons. + + 68 The soluble double sugars (maltose, milk sugar, and cane sugar) are + reduced to the simple sugars (dextrose and levulose). Furthermore + the action on the proteids does not stop with the production of + peptones and proteoses, but these in turn are still further reduced. + + 69 Energy, which is defined as _the ability to do work_, or _to cause + motion_, exists in two general types, or forms, known as kinetic + energy and as potential energy. _Kinetic_ energy is energy at work, + or energy in the act of producing motion; while _potential_ energy + is reserve, or stored, energy. All moving bodies have kinetic + energy, and all stationary bodies which have within them the + _capability_ of causing motion possess potential energy. A bent bow, + a piece of stretched rubber, a suspended weight, the water above a + mill dam, all have the capability of causing motion and all have + potential energy. Examples of kinetic energy are found in the + movements of machinery, in steam and electricity, in winds, and in + currents of water. Kinetic is the active, and potential the + inactive, form of energy. + + 70 As the atoms of hydrogen and oxygen that make up the molecules of + water separate, they unite with atoms of their own kind--the hydrogen + with hydrogen and the oxygen with oxygen atoms. Since these + combinations are weaker than those of the water molecules, energy is + required to bring about the change. But when hydrogen burns in the + oxygen, the change is from a weaker to a stronger combination. The + stored-up energy is then given up or becomes active. + + 71 In the evaporation of water, the energy of the sun is stored with + reference to the force of gravity. In evaporating, water rises as a + gas, or vapor, above the earth's surface, but on condensing into a + liquid, it falls as rain. It then finds its way through streams back + to the ocean. All water above the sea level is in such a position + that gravity can act on it to cause motion, and it possesses, on + this account, potential or stored-up energy. It is because of this + energy that rapids and waterfalls are such important sources of + power. + + 72 Energy, like matter, can neither be created nor destroyed. It can, + however, be transferred from one body to another and transformed + from one form to another form. Whenever work is done, energy is + transferred from the body doing the work, to the body upon which the + work is done. During this process there may, or may not, be a + transformation of energy. In turning a grindstone, kinetic energy is + passed to the stone and used without transformation, but in winding + a clock, the kinetic energy from the hand is transformed into + potential energy in the clock spring. Then as the clock runs down + this is retransformed into kinetic energy, causing the movements of + the wheels. + + Not only is kinetic transformed into potential energy and _vice + versa_, but the different forms of kinetic energy (heat, light, + electricity, sound, and mechanical motion) are readily transformed + the one into the other. With suitable devices, mechanical motion can + be changed into heat, sound, or electricity; heat into motion and + light; and electricity into all the other forms of energy. These + transformations are readily explained by the fact that the different + varieties of kinetic energy are but different forms of motion (Fig. + 84). + + 73 The simplest arrangement of the parts of a gland is that where they + are spread over a plain surface. This arrangement is found in serous + membranes, such as the pleura and peritoneum. These membranes, + however, are not called glands, but secreting surfaces. + + 74 In the oxidations that occur in the body it is not supposed that the + nutrients are immediately converted to carbon dioxide, water, and + urea. On the other hand, it is held that their reduction takes place + gradually, as the reduction of sugar by fermentation, and that the + wastes leaving the body are but the "end products" and show only the + final results. + + 75 Alcohol, if used in considerable quantity, leads to cirrhosis of the + liver and Bright's disease of the kidneys, both very dangerous + diseases. Dr. William Osler in his treatise, _The Practice of + Medicine_, states that alcohol is the chief cause of cirrhosis of + the liver. Dr. T.N. Bogart, specialist in kidney diseases, asserts + that one third of all the cases of Bright's disease coming under his + observation are caused by alcohol. + + 76 Hall, _The Purin Bodies_. + + 77 Review "Main Physiological Problems," page 21. + + 78 In the production of motion in the body, as well as in the + production of any kind of _purposeful_ motion outside of the body, + three conditions must be fulfilled. There is required, in the first + place, a mechanical device or machine which is so constructed as to + produce a certain kind of motion. In the second place, energy is + needed to operate this device. And, finally, there must be some + controlling force, by means of which the motion is made to + accomplish definite results. The driving of a horse hitched to a + wagon will illustrate these conditions. The wagon is the mechanical + device, the horse furnishes the energy, and the driver supplies the + controlling force. In this, as in most cases, the machinery, the + source of energy, and the controlling force are disconnected except + when at work; but in the body all three occur together in the same + structure. + + 79 The dependence of the outer layers of bone cells upon the periosteum + for nourishment causes a destruction of this membrane to affect + seriously the bone beneath, producing in many instances a decay of + the bone substance. + + 80 It has been claimed that the introduction of vertical writing has + reduced the number of cases of spinal curvature originating in the + schoolroom, and statistics appear to prove the claim. It is shown, + on the other hand, that unnatural positions also are unnecessary in + the slanting system of writing, and that in either system the pupil + who is permitted to do so is liable to assume an improper position. + + 81 Lovett, _Lateral Curvature of the Spine and Round Shoulders_. + + 82 See "Hygiene of Muscles," Chapter XV. + + 83 On account of the striations of these cells the muscles which they + form are called striated muscles. + + 84 The striated muscle cells, having many nuclei, are said to be + multi-nucleated. + + 85 Every movement in the body has its opposing movement. This is + necessary both on account of the work to be accomplished and for + preserving the natural form of the body. + + 86 The distance from the fulcrum to the power is called the _power-arm_ + and the distance from the fulcrum to the weight is called the + _weight-arm_ (Fig. 115). + + 87 The foot in lifting the body on tiptoe appears at first thought to + be a lever of the second class, the body being the weight and the + toe serving as the fulcrum. However, if the distance which the body + is raised is compared with the distance which the muscle shortens, + it is found that the _supposed_ weight has moved _farther_ than the + power (Fig. 118). It will also be noted that the muscle which + furnishes the power is attached at its upper end to the "weight." + These facts show clearly that we are not here dealing with a lever + of the second class. The foot in this instance acts as a lever of + the first class with the fulcrum at the ankle joint and the toe + pressing against the earth, which is the _actual_ weight. Since the + earth is immovable, the body is lifted or pushed upward, somewhat as + a fulcrum support is made to move when it is too weak to hold up the + weight that is being lifted. In other words, we have the same lever + action in the foot in lifting the body as we have when one lies face + downward, and, bending the knee, lifts some object on the toes. + +_ 88 Walking_ is considered one of the very best forms of counter-active + exercise for the brain worker (page 328). + + 89 The epidermis does not afford complete protection against chemicals, + many of them being able to destroy it quickly. The rule of washing + the skin immediately after contact with strong chemical agents + should always be followed. + + 90 "Rough calculations have placed the number of sweat glands on the + entire body at about 2,000,000." Rettger, _Studies in Advanced + Physiology_. + + 91 Heat also leaves the body by the lungs, partly by the respired air + and partly through the evaporation of moisture from the lung + surfaces. Respiration in some animals, as the dog, is the chief + means of cooling the body. + + 92 "The story is told of some woodsmen who were overtaken by a severe + snowstorm and had to spend the night away from camp; they had a + bottle of whisky, and, chilled to the bone, some imbibed freely + while others refused to drink. Those who drank soon felt comfortable + and went to sleep in their improvised shelter; those who did not + drink felt very uncomfortable throughout the night and could get no + sleep, but in the morning they were alive and able to struggle back + to camp, while their companions who had used alcohol were frozen to + death.... This, if true, was of course an extreme case; but it + accords with the universal experience of arctic travelers and of + lumbermen and hunters in the northern woods, that the use of alcohol + during exposure to cold, although contributing greatly to one's + comfort for the time being, is generally followed by undesirable or + dangerous results."--HOUGH AND SEDGWICK: _The Elements of Hygiene and + Sanitation_. + + 93 Foods that are difficult to digest, or which cause disturbances of + the digestive organs (a coated tongue being one indication), have a + bad effect upon the skin. It is in this way that the use of tea and + coffee by some people induces a sallow or "muddy" condition of the + complexion. + + 94 A most valuable antiseptic ointment is prepared by the druggist from + the following formula: + + + Lanolin, 25 grams. + Ichthyol, 6 grams. + Yellow vaseline, 20 grams. + + + This is applied as a thin layer on the surface, except in the case + of boils or abscesses. In treating these a heavy layer is spread + over the affected part and then covered with absorbent cotton or a + thin piece of clean cotton cloth. + + 95 In a larger sense adjustment includes all those activities by means + of which the body is brought into proper relations with its + environment, including the changes which the body makes in its + surroundings to _adapt them_ to its purposes. + + 96 Almost to the present time, physiologists have described the nervous + system as being made up of two kinds of structural elements which + were called _nerve cells_ and _nerve fibers_. The nerve cells were + supposed to form the ganglia and the fibers to form the nerves. + Recent investigators, however, employing new methods of microscopic + study, have established the fact that the so-called nerve cell and + nerve fiber are but two divisions of the same thing and that the + nervous system is made up of, not two, but one kind of structural + element. The term "neuron" is used to denote this structural + element, or _complete nerve cell_. + + 97 Many of the axons in the brain and spinal cord have no primitive + sheath. Axons without the medullary sheath are found in the + sympathetic nerves. These are known as non-medullated axons and they + have a gray instead of a white color. + + 98 The difference in weight between the brain of man and that of woman + is due mainly to the fact that man's body is, as a rule, + considerably larger than that of woman's. + + 99 The nervous tissues present, at different places, two colors--one + white, and the other a light gray. Great significance was formerly + attached to these colors, because it was supposed that they + represented two essentially different kinds of nervous matter. It is + now known that the protoplasm in all parts of the neuron + proper--cell-body, axis cylinder, and dendrites--has a grayish color, + while the coverings of most of the fibers are white. Hence gray + matter in any part of the nervous system indicates the presence of + cell-bodies, and white matter the presence of nerve fibers. + + 100 In very early life the spinal cord entirely fills the spinal cavity, + but as the body develops the cord grows less rapidly than the spinal + column, and, as a consequence, separates at the lower end from the + inclosing bony column. + + 101 Fibers passing between the spinal cord and the cerebrum cross to + opposite sides--most of them at the bulb, but many within the cord--so + that the right side of the cerebrum is connected with the left side + of the body, and _vice versa_. This accounts for the observed fact + that disease or accidental injury of one side of the cerebrum causes + loss of motion or of feeling in the opposite side of the body. + + 102 In general, _afferent_ neurons or fibers are those that convey + impulses _toward_ the central nervous system (brain and cord), while + _efferent_ neurons or fibers are those that convey impulses _from_ + the central system. + + 103 At different times the nervous impulse has been regarded as a + current of electricity; as a progressive chemical change, likened to + that in a burning fuse; as a mechanical vibration, such as may be + passed over a stretched rope; and as a molecular disturbance + accompanied by an electrical discharge. The velocity of the nervous + impulse, which is only about one hundred feet per second, proves + that it is not a current of electricity. It takes place with little + or no exhaustion of the cell protoplasm and consequently is not due + to chemical action. And the loose, relaxed condition of the nerves + prevents their transmission of physical vibrations, like those on a + stretched rope. The view that the impulse is a progressive molecular + disturbance, accompanied by an electrical discharge, has much + evidence in its favor, but it has only recently been proposed and is + likely to be modified upon fuller investigation. + + 104 The surface of the body includes the linings of the air passages, + food canal, and certain cavities, as well as the external covering + or skin. + + 105 Derived from the Latin _re_, back, and _flectere_, to turn or bend. + + 106 A frog from which the brain has been removed is suspended with its + feet downward and free to move. If a toe is pinched, the foot is + drawn away, and if dilute acid, or a strong solution of salt, is + placed on the tender skin, the feet are moved as if to take away the + irritating substance. This of course shows that reflex action can + take place independently of the brain. + + Now if the spinal cord is also destroyed, there is no response when + the irritation of the skin is repeated. The animal remains perfectly + quiet, because the destruction of the cord has interrupted the + reflex action pathway. This shows that some part of the central + nervous system is necessary to reflex action. + + 107 Review description of the spinal nerves, page 295. + + 108 Where a deep-seated cause for worry exists, there may be occasion + for grave concern. Many people have become insane through continued + worry about some _one_ thing. In cases of this kind the sufferer + needs the aid of sympathetic friends, and sometimes of the + physician, in getting the mind away from the exciting cause. A + change of scene, a visit, or some new employment is frequently + recommended, where the actual cause for the worry cannot be removed. + + 109 Any part of the body which is overworked or which works at a + disadvantage tends to disturb, more or less, the entire nervous + system and to produce nervousness. Especially is this true of such + delicate and highly sensitive structures as the eyes. If the eyes do + not focus properly or if the muscles that move the eyeballs are out + of their natural adjustment, extra work is thrown upon these + delicate parts. One of the first and sometimes the only indication + of eye strain is that of some disturbance of the nervous system. For + this reason it is important to carefully test the eyes in + determining the cause of nervousness (page 385). + + 110 One form of neck exercise recommended for this purpose is easily + taken on retiring at night. Lying flat on the back, without a + pillow, lift the head slowly from the bed and let it as slowly + settle back to the level of the body. Repeat several times, lying on + the back, and then again on the face and again on each side. + Practice these exercises every night during an interval of a month + or until relief is secured. + + 111 Insurance statistics show that habitual _moderate drinkers_ do not + live so long as abstainers. + + 112 Organs very frequently affected by tobacco are the heart and the + eyes. It induces, as already stated (page 56), a dangerous nervous + derangement called "tobacco heart," and it causes a serious disorder + of the retina (retinitis) which leads in some instances to loss of + vision. Tobacco smoke also acts as an irritant to the delicate + lining of the eyes, especially when the tobacco is smoked indoors. + + 113 Of 4117 boys in the Illinois State Reformatory, 4000 used tobacco, + and over 3000 were cigarette smokers. Dr. Hutchison, of the Kansas + State Reformatory, says: "Using cigarettes is the cause of the + downfall of more of the inmates of this institution than all other + vicious habits combined." + + 114 The term "mind" is used in this and preceding chapters in its + popular, not technical, sense. + + 115 The problem of social adjustment is but a phase of the general + problem of establishing proper relations between the body and its + surroundings. + + 116 A vibrating body is one having a to-and-fro movement, like that of a + clock pendulum or the string of a violin on sounding. Bodies to give + out sound waves must vibrate rapidly, making not less than sixteen + vibrations per second. The upper limit of hearing being about 40,000 + vibrations per second, certain bodies may even vibrate too rapidly + to be heard. + + 117 Somewhat as the waves on a body of water impart motion to the sticks + and weeds along the shore, sound waves are able to cause bodies that + are small or that are delicately poised to vibrate. + + 118 Some idea of how the movements of the cartilages change the tension + of the cords may be obtained by holding the fingers on the larynx, + between the thyroid and cricoid cartilages, and making tones first + of low and then of high pitch. For the high tones the cartilages are + pulled together in front, and for the low tones they separate. As + they pull together in front, they of course separate behind and + above, where the cords are attached. + + 119 It is only the central portion of the pinna that aids the entrance + of sound into the auditory canal. If by accident the outer portion + of the pinna is removed, there is no impairment of the hearing. + + 120 The middle ear is also called the _ear drum_, and, by the same + system of naming, the membrana tympani is referred to as the _drum + membrane_. + + 121 The inner projection of the temporal bone is known as the petrous + process. + + 122 A small opening in the bone at this place is called the _fenestra + rotunda_. + + 123 Consult some work on physics on the different kinds of lenses and + their uses. + + 124 With respect to its adjustments the eye does not differ in principle + from various other optical instruments, such as the microscope, + telescope, photographer's camera, etc., which, in their use, form + images of objects. These all require some adjustment of their parts, + called focusing, which adapts them to the distance. The eye's method + of focusing, however, differs from that of most optical instruments, + in that the adjustment is brought about through changes in the + curvature of a lens. + + 125 The converging power of convex lenses varies as the curvature--the + greater the curvature, the greater the converging power. + + 126 An oculist is a physician who specializes in diseases of the eye. + + 127 Some of the more common symptoms of eye strain are nervousness, + headache, insomnia, irritations of the eyelids, sensitiveness to + bright light, and pain in the use of the eyes. + + 128 Pyle, _Personal Hygiene_. + + 129 "An infectious disease is one in which disease germs infect (that + is, invade) the body from without. Among the infectious diseases are + some that are quite directly and quickly conveyed from person to + person and to these the term contagious is applied. Formerly a sharp + line was drawn between infection and contagion, but to-day it is + recognized that no such line exists."--HOUGH AND SEDGWICK, _The + Elements of Hygiene and Sanitation._ + + 130 The arctic explorer, Nansen, states that during all the time that + his party was exposed to the low temperature of the arctic region, + no one was attacked by a cold, but on returning to a warmer climate + they were subject to colds as usual. The difference he attributes to + the absence of germs in the severe arctic climate. There seems to be + no doubt but that most of our common colds are due to attacks of + germs. + + 131 An interesting biological fact is that the female _Anopheles_, and + not the male, sucks the blood of animals and is the cause of the + spreading of malaria. + + 132 The habit of spitting upon the floors of public buildings and street + cars, and also upon sidewalks, is now recognized as a most dangerous + practice. Not only consumptives, but people with throat affections, + may do no end of harm in the spreading of disease by carelessness in + this respect. + + 133 For further information on the care of consumptives, consult Huber's + _Consumption and Civilization_. + + 134 As typhoid fever is a disease of the small intestine, great care + must be exercised in taking food and in the bodily movements. Solids + greatly irritate the diseased lining of the intestine, and the + weakened walls may actually be broken through by pressure resulting + from moving about. + + 135 Alcoholic beverages include all the various kinds of drinks that owe + their stimulating properties to a substance, ethyl alcohol (C2H5OH), + which is made from sugar by the process of fermentation. They + include _malt liquors_, such as beer and ale, which contain from + three to eight per cent of alcohol; _wines_, such as claret, hock, + sherry, and champagne, which contain from five to twenty per cent of + alcohol; and _distilled liquors_, such as brandy, whisky, rum, and + gin, which contain from thirty to sixty-five per cent of alcohol. + Alcoholic beverages all contain constituents other than alcohol, + these varying with the materials from which they are made and with + the processes of manufacture. The distilled liquors are so called + from the fact that their alcohol has been separated from the + fermenting substances by distillation. + + 136 Duncan, _The Chemistry of Commerce_. + + 137 Alcohol is "denatured" by adding substances to it such as wood + alcohol, which render its use as a beverage impossible. + + 138 The tobacco plant, _Nicotiana tobacum_, is a native of America, and + the use of tobacco began with the American Indians. It was taken + back to Europe by the early explorers, Sir Walter Raleigh being + credited with introducing it to the nobility of England. + + 139 Most headaches are the result either of eye strain or of digestive + disturbances, such as indigestion and constipation, and are to be + relieved through the work of the oculist or through attention to the + hygiene of the digestive system. + + + + +***END OF THE PROJECT GUTENBERG EBOOK PHYSIOLOGY AND HYGIENE FOR SECONDARY SCHOOLS*** + + + + +CREDITS + + +November 2005 + + Project Gutenberg Edition + Joshua Hutchinson, Kevin Handy, John Hagerson, Online + Distributed Proofreading Team + + +A WORD FROM PROJECT GUTENBERG + + +This file should be named 18779-0.txt or 18779-0.zip. + +This and all associated files of various formats will be found in: + + + http://www.gutenberg.org/dirs/1/8/7/7/18779/ + + +Updated editions will replace the previous one -- the old editions will be +renamed. + +Creating the works from public domain print editions means that no one +owns a United States copyright in these works, so the Foundation (and +you!) can copy and distribute it in the United States without permission +and without paying copyright royalties. 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