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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..35f8ee2 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #53347 (https://www.gutenberg.org/ebooks/53347) diff --git a/old/53347-0.txt b/old/53347-0.txt deleted file mode 100644 index 06771a6..0000000 --- a/old/53347-0.txt +++ /dev/null @@ -1,4244 +0,0 @@ -The Project Gutenberg EBook of Physiology, by M. Foster - -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 www.gutenberg.org/license - - -Title: Physiology - -Author: M. Foster - -Release Date: October 22, 2016 [EBook #53347] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK PHYSIOLOGY *** - - - - -Produced by Chuck Greif, deaurider and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - LOCKYER’S ASTRONOMY - - _ELEMENTS OF ASTRONOMY_: - - Accompanied with numerous Illustrations, a Colored Representation - of the Solar, Stellar, and Nebular Spectra, - and Celestial Charts of the Northern - and the Southern Hemisphere. - - By J. NORMAN LOCKYER. - - _American edition_, revised and specially adapted to the Schools - of the United States. - - _12mo. 312 pages. Price, $1.75._ - -The volume is as practical as possible. To aid the student in -identifying the stars and constellations, the fine Celestial Charts of -Arago, which answer all the purposes of a costly Atlas of the Heavens, -are appended to the work--this being the only text-book, as far as the -Publishers are aware, that possesses this great advantage. Directions -are given for finding the most interesting objects in the heavens at -certain hours on different evenings throughout the year. Every device is -used to make the study interesting; and the Publishers feel assured that -teachers who once try this book will be unwilling to exchange it for any -other. - - D. APPLETON & CO., PUBLISHERS, - 549 & 551 BROADWAY, NEW YORK. - - - - - SCIENCE PRIMERS, _edited by_ - _PROFESSORS_ HUXLEY, ROSCOE, _and_ - BALFOUR STEWART. - - VI. - _PHYSIOLOGY._ - -[Illustration: EXPLANATION OF THE PLATE. - -FIG. I.--THE HUMAN SKELETON IN PROFILE. - - _Mn._ The Mandible or Lower Jaw. - - _St._ The Sternum. } - _R._ The Ribs. } In the Thorax. - _R´._ The Cartilages of the Ribs. } - - _Scp._ The Scapula, or Shoulder Blade. - - _Cl._ The Clavicle, or Collar Bone. - - _H._ The Humerus. } - _Ra._ The Radius. } In the Arm. - _U._ The Ulna. } - - _F._ The Femur. } - _Tb._ The Tibia. } In the Leg. - _Fb._ The Fibula. } - -FIG. II. - -A front view of the Sternum, _St._, with the Cartilages of the Ribs, -_R´._, and part of the Ribs themselves _R._] - - - - - Science Primers. - - PHYSIOLOGY. - - BY - M. FOSTER, M.A., M.D., F.R.S., - FELLOW OF TRINITY COLLEGE, CAMBRIDGE. - - WITH ILLUSTRATIONS. - - NEW YORK: - D. APPLETON AND COMPANY, - 549 AND 551 BROADWAY. - 1877. - - - - -PREFACE. - - -This Primer is an attempt to explain in the most simple manner possible -some of the most important and most general facts of Physiology, and may -be looked upon as an introduction to the Elementary Lessons of Professor -Huxley. - -In my descriptions and explanations I have supposed the reader to be -willing to handle and examine such things as a dead rabbit and a sheep’s -heart; and written accordingly, I have done this purposely, from an -increasing conviction that actual observation of structures is as -necessary for the sound learning of even elementary physiology, as are -actual experiments for chemistry. At the same time I have tried to make -my text intelligible to those who think reading verbal descriptions less -tiresome than observing things for themselves. - -It seemed more desirable in so elementary a work to insist, even with -repetition, on some few fundamental truths, than to attempt to skim over -the whole wide field of Physiology. I have therefore omitted all that -relates to the Senses and to the functions of the Nervous System, merely -just referring to them in the concluding article. These the reader must -study in the “Elementary Lessons.” - -M. FOSTER. - - - - -TABLE OF CONTENTS. - - -ART. SECT. - - I. INTRODUCTION. - PAGE - - 1. “ What Physiology is 1 - - 2. “ Animals move of their own accord 1 - - 3. “ Animals are warm 3 - - 4. “ Why animals are warm and move about--they burn 4 - - 5. “ The need of Oxygen 5 - - 6. “ The waste matters 5 - - - II. THE PARTS OF WHICH THE BODY IS MADE UP. - - 7. “ The Tissues 7 - - 8. “ The cavities of the Thorax and Abdomen 9 - - 9. “ The Vertebral Column 12 - -10. “ Head and Neck 15 - -11. “ Nerves 18 - -12. “ General arrangement of all these parts 19 - - - III. WHAT TAKES PLACE WHEN WE MOVE. - -13. “ The Bones of the Arm 21 - -14. “ The structure of the Elbow Joint 23 - -15. “ Other joints in the body 25 - -16. “ The arm is bent by the contraction of the Biceps Muscle 26 - -17. “ How the will makes the Biceps Muscle contract 32 - -18. “ The power of a muscle to contract depends on its being - supplied with blood 35 - -19. “ It is the food in the blood which gives the muscle strength 37 - -20. “ The continual need of food 38 - - - IV. THE NATURE OF BLOOD. - -21. “ The Blood in the Capillaries 40 - -22. “ The Corpuscles of the Blood 42 - -23. “ The clotting of Blood 45 - -24. “ The substances present in Serum 48 - -25. “ The minerals in Blood 50 - - - V. HOW THE BLOOD MOVES. - -26. “ The Arteries, Capillaries, and Veins 51 - -27. “ The Sheep’s Heart 55 - -28. “ The Course of the Circulation 58 - -29. “ Why the blood moves in one direction only; the Valves - of the Veins 64 - -30. “ The Tricuspid Valves of the Heart 66 - -31. “ The pulmonary Semilunar Valves 71 - -32. “ The left side of the Heart 72 - -33. “ What makes the blood move at all: The beat of the Heart 76 - -34. “ The action of the Heart as a whole 79 - -35. “ The Capillaries and the Tissues 82 - - - VI. HOW THE BLOOD IS CHANGED BY AIR: BREATHING. - -36. “ Venous and Arterial Blood 85 - -37. “ The change from Arterial to Venous, and from Venous to - Arterial Blood 87 - -38. “ The Lungs 88 - -39. “ The renewal of Air in the Lungs. How the descent of the - Diaphragm expands the Lungs 90 - -40. “ The natural distension of the Lungs. Inspiration. Expiration 91 - -41. “ How the Diaphragm descends 96 - -42. “ The Chest is also enlarged by the movements of the Ribs - and Sternum 98 - -43. “ Breathing an involuntary act 100 - -44. “ Tidal air; stationary air 101 - - - VII. HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. - -45. “ Why the inside of the mouth is always red and moist 103 - -46. “ Why the Skin is sometimes moist. Sweat Glands 106 - -47. “ The Mucous Membrane of the Alimentary Canal and its Glands 109 - -48. “ The Salivary Glands, Pancreas and Liver 111 - -49. “ Food-stuffs 113 - -50. “ How proteids and starch are changed 115 - -51. “ Lacteals and Lymphatics 117 - -52. “ What becomes of the Food-stuffs 120 - - - VIII. HOW THE BLOOD GETS RID OF WASTE MATTERS. - -53. “ The need of getting rid of Waste Matters 123 - -54. “ The Kidneys get rid of Ammonia in the form of Urea 125 - - -55. IX. THE WHOLE STORY SHORTLY TOLD. 127 - - -56. X. HOW WE FEEL AND WILL. 130 - - - - -SCIENCE PRIMERS. - - - - -_PHYSIOLOGY._ - - - - -INTRODUCTION. § I. - -=1.= Did you ever on a winter’s day, when the ground was as hard as a -stone, the ponds all frozen, and everything cold and still, stop for a -moment, as you were running in play along the road or skating over the -ice, to wonder at yourself and ask these two questions:--“Why am I so -warm when all things around me, the ground, the trees, the water, and -the air, are so cold? How is it that I am moving about, running, -walking, jumping, when nothing else that I can see is stirring at all, -except perhaps a stray bird seeking in vain for food?” - -These two questions neither you nor anyone else can answer fully; but we -may answer them in part, and the knowledge which helps us to the answer -is called =Physiology=. - -=2. You can move of your own accord.= You do not need to wait, like the -boughs or the leaves, till the wind blows upon you, or, like the stones, -till somebody stirs you. The bird, too, can move of its own accord, so -can a dog, so can any animal as long as it is alive. If you leave a -stone in any particular spot, you expect to find the stone there when -you come to it again a long time afterwards; if you do not, you say -somebody or something has moved it. But if you put a sparrow or mouse on -the grass plot, you know that directly your back is turned it will be -off. - -All animals move of themselves. But only so long as they are alive. When -you find the body of a snake on the road, the first thing you do is to -stir it with a stick. If it moves only as you move it, and as far as you -move it, just as a bit of rope might do, you say it is dead. But if, -when you touch it, it stirs of itself, wriggles about, and perhaps at -last glides away, you know it is alive. Every living animal, of whatever -kind, from yourself down to the tiniest creature that swims about in a -little pool of water and cannot be seen without a microscope, moves of -itself. Left to itself, it moves and rests, rests and moves; stirred by -anything, away it goes, running, flying, creeping, crawling, or -swimming. - -Something of the kind sometimes happens with lifeless things. When a -stone is carefully balanced on the top of a high wall, a mere touch will -send it toppling down to the ground. But when it has reached the ground -it stops there, and if you want to repeat the trick you must carry the -stone up to the top of the wall again. You know the toy made like a -mouse, which, when you touch it in a particular place, runs away -apparently of its own accord, as if it were alive. But it soon stops, -and when it has stopped you may touch it again and again without making -it go on. Not until you have wound it up will it go on again as it did -before. And every time you want it to run you must wind it up afresh. -Living animals move again and again, and yet need no winding up, for -they are always winding themselves up. Indeed, as we go on in our -studies we shall come to look upon our own bodies and those of all -animals as pieces of delicate machinery with all manner of springs, -which are always running down but always winding themselves up again. - -=3. You are warm=; beautifully warm, even on the coldest winter day, if -you have been running hard; very warm if you are well wrapped up with -clothing, which, as you say, keeps the cold out, but really keeps the -warmth in. The bed you go to at night may be cold, but it is warm when -you leave it in the morning. Your body is as good as a fire, warming -itself and everything near it. - -The bird too is warm, so is the dog and the horse, and every four-footed -beast you know. Some animals however, such as reptiles, frogs, fish, -snails, insects, and the like do not seem warm when you touch them. Yet -really they are always a little warm, and some times they get quite -warm. If you were to put a thermometer into a hive of bees when they are -busy you would find that they are very warm indeed. All animals are more -or less warm as long as they are alive, some of them, such as birds and -four-footed beasts, being very warm. But only so long as they are alive; -after death they quickly become cold. When you find a bird lying on the -grass quite still, not stirring when it is touched, to make quite sure -of its being dead you feel it. If it is quite cold, you say it has been -dead some time; if it is still warm, you say it is only just -dead--perhaps hardly dead, and may yet revive. - -=4. You are warm, and you move about of yourself. You are able to move -because you are warm; you are warm in order that you may move. How does -this come about?= Just think for a moment of something which is not an -animal, but which is warm and moves about, which only moves when it is -warm, and which is warm in order that it may move. I mean a locomotive -steam-engine. What makes the engine move? The burning coke or coal, -whose heat turns the water into steam, and so works the piston, while at -the same time the whole engine becomes warm. You know that for the -engine to do so much work, to run so many miles, so much coal must be -burnt; to keep it working it must be “stoked” with fresh coal, and all -the while it is working it is warm: when its stock of coal is burnt out -it stops, and, like a dead animal, grows cold. - -Well, your body too, just like the steam-engine, moves about and is -warm, because a fire is always burning in your body. That fire, like the -furnace of the engine, needs fresh fuel from time to time, only your -fuel is not coal, but food. In three points your body differs from the -steam-engine. In the first place, you do not use your fire to change -water into steam, but in quite a different way, as we shall see further -on. Secondly, your fire is a burning not of dry coal, but of wet food, a -burning which although an oxidation (Chemistry Primer, Art. 5) takes -place in the midst of water, and goes on without any light being given -out. Thirdly, the food you take is not burnt in a separate part of your -body, in a furnace like that of the engine set apart for the purpose. -The food becomes part and parcel of your body, and it is your whole body -which is burnt, bit by bit. - -Thus it is the food burning or being oxidized within your body, or as -part of your body, which enables you to move and keeps you warm. If you -try to do without food, you grow chilly and cold, feeble, faint, and too -weak to move. If you take the right quantity of proper food, you will be -able to get the best work out of the engine, your body; and if you work -your body aright, you can keep yourself warm on the coldest winter day, -without any need of artificial fire. - -=5.= But if this be so, in order to oxidize your food, you =have need of -oxygen=. The fire of the engine goes out if it is not fed with air as -well as fuel. So will your fire too. If you were shut up in an air-tight -room, the oxygen in the room would get less and less, from the moment -you entered the room, being used up by you; the oxidation of your body -would after a while flag, and you would soon die for want of fresh -oxygen (see Chemistry Primer, p. 14). - -You have, throughout your whole life, a need of fresh oxygen, you must -always be breathing fresh air to carry on in your body the oxidation -which gives you strength and warmth. - -=6.= When a candle is burnt (Chemistry Primer, p. 6) it turns into -carbonic acid, and water. When wood or coal is burnt, we get ashes as -well. If you were to take all your daily food and dry it, it too would -burn into ashes, carbonic acid, and water (with one or two other things -of which we shall speak afterwards). - -Your body is always giving out carbonic acid (Chemistry Primer, Exp. 7). -Your body is always giving out water by the lungs, as seen when you -breathe on a glass, by the skin, and by the kidneys; and we shall see -that we always give out more water than we take in as food or drink. -Your body too is daily giving out by the kidneys and bowels, matters -which are not exactly ashes, but very like them. We do not oxidize our -food quite into ashes, but very nearly; we burn it into substances which -are no longer useful for oxidation in the body, and which, being -useless, are cast out of the body as waste matters. - -The tale then is complete. By the help of the oxygen of the air which -you take in as you breathe, you oxidize the food which is in your body. -You get rid of the water, the carbonic acid, and other waste matters -which are left after the oxidation, and out of the oxidation you get the -heat which keeps you warm and the power which enables you to move. - -Thus all your life long you are in constant need of oxygen and food. The -oxygen you take in at every breath, the food at every meal. How you get -rid of the waste matters we shall see further on. - -If you were to live, as one philosopher of old did, in a large pair of -delicate scales, you would find that the scale in which you were would -sink down at every meal, and gradually rise between as you got lighter -and hungry. If the food you took were more than you wanted, so that it -could not all be oxidized, it would remain in your body as part of your -flesh, and you would grow heavier and stouter from day to day; if it -were less, you would grow thinner and lighter; if it were just as much -as and no more than you needed, you would remain day after day of -exactly the same weight, the scale in which you sat rising as much -between meals as it sank at the meal time. - -What we have to learn in this Primer is--How the food becomes part and -parcel of your body; how it gets oxidized; how the oxidation gives you -power to move; how it is that you are able to move in all manner of -ways, when you like, how you like, and as much as you like. - -First of all we must learn something about the build of your body, of -what parts it is made, and how the parts are put together. - - - - -THE PARTS OF WHICH THE BODY IS MADE UP. § II. - - -=7.= When you want to make a snow man, you take one great roll of snow to -make the body or trunk. This you rest on two thinner rolls which serve -as legs. Near the top of the trunk you stick in another thin roll on -either side--these you call the two arms: and lastly, on quite the top -of the trunk you place a round ball for a head. Head, trunk, and limbs, -_i.e._ legs and arms--these together make up a complete body. - -In your snow man these are all alike, all balls of snow differing only -in size and form; but in your own body, head, trunk, and limbs are quite -unlike, as you might easily tell on taking them to pieces. Now you -cannot very well take your own body to pieces, but you easily can that -of a dead rabbit. Suppose you take one of the limbs, say a leg, to begin -with. - -First of all there is the =skin= with the hair on the outside. If you -carefully cut this through with a knife or pair of scissors and strip it -off, you will find it smooth and shiny inside. Underneath the skin you -see what you call =flesh=, rather paler, not so red as the flesh of beef -or mutton, but still quite like it. Covering the flesh there may be a -little =fat=. In a sheep’s leg as you see it at the butcher’s there is a -good deal of fat, in the rabbit’s there is very little. - -This reddish flesh you must henceforward learn to speak of as =muscle=. If -you pull it about a little, you will find that you can separate it -easily into parcels or slips running lengthways down the leg, each slip -being fastened tight at either end, but loose between. Each slip is what -is called =a muscle=. You will notice that many of these muscles are -joined, sometimes at one end only, sometimes at both, to white or bluish -white glistening cords or bands; made evidently of different material -from the muscle itself. They are not soft and fleshy like the muscle, -but firm and stiff. These are =tendons=. Sometimes they are broad and -short, sometimes thin and long. - -As you are separating these muscles from each other you will see -(running down the leg between them) little white soft threads, very -often branching out and getting too small to be seen. These are =nerves=. -Between the muscles too are other little cords, red, or reddish black, -and if you prick them, a drop or several drops of blood will ooze out. -These are =veins=, and are not really cords or threads, but hollow tubes, -filled with blood. Lying alongside the veins are similar small tubes, -containing very little blood, or none at all. These are =arteries=. The -=veins= and =arteries= together are called =blood-vessels=, and it will be -easy for you to make out that the larger ones you see are really hollow -tubes. Lastly, if you separate the muscles still more, you will come -upon the hard =bone= in the middle of the leg, and if you look closely you -will find that many of the muscles are fastened to this bone. - -Now try to put back everything in its place, and you will find that -though you have neither cut nor torn nor broken either muscle or -blood-vessel or bone, you cannot get things back into their place again. -Everything looks “messy.” This is partly because, though you have torn -neither muscle nor blood-vessel, you have torn something which binds -skin and muscle and fat and blood-vessels and bone all together; and if -you look again you will see that between them there is a delicate -stringy substance which binds and packs them all together, just as -cotton-wool is used to pack up delicate toys and instruments. This -stringy packing material which you have torn and spoilt is called -=connective= because it connects all the parts together. - -Well, then, in the leg (and it is just the same in the arm) we have -skin, fat, muscle, tendons, blood-vessels, nerves, and bone all packed -together with connective and covered with skin. These together form the -solid leg. We may speak of them as =the tissues= of the leg. - -=8.= If now you turn to the trunk and cut through the skin of the belly, -you will first of all see muscles again, with nerves and blood-vessels -as before. But when you carefully cut through the muscles (for you -cannot easily separate them from each other here), you come upon -something which you did not find in - -[Illustration: FIG. 1.--_The Viscera of a Rabbit as seen upon simply -opening the Cavities of the Thorax and Abdomen without any further -Dissection._ - - _A_, cavity of the thorax, pleural cavity of either side; _B_, - diaphragm; _C_, ventricles of the heart; _D_, auricles; _E_, - pulmonary artery; _F_, aorta; _G_, lungs, collapsed, and occupying - only the back part of the chest; _H_, lateral portions of pleural - membranes; _I_, cartilage at the end of sternum; _K_, portion of - the wall of body left between thorax and abdomen; _a_, cut ends of - the ribs; _L_, the liver, in this case lying more to the left than - the right of the body; _M_, the stomach; _N_, duodenum; _O_, small - intestine; _P_, the cæcum, so largely developed in this and other - herbivorous animals; _Q_, the large intestine. -] - -the leg, a =great cavity=. This is something quite new--there is nothing -like it in the leg--a great cavity, quite filled with something, but -still a great cavity; and if you slit the rabbit right up the front of -its trunk and turn down or cut away the sides as has been done in Fig. -1, you will see that the whole trunk is =hollow= from top to bottom, from -the neck to the legs. - -If you look carefully you will see that the cavity is divided into two -by a cross partition (Fig. 1, _B_) called the =diaphragm=. The part =below= -the diaphragm is the larger of the two, and is called the =abdomen= or -belly; in it you will see a large dark red mass, which is the =liver= -(_L_). Near the liver is the smooth pale =stomach= (_M_), and filling up -the rest of the abdomen you will see the coils of the =intestine= or -bowel, very narrow in some parts (_O_), very broad (_P_ _Q_), broader -even than the stomach, in others. If you pull the bowels on one side as -you easily can do, you will find lying underneath them two small -brownish red lumps, one on each side. These are the =kidneys=. - -In the smaller cavity =above= the diaphragm, called the =thorax= or chest, -you will see in the middle the =heart= (_C_), and on each side of the -heart two pink bodies, which when you squeeze them feel spongy. These -are the two =lungs= (_G_). You will notice that the heart and lungs do not -fill up the cavity of the chest nearly so much as the liver, stomach, -bowels, &c. fill up the cavity of the belly. In fact, in the chest -there seems to be a large empty space. But as we shall see further on, -the lungs did quite fill the chest before you opened it, but shrank up -very much directly you cut into it, and so left the great space you see. - -=9.= The trunk then is really a great chamber containing what are called -the =viscera=, and divided into an upper and lower half, the upper half -being filled with the heart and lungs, the lower with the liver, -stomach, bowels, and some other organs. In front the abdomen is covered -by skin and muscle only. But if all the sides of the trunk were made of -such soft material it would be then a mere bag which could never keep -its shape unless it were stuffed quite full. Some part of it must be -strengthened and stiffened. And indeed the trunk is not a bag with soft -yielding sides, but a box with walls which are in part firm and hard. -You noticed that when you were cutting through the front of the chest -you had to cut through several hard places. These were the =ribs= (Fig. 1, -_a_), made either of hard bone or of a softer gristly substance called -=cartilage=. And if you take away all the viscera from the cavity of the -trunk and pass your finger along the back of the cavity, you will feel -all the way down from the neck to the legs a hard part. This is the -=backbone= or =vertebral column=. When you want to make a straw man stand -upright you run a pole right through him to give him support. Such a -support is the backbone to your own body, keeping the trunk from falling -together. - -In the abdomen nothing more is wanted than this backbone, the sides and -front of the cavity being covered in with skin and muscle only. In the -chest the sides are strengthened by the ribs, long thin hoops of bone -which are fastened to the backbone behind and meet in front in a firm -hard part, partly bone, partly cartilage, called the =sternum=. - -But this backbone is not made of one long straight piece of bone. If it -were you would never be able to bend your body. To enable you to do this -it is made up of ever so many little flat round pieces of bone, laid one -a-top of the other, with their flat sides carefully joined together, -like so many bungs stuck together. Each of these little round flat -pieces of the backbone is called a =vertebra=, and is of a very peculiar -shape. Suppose you took a bung of bone, and fastened on to one side of -its edge a ring of bone. That would represent a vertebra. The solid bung -is what is called the =body=, and the hollow ring is what is called the -=arch= of the vertebra. Now if you put a number of these bodies together -one upon the top of the other, so that the bodies all came together and -the rings all came together, you would have something very like the -vertebral column (see Frontispiece, also Fig. 2). The bungs or bodies -would make a solid jointed pillar, and the rings or arches would make -together a tunnel or canal. And that is really what you have in the -backbone. Only each vertebra is not exactly shaped like a bung and a -ring; the body is very like a bung, but the arch is rough and jagged, -and the bodies are joined together in a particular way. Still we have -all the bodies of the vertebræ forming together a solid pillar which -gives support to the trunk; and the arches forming together a tunnel or -canal which is called the =spinal canal=, (Fig. 2, _C.S._) the use of -which we shall see - -[Illustration: FIG. 2. - - A, a diagrammatic view of the human body cut in half lengthways. - _C.S._, the cavity of the brain and spinal cord; _N_, that of the - nose; _M_, that of the mouth; _Al. Al._, the alimentary canal - represented as a simple straight tube; _H_, the heart; _D_, the - diaphragm. - - B, a transverse vertical section of the head taken along the line - _a b_; letters as before. - - C, a transverse section taken along the line _c d_; letters as - before. -] - -directly. The round flat body of each vertebra is turned to the front -towards the cavity of the trunk, and it is the row of vertebral bodies -which you feel as a hard ridge when you pass your fingers down the back -of the abdomen. The arches are at the back of the bodies, so you cannot -feel them in the abdomen; but if you turn the rabbit on its belly and -pass your finger down its back, you will feel through the skin (and you -can feel the same on your own body) a sharp edge, formed by what are -called the spines, _i.e._ the uneven tips of the arches of the vertebræ -(Fig. 2) all the way down the back. - -So that what we really have in the trunk is this. In front a large -cavity, containing the viscera, and surrounded in the upper part or -thorax by hoops of bone, but not (or only slightly) in the lower part or -abdomen; behind, a much smaller long narrow cavity or canal formed by -the arches of the vertebræ, and therefore surrounded by bone all the way -along, and containing we shall presently see what; and between these two -cavities, separating the one from the other, a solid pillar formed by -the bodies of the vertebræ. So that if you were to take a cross slice, -or transverse section as it is called, of the rabbit across the chest, -you would get something like what is represented in Fig. 2, C, where -_C.S._ is the narrow canal of the arches and where the broad cavity of -the chest containing the heart _H_ is enclosed in the ribs reaching from -the vertebra behind to the sternum in front. Both cavities are covered -up on the outside with muscles, blood-vessels, nerves, connective, and -skin, just as in the leg. - -=10.= We have now to consider the =head and neck=. If you cut through the -skin of the neck of the rabbit, you will see, first of all, muscles and -nerves, and several large blood-vessels; but you will find no large -cavity like that in the trunk. So far the neck is just like the leg. But -if you look carefully you will see two tubes which are not -blood-vessels, and the like of which you saw nowhere in the leg. One of -these tubes is firm, with hardish rings in it; it is the windpipe or -=trachea=; the other is soft, and its sides fall flat together; this is -the gullet or =œsophagus=, leading from the mouth to the stomach. -Behind these and the muscles in which they run you will find, just as in -the trunk, a vertebral column, without ribs, but composed of bodies, and -behind the bodies there is a vertebral canal. This vertebral column and -vertebral canal in the neck are simply continuations of the vertebral -column and canal of the trunk. - -=The neck, then, differs from the leg in having a vertebral column and -canal with a trachea and œsophagus, and differs from the trunk in -having no cavity and no ribs.= - -The head, again, is unlike all these. Indeed, you will not understand -how the head is made unless you take a rabbit’s skull and place it side -by side with the rabbit’s head. If you do this, you will at once see how -the mouth and throat are formed. =You will notice that the skull is all -in one piece=, except a bone which you will at once recognize as the -jawbone, or, to speak more correctly, the lower =jawbone=; for there are -two jawbones. Both these carry teeth, but the upper one is simply part -of the skull, and does not move; the lower one does move; it can be made -to shut close on the upper jaw, or can be separated a good way from it. -The opening between the two jaws is the gap or gape of the mouth, which -as you know can be opened or shut at pleasure. If you try it on yourself -you will find that, as in the rabbit, it is the lower jaw which moves -when you open or shut your mouth. The upper jaw does not move at all -except when your whole head moves. Underneath the skull at the top of -the neck the mouth narrows into the throat, into the upper part of which -the cavity of the nose opens. So that there are two ways into the -throat, one through the mouth and the other through the nose (Fig. 2). - -At the back of the skull you will see a rounded opening, and if you put -a bodkin through this opening you will find it leads into a large hollow -space in the inside of the skull. In the living rabbit this hollow space -is filled up with the =brain=. The skull, in fact, is a box of bone to -hold the brain, a bony brain-case. This bony case fits on to the top of -the vertebræ of the neck in such a way that the rounded opening we spoke -of just now is placed exactly over the top of the tunnel or canal formed -by the rings or arches of the vertebræ. If you were to put a wire -through the arch of the lowest vertebra, you might push it up through -the canal formed by the arches of all the vertebræ, right into the brain -cavity. In fact the brain-case and the row of arches of the vertebræ -form together one canal, which is a narrow tube in the back and in the -neck, but swells out in the head into a wide rounded space (Fig. 2, A -and B, _C.S._) During life this canal is filled with a peculiar white -delicate material, which is called =nervous matter=. The rounded mass of -this material which fills up the cavity of the skull is called the -=brain=; the narrower, rod-like, or band-like mass which runs down the -vertebral canal in the neck and back is called the =spinal cord=. They -have separate names, but they are quite joined together, and the rounded -brain tapers off into the band-like cord in such a way that it is -difficult to say where the one begins and the other ends. - -=11.= In the skull, besides the larger openings we have spoken of, you -will find several small holes leading from the outside of the skull into -the inside of the brain-case. Some of these holes are filled up during -life by blood-vessels, but in others run those delicate white threads or -cords which you have already learnt to call nerves. =Nerves are in fact -branches of nervous material running out from the brain or spinal cord.= -Those from the brain pass through holes in the skull, and at first sight -seem to spread out very irregularly. Those which branch off from the -spinal cord are far more regular. A nerve runs out on each side between -every two vertebræ, little rounded gaps being left for that purpose -where the vertebræ fit together, so that when you look at a spinal cord -with portions of the nerves still connected with it, it seems not unlike -a double comb with a row of teeth on either side. The nerves which -spring in this way from the spinal cord are called =spinal nerves=, and -soon after they leave the vertebral canal they divide into branches, and -so are spread nearly all over the body. In any piece of skin or flesh -you examine, never mind in what part of the body, you will find nerves -and blood-vessels. If you trace the nerves out in one direction, you -will find them joining together to form larger nerves, and these again -joining others, till at last all end in either the spinal cord or the -brain. If you try to trace the same nerves in the other direction, you -will find them branching into smaller and smaller nerves, until they -become too small to be seen. If you take a microscope you will find they -get still smaller and smaller until they become the very finest possible -threads. - -The blood-vessels in a similar way join together into larger and larger -tubes, which last all end, as we shall see, in the heart. =Every part of -the body, with some few exceptions, is crowded with nerves and -blood-vessels. The nerves all come from the brain or spinal cord--the -vessels from the heart. So that every part of the body is governed by -two centres, the heart, and the brain or spinal cord.= You will see how -important it is to remember this when we get on a little further in our -studies. - -=12.= Well, then, the body is made up in this way. First there is the -head. In this is the skull covered with skin and flesh, and containing -the brain. The skull rests on the top of the backbone, where the head -joins the neck. In the upper part of the neck, the throat divides into -two pipes or tubes--one the windpipe, the other the gullet. These -running down the neck in front of the vertebral column, covered up by -many muscles, when they get about as far down as the level of the -shoulders, pass into the great cavity of the body, and first into the -upper part of it, or chest. - -Here the windpipe ends in the lungs, but the gullet runs straight -through the chest, lying close at the back on the backbone, and passes -through a hole in the diaphragm into the abdomen, where it swells out -into the stomach. Then it narrows again into the intestine, and after -winding about inside the cavity of the abdomen a good deal, finally -leaves it. - -You see the =alimentary canal= (for that is the name given to this long -tube made up of gullet, stomach, intestine, &c.) =goes right through the -cavity of the body without opening into it=--very much as the tall narrow -glass of a lamp passes through the large globe glass. You might pour -anything down the narrow glass without its going into the globe glass, -and you might fill the globe glass and yet leave the narrow glass quite -empty. If you imagine both glasses soft and flexible instead of hard and -stiff, and suppose the narrow glass to be very long and twisted about so -as to all but fill the globe, you will have a very fair idea of how the -alimentary canal is placed in the cavity of the body. - -Besides the alimentary canal, there is in the chest, in addition to the -windpipe and lungs, the heart with its great tubes, and in the abdomen -there are the liver, the kidneys, and other organs. - -These two great cavities, with all that is inside them, together with -wrappings of flesh and skin which make up the walls of the cavities, -form the trunk, and on to the trunk are fastened the jointed legs and -arms. These have no large cavities, and the alimentary canal goes -nowhere near them. - -One more thing you have to note. There is only one alimentary canal, one -liver, one heart--but there are two kidneys and two lungs, the one on -one side, the other on the other, and the one very much like the other. -There are two arms and two legs, the one almost exactly like the other. -There is only one head, but one side of the head is almost exactly like -the other. One side of the vertebral column is exactly like the -other--as are also the two halves of the brain and the two halves of the -spinal cord. - -In fact, if you were to cut your rabbit in half from his nose to his -tail, you would find that except for his alimentary canal, his heart, -and his liver, one half was almost exactly the counterpart of the other. - -Such is the structure of a rabbit, and your own body, in all the points -I have mentioned, is made up exactly in the same way. - - - - -WHAT TAKES PLACE WHEN WE MOVE. § III. - - -=13.= Let us now go back to the question. =How is it that we can move about -as we do?= And first of all let us take one particular movement and see -if we can understand that. - -For instance, you can bend your arm. You know that when your arm is -lying flat on the table, you can, if you like, bend the lower part of -your arm (the fore-arm as it is called, reaching from the elbow to the -hand) on the upper arm until your fingers touch your shoulder. How do -you manage to do that? - -Look at the bones of the arm in a skeleton. (Frontispiece; also Fig. 3.) -You will see that in the upper arm there is one rather large bone (_H_) -reaching from the shoulder to the elbow, while in the fore-arm there are -two, one (_U_) being wider and stouter than the other (_Ra_) at the -elbow, but smaller and more slender at the wrist. The bone in the upper -arm is called the =humerus=; the bone in the fore-arm, which is stoutest -at the elbow, is called the =ulna=; the one which is stoutest at the -wrist is called the =radius=. If you look carefully you will see that the -end of the humerus at the elbow is curiously rounded, and the end of the -ulna at the elbow curiously scooped out, in such a way that the one fits -loosely into the other. - -[Illustration: FIG 3.--_The Bones of the Upper Extremity with the Biceps -Muscle._ - - The two tendons by which this muscle is attached to the scapula, or - shoulder-blade, are seen at _a_. P indicates the attachment of the - muscle to the radius, and hence the point of action of the power; - F, the fulcrum, the lower end of the humerus on which the upper end - of the radius (together with the ulna) moves; W, the weight (of the - hand). -] - -If you try to move them about one on the other, you will find that you -can easily double the ulna very closely on the humerus without their -ends coming apart, and if you notice you will see that as you move the -ulna up and down, its end and the end of the humerus slide over each -other. But they will only slide one way, what we may call up and down. -If you try to slide them from side to side, you will find that they get -locked. They have only one movement, like that of a door on its hinge, -and that movement is of such a kind as to double the ulna on the -humerus. - -Moreover, if you look a little more carefully you will find that, though -you can easily double the ulna on the front of the humerus, and then -pull it back again until the two are in a straight line, you cannot bend -the ulna on the back of the humerus. On examining the end of the ulna -you will find at the back of it a beak-like projection (Fig. 3, also -Frontispiece), which when the bones are straightened out locks into the -end of the humerus, and so prevents the ulna being bent any further -back. This is the reason why you can only bend your arm one way. As you -very well know, you can bend your arm so as to touch the top of your -shoulder with your fingers, but you can’t bend it the other way so as to -touch the back of your shoulder; you can’t bring it any further back -than the straight line. - -=14.= Well, then, at the elbow the two bones, the humerus and ulna, are so -shaped and so fit into each other that the arm may be straightened or -bent. In the skeleton the two bones are quite separate, _i.e._ they have -to be fastened together by something, else they would fall apart. Most -probably in the skeleton you have been examining they are fastened -together by wires or slips of brass. But they would hold together if you -took away the wire or brass slips and bound some tape round the two -ends, tight enough to keep them touching each other, but loose enough to -allow them to move on each other. You might easily manage it if you took -short slips of tape, or, better still, of india-rubber, and placed them -all round the elbow, back, front, and sides, fastening one end of each -slip to the humerus and the other to the ulna. If you did this you -would be imitating very closely the manner in which the bones at the -elbow are kept together in your own arm. Only the slips are not made of -india-rubber, but are flat bands of that stringy, or as we may now call -it fibrous stuff, which in the preceding lessons you learnt to call -connective tissue. These flat bands have a special name, and are called -=ligaments=. - -At the elbow the two ends of the ulna and humerus are kept in place by -ligaments or flat bands of connective tissue. - -In the skeleton, the surfaces of the two bones at the elbow where they -rub against each other, though somewhat smooth, are dry. If you ever -looked at the knuckle of a leg of mutton before it was cooked, you will -have noticed that you have there two bones slipping over each other -somewhat as they do at the elbow, and will remember that where the bones -meet they are wonderfully smooth, and very moist, so as to be quite -slippery. It is just the same in your own elbow; the end of the ulna and -the end of the humerus are beautifully smooth and quite moist, so that -they slip over each other as easily as possible. You know that your eye -is always moist. It is kept moist by tears, though you don’t speak of -tears until your eyes overflow with moisture; but in reality you are -always crying a little. Well, there are, so to speak, tears always being -shed inside the wrapping of ligaments around the elbow, and they keep -the two surfaces of the bones continually moist. - -The ends of bones where they touch each other are also smooth, because -they are coated over with what is called gristle or cartilage. Bone is -very hard and very solid; there is not much water in it. Bones dry up -very little. Cartilage is not nearly so hard as bone; there is very much -more water in it. When it is quite fresh it is very smooth, but because -it has a good deal of water in it, it shrinks very much when it dries -up, and when dried is not nearly so smooth as when it is fresh. You can -see the dried-up cartilage on the ends of the bones in the skeleton--it -is somewhat smooth still, but you can form no idea of how smooth it is -in the living body by simply seeing it on the dried skeleton. - -=At the elbow, then, we have the ends of two bones fitting into each -other, so that they will move in a certain direction; these ends are -smoothed with cartilage, kept moist with a fluid, and held in place by -ligaments. All this is a called a joint.= - -=15.= There are a great many other joints in the body besides the -elbow-joint: there is the shoulder-joint, the knee-joint, the hip-joint, -and so on. These differ from the elbow-joint in the shape of the ends of -the bone, in the way the bones move on each other, and in several other -particulars, but we must not go into these differences now. They are all -like the elbow, since in each case one bone fits into another, the -surfaces are coated with cartilage, are kept moist with fluid (what the -grooms call joint-oil, though it is not an oil at all), and are held in -place by ligaments. - -I dare say you will have noticed that though I have been speaking only -of the humerus and ulna at the elbow, the other bone of the -fore-arm--the radius--has something to do with the elbow too. I left it -out in order to simplify matters, but it is nevertheless quite true -that the end of the humerus moves over the end of the radius as well as -over the end of the ulna, and that the end of the radius is also coated -with cartilage and is included in the wrapping of the ligaments. I might -add that the radius also moves independently on the ulna, but I don’t -want to trouble you with this just now. What I wanted to show you was -that the elbow is a joint, a joint so constructed that it allows the -fore-arm to be bent on the upper arm. - -=16. In order that the arm may be bent, some force must be used.= The ulna -or radius--for the two move together--must be pushed or pulled towards -the humerus, or the humerus must be pushed or pulled towards the radius -and ulna. How is this done in your own arm? - -Take the bones of the arm; fix the top end of the humerus; tie it to -something so that it cannot move. Fasten a piece of string to either the -radius or ulna (it doesn’t matter which), rather near the elbow. Bore a -hole through the top of the humerus and pass the string through it. Your -string must be long enough to let the arm be quite straight without any -strain on the string. Now, taking hold of the string where it comes out -through the humerus, pull it. The fore-arm will be bent on the arm. Why? -=Because you have been working a lever of the third order.= - -The radius and ulna form the lever; its fulcrum is the end of the -humerus in the elbow (Fig. 3, F); the weight to be moved is the weight -of the radius and ulna (with that of the bones of the hand if present), -and this may be represented by a weight applied at about the middle of -the fore-arm; the power is the pull you give the string, and that is -brought to bear on your lever at the point where the string is fastened -to the radius, _i.e._ nearer the fulcrum than the point where the weight -is applied; and you know that when you have the fulcrum at one end and -the power between the fulcrum and the weight, you have a lever of the -third order. - -Now, in order to make the thing a little more like what takes place in -your own arm, instead of boring a hole through the humerus, let the -string glide in a groove which you will see at the top of the humerus, -and fasten the end of it to the shoulder-blade or anything you like -above the humerus, and let the string be just long enough to let the arm -be quite straightened out, but no longer, so that when the arm is -straight the string is just about tight, or at least not loose. - -Now shorten the string by pinching it up into a loop. Whenever you do -this you will bend the fore-arm on the arm. Suppose you used a string -which you had not to pinch up, but which, when you pleased, you could -make to shorten itself. =Every time it shortened itself it would pull the -fore-arm up and would bend the arm--and every time it slackened again, -the arm would fall back into the straight position.= - -In your arm there is not a string, but a body, placed very much as our -string is placed, and which has the power of shortening itself when -required. Every time it shortens itself it bends the arm, and when it -has done shortening and lengthens again, the arm falls back into its -straight position. This body which thus can shorten and lengthen itself -is called a muscle. - -If you put one hand on the front of your other upper arm, about half-way -between your shoulder and elbow, and then bend that arm, you will feel -something rising up under your hand. This is the muscle, which bends the -arm, shortening, or, as we shall learn to call it, =contracting=. - -In your own arm, as in the limb of the rabbit which you studied in your -last lesson, the flesh is arranged in masses or bundles of various sizes -and shapes, and each mass or bundle is called a muscle. There are -several muscles in the arm, but there is in particular a large one -occupying the front of the arm, called the =biceps=. It is a rounded mass -of red flesh, considerably longer than it is broad or thick, and -tapering away at either end. It is represented in Fig. 3. - -You may remember that while examining the leg of the rabbit you noticed -that in many of the muscles, the soft flesh, which made up the greater -part of the muscle, at one or both ends of the muscle suddenly left off, -and changed into much firmer material which was white and glistening. -This firmer white part you were told was called the tendon of the -muscle. The rest of the muscle, generally called “the belly,” is made up -of what you are accustomed to call flesh, or lean meat, but which you -must now learn to speak of as =muscular substance=. Every muscle, in fact, -consists in the first place of a mass of muscular substance. =This -muscular substance is made up of an immense number of soft strings or -fibres, all running in one direction and done up into large and small -bundles.= At either end of the muscle these soft muscular fibres are -joined on to firmer but thinner fibres of connective or fibrous tissue. -And these thinner but firmer fibres make up the cord or band of tendon -with which the muscle finishes off at either end. - -=It is by these tendons that the soft muscles are joined on to the hard -bones, or to some of the other firm textures of the body.= The tendons -are sometimes round and cord-like, sometimes flat and spread out. -Sometimes they are very long, sometimes very short, so as to be scarcely -visible. But always you have some amount of the firmer fibres of -connective tissue joining the soft muscular fibres on to the bones, and -generally the tendons are not only firmer but much thinner and more -slender than the belly of the muscle. - -The muscular belly of the biceps is placed in the front of the upper -arm. Some little way above the elbow-joint it ends in a small round -strong tendon which slips over the front of the elbow and is fastened -to, _i.e._ grows on to, the radius at some little distance below the -joint (Fig. 3, P). The upper part of the muscular belly ends a little -below the shoulder, not in one tendon but in two[1] tendons (Fig. 3, -_a_), which gliding over the end of the humerus are fastened to the -shoulder-blade (or =scapula= as it is called), into which the humerus fits -with a joint. - -We have then in the biceps a thick fleshy muscular belly placed in the -front of the arm and fastened by tendons, at one end to the -shoulder-blade, and at the other to the fore-arm. What would happen if -when the arm is straight and the shoulder-blade fixed, the biceps were -suddenly to grow very much shorter than it was? Evidently the same -thing that happened when you pinched up and shortened the string which, -if you look back you will see, we supposed to be placed very much as the -biceps with its tendons is placed. =The radius and ulna would be pulled -up, the fore-arm would be bent on the arm.= - -Now tendons have no power of shortening themselves, but muscular -substance does possess this remarkable power of suddenly shortening -itself. Under certain circumstances each soft muscular fibre of which -the muscle is made will suddenly become shorter, and thus the whole -muscle becomes shorter, and so pulls its two tendinous ends closer -together, and if one end be fastened to something fixed, and the other -to something moveable, the moveable thing will be moved. - -This way that a muscle or a muscular fibre has of suddenly shortening -itself is called a =muscular contraction=. =All muscles, all muscular -fibres, have the power of contracting.= Now a mass of substance like the -biceps might grow shorter in two ways. It might squeeze itself together -and become smaller altogether, it might squeeze itself as you would -squeeze a sponge into a smaller bulk. Or it might change its form and -not its bulk, becoming thicker as it became shorter, just as you might -by pressing the two ends together squeeze a long thin roll of soft wax -into a short thick one. It might get shorter in either of these two -ways, but it does actually do so in the latter way; it gets thicker at -the same time that it gets shorter, and gets nearly as much thicker as -it gets shorter. And that is why, when you put your hand on the arm -which is being bent, you feel something rise up. =You feel the biceps -getting thicker as it is getting shorter in order to bend the arm.= - -The shortening does not last for ever. Sooner or later the muscle -lengthens again, getting thinner once more, and so returns to its former -state. The lengthened condition of the muscle is the natural condition, -the condition of rest. The shortening or contraction is an effort which -can only be continued for a certain time. The contraction bends the arm, -and as long as the muscle remains shortened the arm keeps bent; but as -the muscle lengthens, the weight of the hand and fore-arm, if there is -nothing to prevent, straightens the arm out again. - -=It is in the muscle alone, in the belly made up of muscular fibres, that -the shortening takes place.= The tendons do not shorten at all. On the -contrary, if anything they lengthen a little, but only a very little, -when the muscle pulls upon them. Their purpose is to convey to the bone -the pull of the muscle. They are not necessary, only convenient. It -would be possible but awkward to do without them. Suppose the fleshy -fibres of the biceps reached from the shoulder-blade to the fore-arm: -you could bend your arm as before, but it would be very tiresome to have -the muscle swelling up in the inside of the elbow, or on the top of the -shoulder; in either place it would be very much in the way. By keeping -the fleshy, the real contracting muscle, in the arm, and carrying the -thin tendons to the arm and to the shoulder, you are enabled to do the -work much more easily and conveniently. - -Well, then, we have got thus far in understanding how the arm is bent. -The biceps muscle contracts and shortens, tries to bring its two -tendinous ends together. The upper tendons, being fastened to the fixed -shoulder-blade, cannot move; but the lower tendon is fixed to the -radius; the radius, with the ulna to which it is fastened, readily moves -up and down on the elbow-joint--the shape of bones in the joint and all -the arrangements of the joint, as we have seen, readily permitting this. -When the muscle, then, pulls on its lower tendon, its pulls on the -radius at the point where the tendon is fastened on to the bone. The -radius thus pulled on forms with the ulna a lever of the third order, -working on the end of the humerus as a fulcrum; and thus as the tendon -is pulled the fore-arm is bent. - -=17.= But now comes the question. What makes the muscle shorten or -contract? You willed to move your arm, and moved it, as we have seen, by -making the biceps contract; but =how did your will make the biceps -contract=? - -If you could examine your arm as you did the leg of the rabbit, you -would find running into your biceps muscle, one or more of those soft -white threads or cords, which you have already learnt to recognize as -=nerves=. - -These nerves seem to grow into and be lost in the biceps muscle. We need -not follow them any further in that direction, but if we were to trace -them in the other direction, up the arm, we should find that they soon -meet with other similar nerves, and that the several nerves joining -together form stouter and thicker nerve-cords. These again join others, -and so we should proceed until we came to quite stoutish white -nerve-trunks as they are called, which we should find passed at last -between the vertebræ, somewhere in the neck, into the inside of the -vertebral canal, where they became mixed up with the mass of nervous -material we have already spoken of as the spinal cord. - -What have these nerves to do with the bending of the arm? Why simply -this. Suppose you were able without much trouble to cut across the -delicate nerves going to your biceps, and did so: what would happen? You -would find that you had lost all power of bending your arm; however much -you willed it, there would be no swelling rise up in your arm. Your -biceps would remain perfectly quiet, and would not shorten at all, would -not contract in obedience to your will. - -What does this show? It proves that when you will to bend your arm, -something passes along the nerves going to the biceps muscle, which -something causes that muscle to contract? The nerve, then, is a bridge -between your will and the muscle--so that when the bridge is broken or -cut away, the will cannot get to the muscle. - -If anywhere between the muscle and the spinal cord you cut the nerve -which goes, or branches from which go, to the muscle, you destroy the -communication between the will and the muscle. - -The spinal cord, as we have seen, is a mass of nervous substance -continuous with the brain; from the spinal cord nearly all the nerves of -the body are given off; those nerves whose branches go to the biceps -muscle in the arm leave the spinal cord somewhere in the neck. - -If you had the misfortune to have your spinal cord cut across or -injured in your neck, you might still live, but you would be paralysed. -You might will to bend the arm, but you could not do it. You would know -you were willing, you would feel you were making an effort, but the -effort would be unavailing. The spinal cord is part of the bridge -between the will and the muscle. - -When you bend your arm, then, this is what takes place. =By the exercise -of your will a something is started in your brain. That something--we -will not stop now to ask what that something is--passes from your brain -to the spinal cord, leaves the spinal cord and travels along certain -nerves, picking its way among the intricate bundles of delicate nervous -threads which run from the upper part of the spinal cord to the arm -until it reaches the biceps muscle. The muscle, directly that -“something” comes to it along its nerves, contracts, shortens, and grows -thick; it rises up in the arm; its lower tendon pulls at the radius; the -radius with the ulna moves on the fulcrum of the humerus at the -elbow-joint, and the arm is bent.= - -You wish to leave off bending the arm. Your will ceases to act. The -something to which your will had given rise dies away in the brain, dies -away in the spinal cord, dies away in the nerves, even in the finest -twigs. The muscle, no longer excited by that something, ceases to -contract, ceases to swell up, ceases to pull at the radius, and the -fore-arm by its own weight falls into its former straightness, -stretching, as it falls, the muscle to its natural length. - -=18.= So far I hope you have followed me, but we are still very far from -being at the bottom of the matter. Why does the muscle contract when -that something reaches it through the nerves? We must content ourselves -by saying that it is the property of the muscle to do so. Does the -muscle always possess this property? No, not always. - -Suppose you were to tie a cord very tightly round the top of your arm, -close to the shoulder. What would happen? If you tied it tight enough (I -don’t ask you to do it, for you might hurt yourself) the arm would -become pale, and very soon would begin to grow cold. It would get -numbed, and would gradually seem to grow very heavy and clumsy; your -feeling in it would be blunted, and after a while be altogether lost. -When you tried to bend your arm you would find great difficulty in doing -so. Though you tried ever so much, you could not easily make the biceps -contract, and at last you would not be able to do so at all. You would -discover that you had lost all power of bending your arm. And then if -you undid the cord you would find that after some very uncomfortable -sensations, little by little the power would come back to you; the arm -would grow warm again, the heaviness and clumsiness would pass away, the -feeling in it would return, you would be able to bend it, and at last -all would be as it was before. - -What did you do when you tied the cord tight? The chief thing you did -was to press on the blood-vessels in the arm and so stop the blood from -moving in them. If instead of tying the cord round the whole arm you had -tied a finer thread round the blood-vessels only, you would have -brought about very nearly the same effect. We saw in the last lesson how -all parts of the body are supplied with blood-vessels, with veins, and -arteries. In the arm there is a very large artery, branches from which -go all over the arm. Some of these branches go to the biceps muscle. -What would happen if you tied these branches only, tying them so tight -as to stop all the blood in them, but not interfering with the -blood-vessels in the rest of your arm? The arm as a whole would grow -neither pale nor cold, it would not become clumsy or heavy, you would -not lose your feeling in it, but nevertheless if you tried to bend your -arm you would find you could not do it. You could not make the biceps -contract, though all the rest of the arm might seem to be quite right. - -What does this teach us? =It teaches us that the power which a muscle has -of contracting when called upon to do so, may be lost and regained, and -that it is lost when the blood is prevented from getting to it.= When a -cord is tied round the whole arm, the power of the whole arm is lost. -This loss of power is the beginning of death, and indeed if the cord -were not unloosed the arm would quite die--would mortify, as it is said. -When only those blood-vessels which go to the biceps are tied, the -biceps alone begins to die, all the rest of the arm remaining alive, and -the first sign of death in the biceps is the loss of the power to -contract when called upon to do so. - -In order that you may bend your arm, then, you must not only have a -biceps muscle with its nerves, its tendons, and all its arrangements of -bones and joints, but the muscle must be supplied with blood. - -=19.= We can now go a step further and ask the question, =What is there in -the blood that thus gives to the muscle the power of contracting, that -in other words keeps the muscle alive?= The answer is very easily found. -What is the name commonly given to this power of a muscle to contract? -We generally call it strength. Lay your arm straight out on the table, -put a heavy weight in your hand, and try to bend your arm. If you could -do it, one would say you were strong; if you could not, one would say -you were weak--all the stronger or weaker, the heavier or lighter the -weight. In the one case your biceps had great power of contracting; in -the other, little power. Try and find out the heaviest weight you can -raise in this way by bending your arm, some morning, not too long after -breakfast, when you are fresh and in good condition. Go without any -dinner, and in the afternoon or evening, when you are tired and hungry, -try to raise the same weight in the same way. You will not be able to do -it. Your biceps will have lost some of its power of contracting, will be -weaker than it was in the morning. What makes it weak? The want of food. -But how can the food affect the muscle? You do not place the food in the -muscle; you put it into your mouth, and from thence it goes into your -stomach and into the rest of your alimentary canal, and there seems to -disappear. How does the food get at the muscle? By means of the blood. -The food becomes blood. =The things which you eat as food become changed -into other things which form part of the blood. Those things going to -the muscle give it strength and enable it to contract.= And that is why -food makes you strong. - -=20.= But you are always wanting food day by day, from time to time. Why -is that? Because the muscle in getting strength out of the food changes -it, uses it up, and so is always wanting fresh blood and new food. We -have seen in Art. 1 that food is fuel. We have also seen that muscle -(and other parts of the body do the same) is always burning, burning -without flame but with heat, burning slowly but burning all the same, -and doing the more work the more it burns. The fuel it burns is not dry -wood or coal, but wet, watery blood, a special kind of fuel prepared for -its private use, in the workshop of the stomach or elsewhere, out of the -food eaten by the mouth. This it is always using up; of this it must -always have a proper supply, if it is to go on working. Hence there must -always be fresh blood preparing; hence there must from time to time be -fresh supplies of food out of which to manufacture fresh blood. - -To understand then fully what happens when you bend your arm, we have to -learn not only what we have learnt about the bones and the joint and the -muscle and the nerves, about the machinery and the engine, we have to -study also how the food is changed into blood, how the blood is brought -to the muscle, what it is in the blood on which the muscle lives, what -it is which the muscle burns, and how the things which result from the -burning, the ashes and the smoke or carbonic acid and the rest of them, -are carried away from the muscle and out of the body. - -Meanwhile let me remind you that for the sake of being simple I have -been all this while speaking of one muscle only, the biceps in the arm. -But there are a multitude of muscles in the body besides the biceps, as -there are many bones besides those of the arm, and many joints besides -the elbow. But what I have said of the one is in a general way true of -all the rest. The muscles have various forms, they pull upon the bones -in various ways, they work on levers of various kinds. The joints differ -much in the way in which they work. All manner of movements are produced -by muscles pulling sometimes with and sometimes against each other. But -you will find when you come to examine them that all the movements of -which your body is capable depend at bottom on this--=that certain -muscular fibres, in obedience to a something reaching them through their -nerves, contract, shorten, and grow thick, and so pull their one end -towards the other, and that to do this they must be continually supplied -with pure blood=. - -Moreover, what I have said of the relations of muscle to blood is also -true of all other parts of the body. Just as the muscle cannot work -without a due supply of blood, so also the brain and the spinal cord and -the nerves have even a more pressing need of pure blood. The weakness -and faintness which we feel from want of food is quite as much a -weakness of the brain and of the nerves as of the muscles,--perhaps -rather more so. And other parts of the body of which we shall have to -speak later on need blood too. - -The whole history of our daily life is shortly this. The food we eat -becomes blood, the blood is carried all over the body, round and round -in the torrent of the circulation; as it sweeps past them, or rather -through them, the muscle, the brain, the nerve, the skin pick out new -food for their work and give back the things they have used or no longer -want. As they all have different works, some use up what others have -thrown away. There are, besides, scavengers and cleaners to pick up -things no longer wanted anywhere and to throw them out of the body. Thus -the blood is kept pure as well as fresh. Through the blood thus ever -brought to them, each part does its work: the muscle contracts, the -brain feels and wills, the nerves carry the feeling and the willing, and -the other organs of the body do their work too, and thus the whole body -is kept alive and well. - - - - -THE NATURE OF BLOOD. § IV. - - -=21. What, then, is this blood which does so much?= - -Did you ever look through a good microscope at the thin transparent web -of a frog’s foot, and watch the red blood coursing along its narrow -channels? If not, go and look at it at once; you will never understand -any physiology till you have done so. There you will see a network of -delicate passages far finer than any of your own hairs, and through -those passages a tumbling crowd of tiny oval yellow globules hurrying -and jostling along. Some of the passages are wider than others, and -through some of the wider ones you will see a thick stream of globules -rushing onwards towards the smaller channels, and spreading out among -them. The globules which you see are floating in a fluid so clear that -you cannot see it. Some of the smaller channels are so narrow that only -one globule or =corpuscle=, as we may call it, can pass through at a time, -and very frequently you may see them passing in single file. Watching -them as they glide along these narrow paths, you will note that at last -they tumble again into wider passages, somewhat like those from which -they came, except that the stream runs away from instead of towards the -narrower channels; and in the stream the corpuscle you are watching -shoots out of sight. The finest passages are called =capillaries=; they -are guarded by delicate walls which you can hardly see; they seem to you -passages only, and how fine and small they are will come home to you -when you recollect that all you are looking at is going on in the depths -of a skin which is so thin that perhaps you would be inclined to say it -has no thickness at all. - -The larger channels which are bringing the blood down to the capillaries -are the ends of vessels like those which in the rabbit you learnt to -call arteries, and the other larger channels through which the blood is -rushing away from the capillaries are the beginnings of veins. - -When you have watched this frog’s foot for some little time, turn away -and reflect that in almost every part of your own body, in every square -inch, in almost every square line, something very similar might be seen -could the microscope be brought to bear upon it, only the corpuscles are -smaller and round, the capillaries narrower and for the most part more -thick-set, and the race a swifter one. In the muscle of which we were -speaking in the last lesson, each of the soft long fibres of which the -muscle is composed is wrapped round with a close network of these tiny -capillaries, through which, as long as life lasts, for ever rushes a -swift stream of blood, reddened by countless numbers of tiny corpuscles. - -In every part of your flesh, in your brain and spinal cord, in your -skin, your bones, your lungs, in all organs and in nearly every part of -your body, there is the same hurrying rush through narrow tubes of red -corpuscles and of the clear fluid in which these swim. - -If you prick your finger it bleeds. Almost any part of your body would -bleed were you to prick it. So thick-set are the little blood-vessels, -that wherever you thrust a needle, be it as fine a needle as you please, -you will be sure to pierce and tear some little blood channel, either -artery or capillary or vein, and out will come the ruddy drop. - -=22. What is blood?= It is a fluid; it runs about like water: yet it is -thicker than water, thicker for two reasons. In the first place, water, -that is pure water, is all one substance. If you were to look at it with -ever so powerful a microscope, you would see nothing in it. It is -exceedingly transparent--you can see very well through ever such a -thickness of clean water. But if you were to try and look through even a -very thin sheet of blood spread out between two glass plates, you would -find that you could see very little; =blood is very opaque=. If again you -examine a drop of your blood with a microscope, what do you see? =A -number of little= - -[Illustration: FIG. 4.--_Red and White Corpuscles of the Blood -magnified._ - - _A._ Moderately magnified. The red corpuscles are seen lying in - rows like rolls of coins; at _a_ and _a_ are seen two white - corpuscles. - - _B._ Red corpuscles much more highly magnified, seen in face; _C._ - ditto, seen in profile; _D._ ditto, in rows, rather more highly - magnified; _E._ a red corpuscle swollen into a sphere by imbibition - of water. - - _F._ A white corpuscle magnified same as _B._; _G._ ditto, throwing - out some blunt processes; _K._ ditto, treated with acetic acid, and - showing nucleus, magnified same as _D._ - - _H._ Red corpuscles puckered or crenate all over. - - _I._ Ditto, at the edge only. -] - -=round bodies, the blood discs or blood corpuscles= (Fig. 4, _A_). If you -look carefully you will notice that most of them are round, as _B_; but -every now and then you see something like _C_. That is one of the round -ones seen sideways; for they are not round or spherical like a ball, but -circular and dimpled in the middle, something like certain kinds of -biscuit. When you see one by itself it looks a little yellow in colour, -that is all; but when you see them in a lump, the lump is clearly red. -Remember how small they are: three thousand of them put flat in a line, -edge to edge, like a row of draughts, would just about stretch across -one inch. All the redness there is in blood belongs to them. When you -see one of them, you see so little of the redness that it seems yellow. -If you were to put a drop of blood into a tumbler of water, the water -would not be stained red, but only just turned of a yellowish tint, so -little redness would be given to it by the drop of blood. In the same -way a very very thin slice of currant jelly would look yellowish, not -red. - -These red corpuscles are not hard solid things, but delicate and soft, -very tender, very easily broken to pieces, more like the tiniest lumps -of red jelly than anything else, and yet made so as to bear all the -squeezing which they get as they are driven round and round the body. - -Besides these red corpuscles, you may see if you look attentively =other -little bodies, just a little bigger than the red corpuscles, not -coloured at all, and not circular and flat, but quite round like a ball= -(Fig. 4, _a_, _F_, _G_). That is to say, these are very often quite -round, only they have a curious trick of changing their form. Imagine -you were looking at a suet dumpling so small that about two thousand -five hundred of them could be placed side by side in the length of one -inch--and suppose the round dumpling while you were looking at it -gradually changed into the shape of a three-cornered tart, and then -into a rounded square, and then into the shape of a pear, and then into -a thing that had no shape at all, and then back again into a round ball, -and kept doing this apparently all of its own accord while you were -looking at it--wouldn’t you think it very curious? Well, one of these -little bodies in the blood of which we are speaking, and which are -called white corpuscles, may be seen, when a drop of blood is watched -under the microscope, to go on in this way, continually changing its -shape. But of these =white corpuscles= of the blood, and of their -wonderful movements, you will learn more as you go on in your -physiological studies. - -=23.= Besides these red and white corpuscles there is nothing else very -important in the blood that you can _see_ with the microscope; but their -being in the blood is one reason why blood is thicker than water. - -Did you ever see a pig or sheep killed? If so, you would be sure to -notice that the blood ran quite fluid from the blood-vessels in the -neck, ran and was spilt like so much water--but that very soon the blood -caught in the pail or spilt on the stones became quite solid, so that -you could pick it up in lumps. Whenever blood is shed from the living -body, within a short time it becomes solid. This becoming solid is -called the =clotting= or =coagulation of blood=. - -What makes it clot? Suppose while the blood was running from the pig’s -neck into the butcher’s pail, and while it was still quite fluid, you -were to take a bunch of twigs and keep slowly stirring the blood round -and round in the pail. You would naturally expect that the blood would -soon begin to clot, would get thicker and thicker and more and more -difficult to stir. But it does not; and if you keep on stirring long -enough you will find that it never clots at all. =By continually stirring -it you will prevent its clotting.= Now take out your bundle of twigs: you -will find it covered all over with a thick reddish mass of some soft -sticky substance; and if you pump on the red mass you will be able to -wash away all its red colour, and will have nothing left but a quantity -of white, soft, sticky, stringy material, all entangled and matted -together among the twigs of your bundle. This stringy material is in -reality made up of a number of fine, delicate, soft, elastic threads or -fibres, and is called =fibrin=. - -=You see, by stirring, or, as it is frequently called, whipping the blood -with the bundle of twigs, you have taken the fibrin out of the blood, -and so prevented its clotting.= - -If you were to take one of the clotted lumps of blood that were spilt on -the ground or a bit of the clot from a pail in which the blood had not -been whipped, and wash it long enough, you would find at last that all -the colour went away from the lump, and you had nothing left but a small -quantity of white stringy substance. This white stringy substance is -fibrin--exactly the same thing you got on your bundle of twigs. - -If the blood is carefully caught in a pail, and afterwards not disturbed -at all, it clots into a solid mass. The whole of the blood seems to have -changed into a complete jelly; and if you turn it out of the pail, as -you may do, it keeps its shape, and gives you quite a mould of the pail, -a great trembling red jelly just the shape of the inside of the pail. - -But if you were to leave the blood in the pail for a few hours or for a -day, you would find, instead of the large jelly quite filling the pail, -a smaller but firmer jelly covered by or floating in a colourless or -very pale yellow liquid. This smaller, firmer jelly, which in the course -of a day or so would get still firmer and smaller, would in fact go on -shrinking in size, you may still call the =clot=; the clear fluid in which -it is floating is called =serum=. - -What has taken place is as follows. Soon after blood is shed there is -formed in it a something which was not present in it before. This -something, which we call =fibrin=, starts as a multitude of fine tender -threads which run in all directions through the mass of blood, forming a -close network everywhere. So the blood is shut up in an immense number -of little chambers formed by the meshes of the fibrin; and it is this -which makes it seem a jelly. But each thread of fibrin as soon as it is -formed begins to shrink, and the blood in each of these little chambers -is squeezed by the shrinking of its walls of fibrin, and tries to make -its way out. The corpuscles get caught in the meshes, but all the rest -of the blood passes between the threads and comes out on the top and -sides of the pail. And this goes on until you have left in the clot very -little besides corpuscles entangled in a network of fibrin, and all the -rest of the blood has been squeezed outside the clot, and is then called -serum. =Serum, then, is blood out of which the corpuscles have been -strained by the process of clotting.= - -Now I dare say you are ready to ask the question, If blood clots so -readily when it is shed, why does it not clot inside the body? Why is -our blood ever fluid? This is rather a difficult question to answer. -When blood is shed from the warm body it soon gets cool. But it does not -clot and become solid because it gets cool, as ordinary jelly does. If -you keep it from getting cool it clots all the same, in fact quicker, -and if kept cold enough will not clot at all. Nor does it clot when -shed, because it has become still, and is no longer rushing round -through the blood-vessels. Nor is it because it is exposed to the air. -Perhaps we don’t know exactly why it is, and you will have much to learn -hereafter about the coagulation of blood. All I will say at present is -that as long as the blood is in the body there is something at work to -keep it from clotting. It does clot sometimes in the body, and -blood-vessels get plugged with the clots; but that constitutes a very -dangerous disease. - -=24.= Well, blood is thicker than water because it contains solid -corpuscles and fibrin. But even the serum, _i.e._ blood out of which -both fibrin and corpuscles have been taken, is thicker than water. - -You know that if you were to take a basinful of pure water and boil it, -it would boil away to nothing. It would all go off in steam. But if you -were to try to boil a basinful of serum, you would find several curious -things happen. - -In the first place you would not be able to boil it at all. Before you -got it as hot as boiling water, your serum, which before seemed quite as -liquid as water, only feeling a little sticky if you put your finger in -it, would all become quite solid. You know the difference between a raw -and a boiled egg. The white of the raw egg, though very sticky and ropy, -or viscid as it is called, is still liquid; you will find it hard work -if you try to cut it with a knife. The white of the hard boiled egg, on -the other hand, is quite solid, and you can cut it into ever so thin -slices. It has been “set” by boiling. Well, the serum of blood is in -this respect very like white of egg. In fact they both contain the same -substance, called =albumin=, which has this property of “setting” or -becoming solid when heated nearly to boiling-point. Both the serum of -blood and white of egg even when “set” are wet, _i.e._ contain a great -deal of water. You may dry them in the proper manner into a transparent -horny substance. When quite dry they will readily burn. They are -therefore things which can be oxidized. When burnt they give off -carbonic acid, water, and ammonia; the latter you might easily recognize -by its effect on your nose if you were to burn a piece of dried blood in -a flame. Now, when I say that =albumin= in burning gives off carbonic -acid, water, and ammonia, you know from your Chemistry that it must -contain carbon to form the carbonic acid, hydrogen to form water, and -nitrogen to form ammonia. It need not contain oxygen, for as you know it -could get all the oxygen it wanted from the air; still it does contain -some oxygen. =Albumin, then, is an oxidizable or combustible body made up -of nitrogen, carbon, hydrogen, and oxygen.= It is important you should -remember this; but I will not bother you with how much of each--it is a -very complex substance, built up in a wonderful way, far more complex -than any of the things you had to learn about in your Chemistry Primer. -And this albumin, dissolved in a great deal of water, forms the serum of -blood. - -I did not say anything about what fibrin was made of; but it, like -albumin, is made up of nitrogen, carbon, hydrogen, and oxygen. It is not -quite the same thing as albumin, but first cousin to it. There is -another first cousin to both of them, also containing nitrogen, carbon, -hydrogen, and oxygen, which together with a great deal of water forms -muscle; another forms a great part of the red corpuscles; and scattered -all over the body in various places, there are first cousins to albumin, -all containing nitrogen, carbon, hydrogen, and oxygen, all combustible, -and all when burnt giving off carbonic acid, water, and ammonia. All -these first cousins go under one name; they are all called =proteids=. - -=25.= Well, then, blood is thicker than water by reason of the proteids in -the corpuscles, in the fibrin, and in the serum, but there is something -else besides. I will not trouble you with the crowd of things of which -there are perhaps just a few grains in a gallon of blood, like the -little pinches of things a cook puts into a savoury dish; though, as you -go on in your studies, you will find that these, like many other little -things in the world, are of great importance. - -But I will ask you to remember this. If you take some dried blood and -burn it, though you may burn all the proteids (and some other of the -trifles I spoke of just now) away, you will not be able to burn the -whole blood away. Burn as long as you like, you will always have left a -quantity of what you have learnt from your Chemistry to call =ash, and if -you were to examine this ash you would find it contained ever so many -elements; sulphur, phosphorus, chlorine, potassium, sodium, calcium, and -iron, being the most abundant and most important=. - -Blood, then, is a very wonderful fluid: wonderful for being made up of -coloured corpuscles and colourless fluid, wonderful for its fibrin and -power of clotting, wonderful for the many substances, for the proteids, -for the ashes or minerals, for the rest of the things which are locked -up in the corpuscles and in the serum. - -But you will not wonder at it when you come to see that the blood is the -great circulating market of the body, in which all the things that are -wanted by all parts, by the muscles, by the brain, by the skin, by the -lungs, liver, and kidney, are bought and sold. What the muscle wants, -it, as we have seen, buys from the blood; what it has done with it sells -back to the blood; and so with every other organ and part. As long as -life lasts this buying and selling is for ever going on, and this is why -the blood is for ever on the move, sweeping restlessly from place to -place, bringing to each part the things it wants, and carrying away -those with which it has done. When the blood ceases to move, the market -is blocked, the buying and selling cease, and all the organs die, -starved for the lack of the things which they want, choked by the -abundance of things for which they have no longer any need. - -We have now to learn how the blood is thus kept continually on the move. - - - - -HOW THE BLOOD MOVES. § V. - - -=26.= You have already learnt to recognize the blood-vessels of the -rabbit, and to distinguish two kinds of blood-vessels--the arteries, -which in a dead animal generally contain little or no blood, and have -rather firm stout walls; and the veins, which are generally full of -blood, and have thinner and flabby walls. The arteries when you cut them -generally gape and remain open; the veins fall together and collapse. -The larger the arteries, the stouter and firmer they are, and the -greater the difference between them and the veins. - -You have also studied the capillaries in the frog’s foot; you have seen -that they are minute channels, with the thinnest and tenderest walls, -forming a close network in which the smallest arteries end, and from -which the smallest veins begin. - -You have moreover been told that all over your own body, in every part, -there are, though you cannot see them, networks of capillaries like -those in the frog’s foot which you can see; that all the arteries of -your body end in capillaries, and all the veins begin in capillaries. -Let me repeat that, one or two structures excepted, there is no part of -your body in which, could you put it under a microscope, you would not -see a small artery branching out and losing itself in a network of -capillaries, out of which, as out of so many roots, a small vein gathers -itself together again. - -In some places the network is very close, the capillaries lying closer -together than even in the frog’s foot; in others the network is more -open, and the capillaries wider apart; but everywhere, with a few -exceptions which you will learn by and by, there are capillaries, -arteries, and veins. - -Suppose you were a little lone red corpuscle, all by yourself in the -quite empty blood-vessels of a dead body, squeezed in the narrow pathway -of a capillary, say of the biceps muscle of the arm, able to walk -about, and anxious to explore the country in which you found yourself. -There would be two ways in which you might go. Let us first imagine that -you set out in the way which we will call backwards. Squeezing your way -along the narrow passage of the capillary in which you had hardly room -to move, you would at every few steps pass, on your right hand and on -your left, the openings into other capillary channels as small as the -one in which you were. Passing by these you would presently find the -passage widening, you would have more room to move, and the more -openings you passed, the wider and higher would grow the tunnel in which -you were groping your way. The walls of the tunnel would grow thicker at -every step, and their thickness and stoutness would tell you that you -were already in an artery, but the inside would be delightfully smooth. -As you went on you would keep passing the openings into similar tunnels, -but the further you went on, the fewer they would be. Sometimes the -tunnels into which these openings led would be smaller, sometimes -bigger, sometimes of the same size as the one in which you were. -Sometimes one would be so much bigger, that it would seem absurd to say -that it opened into your tunnel. On the contrary, it would appear to you -that you were passing out of a narrow side passage into a great wide -thoroughfare. I dare say you would notice that every time one passage -opened into another the way suddenly grew wider, and then kept about the -same size until it joined the next. Travelling onwards in this way, you -would after a while find yourself in a great wide tunnel, so big that -you, poor little corpuscle, would seem quite lost in it. Had you anyone -to ask, they would tell you it was the main artery of the arm. Toiling -onwards through this, and passing a few but for the most part large -openings, you would suddenly tumble into a space so vast that at first -you would hardly be able to realize that it was the tunnel of an artery -like those in which you had been journeying. This you would learn to be -the =aorta=, the great artery of all; and a little further on you would be -in the heart. - -Suppose now you retraced your steps, suppose you returned from the aorta -to the main artery of the arm, and thus back through narrower and -narrower tunnels till you came again to the spot from which you started, -and then tried the other end of the capillary. You would find that that -led you also, in a very similar way, into wider and wider passages. Only -you could not help noticing that though the inside of all the passages -was as smooth as before, the walls were not nearly so thick and stout. -You would learn from this that you were in the veins, and not in the -arteries. You would meet too with something, the like of which you did -not see in the arteries (except perhaps just close to the heart). Every -now and then you would come upon what for all the world looked like one -of those watch-pockets that sometimes are hung at the head of a -bedstead, a watch-pocket with its opening turned the way you were going. -This you would find was called a =valve=, and was made of thin but strong -membrane or skin. Sometimes in the smaller veins you would meet with one -watch-pocket by itself, sometimes with two or even three abreast, and I -dare say you would notice that very frequently, directly you had passed -one of these valves, you came to a spot where one vein joined another. - -Well, but for these differences, your journey along the veins would be -very like your journey along the arteries, and at last you would find -yourself in a great vein, whose name you would learn to be the =vena -cava=, or hollow vein (and because, though there is but one aorta, there -are two great “hollow veins,” =the superior vena cava= or =upper hollow -vein=), and from thence your next step would be into the heart again. So -you see, starting from the capillary (you started from a capillary in -the arm, but you might have started from any capillary anywhere), -whether you go along the arteries or whether you go along the veins, you -at last come to the heart. - -Before we go on any further we must learn something about the heart. - -=27.= Go and ask the butcher for a sheep’s pluck. There will most probably -be one hanging up in his shop. Look at it before he takes it down. The -hook on which it is hanging has been thrust through the windpipe. You -will see that the sheep’s windpipe is, like the rabbit’s, all banded -with rings of cartilage, only very much larger and coarser. Below the -windpipe come the spongy lungs, and between them lies the heart, which -perhaps is covered up with a skin and so not easily seen. Hanging to the -heart and lungs is the great mass of the liver. When you have got the -pluck home, cut away the liver, cut away the skin (pericardium, it is -called) which is covering the heart, if it has not been cut away -already, and lay the lungs out on a table with the heart between them. -You will then have something very much like what - -[Illustration: - - FIG. 5.--_Heart of Sheep, as seen after Removal from the Body, - lying upon the Two Lungs. The Pericardium has been cut away, but no - other Dissection made._ - -_R.A._ Auricular appendage of right auricle; _L.A._ auricular appendage -of left auricle; _R.V._ right ventricle; _L.V._ left ventricle; _S.V.C._ -superior vena cava; _I.V.C._ inferior vena cava; _P.A._ pulmonary -artery; _Ao_, aorta; _Áó_, innominate branch from aorta dividing into -subclavian and carotid arteries; _L._ lung; _Tr._ trachea. 1, solid cord -often present, the remnant of a once open communication between the -pulmonary artery and aorta. 2, masses of fat at the bases of the -ventricle hiding from view the greater part of the auricles. 3, line of -fat marking the division between the two ventricles. 4, mass of fat -covering the trachea.] - -is represented in Fig. 5. If you could look through the front of your -own chest, and see your own heart and lungs in place, you would see -something not so very very different. - -If now you handle the heart--and if you want to learn physiology you -must handle things--you will have no great difficulty in finding the -great yellowish tubes marked _Ao_ and _Áó_ in the figure. Your butcher -perhaps may not have cut them across exactly where mine has done, but -that will not prevent your recognizing them. You will notice what thick -stout walls they have, and how they gape where they are cut. _Ao_ is the -=aorta=, and _Áó_ is a great branch of the aorta, going to the head and -neck of one side, perhaps the branch along which we imagined just now -that you, a poor little red blood-corpuscle, were travelling. If you -were to put a wire through _Áó_ you would be able to bring it out -through _Ao_, or _vice versâ_. But what is _P.A._ which looks so much -like the aorta, though you will find that it has no connection with it? -You cannot pass a wire from the aorta into it. It also is an artery, the -=pulmonary[2] artery=. We shall have more to say about it directly. - -Now try and find what are marked in the figure as _S.V.C._ and _I.V.C._ -You will perhaps have a little difficulty in this; and when you have -found them you will understand why. They are the great veins of the -body. _S.V.C._ is the =superior vena cava=, to form which all the veins -from the head and neck and arms join, the vein in which you were -journeying a little while ago. _I.V.C._ is the =inferior vena cava=, made -out of all the veins from the trunk and the legs. Being veins, they have -thin flabby walls; and their sides fall flat together, so that they seem -nothing more than little folds of skin, and it becomes very hard to find -the passage inside them. But when you have found the opening into them, -you will see that you can stretch them out into quite wide tubes, and -that their walls, though very much thinner than those of the aorta, so -thin indeed that they are almost transparent, are still after a fashion -strong. If you put a penholder or thin rod through either you will find -that they both seem to lead right into the middle of the heart. With a -little care you can pass a rod up _I.V.C._ and bring the end of it out -at the top of _S.V.C._ Of course you will understand that both of these -veins have been cut off short. - -=28.= Before we go on any further with the sheep’s heart, let me tell you -something about it, by help of the diagram in Fig. 6, which is meant to -represent the whole circulation. You must remember that this figure is a -=diagram=, and not a picture; it does not represent the way the -blood-vessels are really arranged in your own body. If you had no arms -and no legs, and if you only had a few capillaries at the top of your -head and at the bottom of your body, it might be more like than it is. - -In the centre of the figure is the heart. This you will see is -completely divided by an upright partition into two halves, a right half -and a left half. Each half is further marked off, but not completely -divided, into - -[Illustration: - - FIG. 6.--_Diagram of the Heart and Vessels, with the Course of the - Circulation, viewed from behind so that the proper left of the - Observer corresponds with the left side of the Heart in the - Diagram._ - -_L.A._ left auricle; _L.V._ left ventricle; _Ao._ aorta; _A_^{1}. -arteries to the upper part of the body; _A_^{2}. arteries to the lower -part of the body; _H.A._ hepatic artery, which supplies the liver with -part of its blood; _V_^{2}. veins of the upper part of the body; -_V_^{2}. veins of the lower part of the body; _V.P._ vena portæ; _H.V._ -hepatic vein; _V.C.I._ inferior vena cava; _V.C.S._ superior vena cava; -_R.A._ right auricle; _R.V._ right ventricle; _P.A._ pulmonary artery; -_Lg._ lung; _P.V._ pulmonary vein; _Lct._ lacteals; _Ly._ lymphatics; -_Th.D._ thoracic duct; _Al._ alimentary canal; _Lr._ liver. The arrows -indicate the course of the blood, lymph, and chyle. The vessels which -contain arterial blood have dark contours, while those which carry -venous blood have light contours.] - -two chambers, an upper chamber and a lower chamber; so that altogether -we have four chambers,--two upper chambers, one on each side, marked -_R.A._ and _L.A._, these are called the =right and left auricles=; and two -lower chambers, one on each side, marked _R.V._ and _L.V._, these are -called the =right and left ventricles=. The right auricle, _R.A._, opens -in the direction of the arrow into the right ventricle, _R.V._, the -opening being guarded, as we shall see, by a valve. The left auricle, -_L.A._, opens into the left ventricle, _L.V._, the opening being -likewise guarded by a valve; but you have to go quite a roundabout way -to get from either the right auricle or ventricle to the left auricle or -ventricle. Let us see how we can get round the figure. Suppose we begin -with the two tubes marked _V.C.S._ and _V.C.I._, the walls of which are -drawn with thin lines. These both open into the right auricle. They are -the vena cava superior and inferior, which you have just made out in the -sheep’s heart. From the right auricle you pass easily into the right -ventricle; thence, following the arrow, the way is straight into the -tube marked _P.A._ This is the pulmonary artery, the outside of which -you saw in the sheep’s heart (Fig. 5, _P.A._) Travelling along this -pulmonary artery, you come to the lungs, and after passing through -branches not represented in the figure, picking your way through -arteries which continually get smaller and smaller, you find yourself -at last in the capillaries of the lungs. Squeezing your way through -these, you come out into veins, and gradually advancing through larger -and larger veins, you, still following the arrow, find yourself in one -of four large veins (only one of them is represented in the diagram) -which land you in the left auricle. From the left auricle it is but a -jump into the left ventricle. From the left ventricle the way is open, -as indicated by the arrow, into the tube marked _Ao_. This is intended -to represent the aorta, which you have already seen in the sheep’s heart -(Fig. 5, _Ao_). It is here drawn for simplicity’s sake as dividing into -two branches, but you have already been told, and must bear in mind, -that it does not in reality divide in this way, but gives off a good -many branches of various sizes. However, taking the figure as it stands, -suppose we travel along _A_^{2}. Following the arrow, and shooting -through arteries which continually get smaller and smaller, we come at -last to capillaries somewhere, in the skin or in some muscle, or in a -bone, or in the brain, or almost anywhere, in fact, in the upper part of -the body. Out of the capillaries we pass into veins, which, joining -together and so forming larger and larger trunks, bring us at last to -the point from which we started, the superior vena cava, _V.C.S._ If we -had taken the other road, _A_^{2}, we should have passed through -capillaries somewhere in the lower part of the body instead of the -upper, and come back by the vena cava inferior, _V.C.I._, instead of the -vena cava superior. =Starting from the right auricle, whichever way we -took we should always come back to the right auricle again, and in our -journey should always pass through the following things in the following -order: right auricle, right ventricle, pulmonary artery, arteries, -capillaries, and veins of the lungs, pulmonary vein, left auricle, left -ventricle, aorta, arteries, capillaries, and veins somewhere in the -body, and either superior or inferior vena cava.= That is the course of -the circulation. But there is something still to be added. Among the -many large branches, not drawn in the diagram, given off by the aorta to -the lower part of the body, there are two branches which are drawn and -which deserve special notice. - -One is a large branch carrying blood to the tube _A.L._, which is meant -in the diagram to stand for the stomach, intestines, and some other -organs. This branch, like all other branches of the aorta, divides into -small arteries, and these into capillaries, which again are gathered up -into veins, forming at last a large vein marked in the diagram _V.P._ -and called the =vena portæ= or =portal vein=. Now the remarkable thing is -that this vein does not, like all the other veins, go straight to join -the vena cava, but makes for the liver, where it divides into smaller -and smaller veins, until at last it breaks completely up in the liver -into a set of capillaries again. These capillaries gather once more into -veins, forming at last the large trunk, called the =hepatic[3] vein=, -_H.V._, which does what the portal vein ought to have done but did not; -it opens straight into the vena cava. - -The other branch of the aorta of which we are speaking goes straight to -the liver, and is called the =hepatic artery=, _H.A._: there it breaks up -in the liver into small arteries, and then into capillaries, which -mingle with the capillaries of the portal vein, and form one system, out -of which the hepatic veins spring. So you see it makes a great -difference to a red corpuscle which is travelling along the lower part -of the aorta _A_^{2}, whether it takes a turn into the branch going to -the alimentary canal, or whether it goes straight on into, for instance, -a branch going to some part of the leg. In the latter case, having got -through a set of capillaries, it is soon back into the vena cava and on -its road to the heart. But if it takes the turn to the alimentary canal, -it finds after it has passed through the capillaries and got into the -portal vein, that it has still to go through another set of capillaries -in the liver before it can pass through the hepatic vein into the vena -cava. - -This then is the course of the circulation. Right side of the heart, -pulmonary artery, capillaries of the lungs, pulmonary vein, left side of -the heart, aorta, capillaries somewhere, sometimes two sets, sometimes -one, vena cava, right side of the heart again. A little corpuscle cannot -get from the right to the left side of the heart without going through -the capillaries of the lungs. It cannot get from the left side of the -heart to the right without going through some capillaries somewhere in -the body, and if it should happen to take the turn to the stomach, it -has to go through two sets of capillaries instead of one. - -You see, you really have two circulations, and you have two hearts -joined together into one. If you were very skilful you might split the -heart in half and pull the two sides asunder, and then you would have -one heart receiving all the veins from the body and sending its arteries -(branches of the pulmonary artery) all to the lungs, and another heart -receiving all the veins from the lungs and sending its arteries -(branches of the aorta) all over the body. And you would have two -circulations, one through the lungs, and another through the rest of the -body, both joining each other. Very often two circulations are spoken -of, and because the lungs are so much smaller than the rest of the body, -the circulation through the lungs is called the lesser circulation, that -through the rest of the body the greater circulation. - -=29.= I have described the circulation as if the blood always went in one -direction from the right side of the heart to the left, from arteries to -veins, the way the arrows point in the diagram. And so it does. It -cannot go the other way round. =Why does it go that way? Why cannot it go -the other way round?= - -The reasons are to be found partly in the heart, partly in the veins. - -=In the veins the blood will only pass from the capillaries to the heart.= -Why not from the heart to the capillaries? You remember the little -watch-pocket-like valves, here and there, sometimes singly, sometimes -two or three abreast. =You remember that the mouths of the watch-pockets -were always turned towards the heart.= Now suppose a crowd of little -corpuscles hurrying along a vein towards the heart. When they came to -one of these watch-pocket valves they would simply trample it down flat, -and so pass over it without hardly knowing it was there, and go on -their way as if nothing had happened. But suppose they were journeying -the other way, from the heart to the capillaries. When they came to the -open mouth of a watch-pocket valve, some of them would be sure to run -into the pocket, and then the pocket would bulge out, and the more it -bulged out the more blood would run into it, until at last it would be -so full of blood that it would press close against the top of the vein, -as is shown in Fig. 7 (or, if there were two or three, they would all -meet together), and so quite block the vein up. If you doubt this, make -a watch-pocket out of a piece of silk or cotton, fasten it on to a piece -of brown paper, and roll the paper up into a tube, so that the valve is -nicely inside the tube. If you pour some peas down the tube with the -mouth of the valve looking away from you, they will run through at once; -but if you try to pour them the other way, your tube will soon be -choked, and if you carefully unroll the tube you will find the -watch-pocket crammed full of peas. - -[Illustration: FIG. 7.--_Diagrammatic Sections of Veins with Valves._ - - In the upper, the blood is supposed to be flowing in the direction - of the arrow, towards the heart; in the lower, the reverse way. C, - capillary side; H, heart side. -] - -=The valves in the veins, then, let the blood pass easily from the -capillaries to the heart, but won’t let it go the other way.= If you bare -your arm you may see some of the veins in the skin, in which the blood -is running up from the hand towards the shoulder. If with your finger -you press one of these veins back towards the hand it will swell up, and -if you look carefully you may see little knots here and there caused by -the bulging out of the watch-pocket valves. If you press it the other -way, towards the elbow, you will empty it easily, and if with another -finger you prevent the blood getting into it from behind, that is from -the hand, the vein will remain empty a very long time. - -The presence of valves in the veins, then, is one reason why the blood -moves in one direction, but other reasons, and these the chief ones, are -to be found in the heart. - -Let us now go back to the sheep’s heart. - -=30.= You know from the diagram that the two great veins, the superior and -inferior vena cava, open into the right auricle. If you slit up these -two veins in the sheep’s heart, you will find that they end by separate -openings in a small cavity, the inside of which is for the most part -smooth, and the walls of which, made, as you will at once see, of -muscle, are not very thick. This small cavity is the right auricle, -shown in Fig. 8, _R.A._, where the great veins have not been slit up, -but the front of the auricle has been cut away. In this auricle, beside -the openings into the two great veins and another one which belongs to a -vein coming from the heart itself (Fig. 8, _b_) there is quite a large -one, leading straight downwards, into which you - -[Illustration: FIG. 8.--_Right Side of the Heart of a Sheep._ - - _R.A._ cavity of right auricle; _S.V.C._ superior vena cava; - _I.V.C._ inferior vena cava; (a piece of whalebone has been passed - through each of these;) _a_, a piece of whalebone passed from the - auricle to the ventricle through the auriculo-ventricular orifice; - _b_, a piece of whalebone passed into the coronary vein. - - _R.V._ cavity of right ventricle; _tv_, _tv_, two flaps of the - tricuspid valve: the third is dimly seen behind them, the _a_, - piece of whalebone, passing between the three. Between the two - flaps, and attached to them by _chordæ tendineæ_, is seen a - papillary muscle, _PP_, cut away from its attachment to that - portion of the wall of the ventricle which has been removed. Above, - the ventricle terminates somewhat like a funnel in the pulmonary - artery, _P.A._ One of the pockets of the semilunar valve, _sv_, is - seen in its entirety, another partially. - - 1, the wall of the ventricle cut across; 2, the position of the - auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of - fat lodged between the auricle and pulmonary artery. -] - -can put your three fingers. This is the opening into the right -ventricle; and you will have no difficulty in putting your fingers from -the auricle into the ventricle and bringing them out again. - -[Illustration: FIG. 9.--_The Orifices of the Heart seen from above, the -Auricles and Great Vessels being cut away._ - - _P.A._ pulmonary artery, with its semilunar valves; _Ao._ aorta, - do. - - _R.A.V._ right auriculo-ventricular orifice with the three flaps - (_lv._ 1, 2, 3) of tricuspid valve. - - _L.A.V._ left auriculo-ventricular orifice, with _m.v._ 1 and 2, - flaps of mitral valve; _b_, piece of whalebone passed into coronary - vein. On the left part of _L.A.V._ the section of the auricle is - carried through the auricular appendage; hence the toothed - appearance due to the portions in relief cut across. -] - -But hold the heart in one hand with the auricle upwards, and try to pour -some water into the ventricle. The first few spoonfuls will go in all -right, and then you will see some thin white skin or membrane come -floating up into the opening and quite block up the entrance from the -auricle into the ventricle; the - -[Illustration: FIG. 10.--_View of the Orifices of the Heart from below, -the whole of the Ventricles having been cut away._ - - _R.A.V._ right auriculo-ventricular orifice surrounded by the three - flaps, _t.v._ 1, _t.v._ 2, _t.v._ 3, of the tricuspid valve; these - are stretched by weights attached to the _chordæ tendineæ_. - - _L.A.V._ left auriculo-ventricular orifice surrounded in same way - by the two flaps, _m.v._ 1, _m.v._ 2, of mitral valve; _P.A._ the - orifice of pulmonary artery, the semilunar valves having met and - closed together; _Ao._ the orifice of the aorta with its semilunar - valves. The shaded portion, leading from _R.A.V._ to _P.A._, - represents the funnel seen in Fig. 8. -] - -water will immediately fill the auricle and run over. If you look at the -membrane carefully as it comes bulging up, you will notice that it is -made up of three pieces joined together as is shown in Fig. 9 (_lv._ 1, -_lv._ 2, _lv._ 3). These three pieces form the valve between the right -auricle and ventricle, called the =tricuspid=, or three-peaked valve. Why -it is so called you will understand if you lay open the right ventricle -by cutting with a pair of scissors from the auricle into the ventricle -along the side of the heart, or by cutting away the front of the -ventricle as has been done in Fig. 8. You will then see that the valve -is made up of three little triangular flaps, which grow together round -the opening with their points hanging down into the cavity of the -ventricle (Fig. 10, _t.v._) They do not, however, hang quite loosely. -You will notice fastened to the sides of the flaps, thin delicate -threads, the other ends of which are fastened to the sides of the -ventricle, and often to little fleshy projections called papillary -muscles (Fig. 8, _P.P._) - -How do these valves act? In this way. When the ventricle is empty, and -blood or water or any other fluid is poured into it from the auricle, -the valves are pushed on one side against the walls of the ventricle, -and thus there is a great wide opening from the auricle into the -ventricle. But as the ventricle fills, the blood or water gets behind -the flaps and floats them up towards the auricle. The more fluid in the -ventricle the higher they float, until when the ventricle is quite full -they all meet together in the middle of the opening between the auricle -and ventricle and completely block it up. But why do they not turn right -over into the auricle, and so open up again the wrong way? Because of -those little threads (the _chordæ tendineæ_, as they are called) which -fasten them to the walls of the ventricle. The flaps float back until -these threads are stretched quite tight, and the threads are just long -enough to let the flaps reach to the middle of the opening, but no -further. The tighter the threads are stretched the closer the flaps fit -together, and the more completely do they block the way from the -ventricle back into the auricle. - -=The tricuspid valve, then, lets blood flow easily from the right auricle -into the right ventricle, but prevents it flowing from the ventricle -into the auricle.= - -=31.= Now look at the cavity of the ventricle. Its walls are fleshy, that -is muscular, and you will notice that they are much stouter and thicker -than those of the auricle. Besides the opening from the auricle there is -but one other, which is at the top of the ventricle, side by side with -the former. If you put a penholder or your finger through this second -opening, you will find that it leads into the large vessel which you -have already learnt to recognize as the pulmonary artery (Fig. 5, -_P.A._) - -Slit up the pulmonary artery from the ventricle with a pair of scissors, -as has been done in Fig. 8, _P.A._ You will notice at once the line -where the red soft flesh of the muscular ventricle leaves off, and the -yellow firmer material of which the artery is made begins. Just at that -line you will see a row of three (perhaps you may have cut one of the -three with your scissors) most beautiful, watch-pocket valves, made on -just the same principle as those in the veins, only larger, and more -exquisitely finished. These are called =semilunar valves=, because each -pocket is of the shape of a half-moon. Lift them up carefully and see -how tender and yet how strong they are. There is no need to tell you the -use of these. You know it at once. =They are to let the blood flow from -the ventricle into the pulmonary artery, and to prevent the blood going -back from the artery into the ventricle.= - -On the right side of the heart we have, then, two great valves, the -tricuspid valve between the auricle and the ventricle, and the semilunar -valve between the ventricle and the pulmonary artery. These let the -blood flow easily one way, but not the other. If you doubt this, try it. -Put a tube into either the superior or inferior vena cava of a fresh -heart, tying the other vena cava and another tube into the pulmonary -artery. If with a funnel you pour water into the tube in the vein, it -will run through auricle and ventricle and out through the tube of the -pulmonary artery as easily as possible; but if you try to pour water the -other way down the pulmonary artery, you will find you cannot do it; the -tube gets blocked directly, and only a few drops come back through the -heart into the vein. - -Now slit up the pulmonary artery as far as you can, and note when you -cut it how stout and firm are its walls. You will find that it soon -divides into two branches, one for the right lung, one for the left. -Each of these, when it gets to the lung, divides into branches, and -these again into others, as far as you can follow them. You know from -what you have learnt already that these branches end in capillaries all -over the lungs. - -=32.= Not far from the two main branches of the pulmonary artery you will -find, covered up perhaps with fat and other matters, some tubes which -you will at once recognize as veins, and if you open any one of these -you will find that you can put a thin rod into it, and that it leads in -one direction to the lungs, and in the other into the left side of the -heart. These are the _pulmonary veins_, and if you slit them right up -you will find they open (by four openings) into a cavity on the left -side of the heart, almost exactly like that cavity on the right side -which we called the right auricle (Fig. 11). This cavity is, in fact, -the =left auricle=; out of it there is an opening into the left ventricle, -very like the opening from the right auricle into the right ventricle. -It too is guarded by flap valves, exactly like the tricuspid valve, only -there are but two flaps instead of three (Fig. 9, _m.v._ 1, _m.v._ 2). -Hence this valve is called the =bicuspid=, or more frequently the =mitral= -valve. Its flaps have little threads by which they are fastened to the -walls of the ventricle, and in fact, except for there being two flaps -instead of three, the mitral valve is exactly like the tricuspid valve, -and acts exactly the same way. - -If you cut with a pair of scissors from the auricle into the ventricle, -you will find the left ventricle (Fig. 11) very much like the right -ventricle, only its walls are very much thicker, so much thicker that -the left ventricle takes up the greater part of the heart. You will see -this if you now look at the outside of a fresh heart. - -The auricles are so small and so covered up by fat that from the outside -you can hardly see them at all. What you chiefly see are two little -fleshy corners, one of each auricle (Fig. 5, _R.A._ _L.A._), often -called “the auricular appendages.” By far the greater part is taken up -by the ventricles--and if you look you will see a band of fat slanting -across the heart (Fig. 5, 3). This marks the line of the fleshy -division, or =septum= as it is called, between the two ventricles. You -will notice that the point or apex of the heart belongs altogether to -the left ventricle. - -[Illustration: FIG. 11.--_Left Side of the Heart of a Sheep (laid -open)._ - - _P.V._ pulmonary veins opening into the left auricle by four - openings, as shown by the styles or pieces of whalebone placed in - them: _a_, a style passed from auricle into ventricle through the - auriculo-ventricular orifice; _b_, a style passed into the coronary - vein, which, though it has no connection with the left auricle, is, - from its position, necessarily cut across in thus laying open the - auricle. - - _M.V._ the two flaps of the mitral valve (drawn somewhat - diagrammatically): _pp_, papillary muscles, belonging as before to - the part of the ventricle cut away; _c_, a style passed from - ventricle in _Ao._ aorta; _Ao_^{2}. branch of aorta (see Fig. 5, - _Áó_); _P.A._ pulmonary artery; _S.V.C._ superior vena cava. - - 1, wall of ventricle cut across; 2, wall of auricle cut away around - auriculo-ventricular orifice; 3, other portions of auricular wall - cut across; 4, mass of fat around base of ventricle (see Fig. 5, - 2). -] - -To return to the inside of the left ventricle. Up at the top of the -ventricle, close to the opening from the auricle, there is one other -opening, and only one. If you put your finger into this, you will find -that it leads into a tube which first of all dips under or behind the -pulmonary artery and then comes up and to the front again. This tube is -what you already know as the =aorta=. If you slit it up from the ventricle -(and to do this you must cut through the pulmonary artery), you will -find that on the left side, as on the right, the red fleshy wall of the -ventricle suddenly changes into the yellow firm wall of the artery, and -that just at this line there are three semilunar valves exactly like -those in the pulmonary artery. - -=On the left side of the heart, then, we have also two valves, the mitral -between the auricle and the ventricle, and the semilunar between the -ventricle and the aorta. These let the blood pass one way and not the -other. You can easily drive fluid from the pulmonary veins through -auricle and ventricle into the aorta, but you cannot send it back the -other way from the aorta.= - -These then are the reasons why the blood will only pass one way, the way -I said it did. There are sets of valves opening one way and shutting the -other. These valves are the tricuspid between the right auricle and -right ventricle, the pulmonary semilunar valves between the right -ventricle and the pulmonary artery, the mitral valve between the left -auricle and the left ventricle, the aortic semilunar valves between the -left ventricle and the aorta, and the valves which are scattered among -the veins of the body. Of these by far the most important are the -valves in the heart: they do the chief work; those in the veins do -little more than help. - -=33.= Well, then, we understand now, do we not? why the blood, if it moves -at all, moves in the one way only. There still remains the question, =Why -does the blood move at all?= - -You know that during life it does keep moving. You have seen it moving -in the web of a frog’s foot--and whenever any part of the body can be -brought under the microscope, the same rush of red corpuscles through -narrow channels may be seen. You know it moves because when you cut a -blood-vessel the blood runs out. If you cut an artery across, the blood -gushes out from the end which is nearest the heart; if you cut a vein -across, the blood comes most from the end nearest the capillaries. If -you want to stop an artery bleeding, you tie it between the cut and the -heart; if you want to stop a vein bleeding, you tie it between the cut -and the capillaries. You understand now why there is this difference -between a cut artery and a cut vein. And you see that this is by itself -a proof that the blood moves in the arteries from the heart to the -capillaries, and in the veins from the capillaries to the heart. - -The blood is not only always moving, but moves very fast. It flies along -the great arteries at perhaps ten inches in a second. Through the little -bit of capillaries along which it has to pass it creeps slowly, but -manages sometimes to go all the way round from vein to vein again in -about half a minute. - -It is always moving at this rapid rate, and when it ceases to move, you -die. - -=What makes it move?= - -Suppose you had a long thin muscle, fastened at one end to something -firm, and with a weight hanging at the other end. You know that every -time the muscle contracted it would pull on the weight and draw it up. -But suppose, instead of hanging a weight on to the muscle, you wrapped -the muscle round a bladder full of water. What would happen then each -time the muscle contracted? Why, evidently it would squeeze the bladder, -and if there were a hole in the bladder some of the water would be -squeezed out. That is just what takes place in the heart. You have -already learnt that the heart is muscular. Each cavity of the heart, -each auricle, and each ventricle is, so to speak, a thin bag with a -number of muscles wrapped round it. In an ordinary muscle of the body, -the bundles of fibres of which the muscle is made up are placed -carefully and regularly side by side. You can see this very well in a -round of boiled beef, which is little more than a mass of great muscles -running in different directions. You know that if you try to cut a thin -slice right across the round, at one part your carving-knife will go -“with the grain” of the meat, _i.e._ you will cut the fibres lengthways; -at another part it will go “against the grain,” _i.e._ you will cut the -fibres crossways. In both parts, the bundles of fibres will run very -regularly. But in the heart the bundles are interlaced with each other -in a very wonderful fashion, so that it is very difficult to make out -the grain. They are so arranged in order that the muscular fibres may -squeeze all parts of each bag at the same time. - -Each cavity of the heart, then, auricle or ventricle, is a thin bag -with a network of muscles wrapped round it, and each time the muscles -contract they squeeze the bag and try to drive out whatever is in it. -There are more muscles in the ventricles than in the auricles, and more -in the left ventricle than in the right, for we have already seen how -much thicker the ventricles are than the auricles, and the left -ventricle than the right; and the thickness is all muscle. - -And now comes the wonderful fact. These muscles of the auricles and -ventricles are always at work contracting and relaxing, shortening and -lengthening, of their own accord, as long as the heart is alive. The -biceps in your arm contracts only when you make it contract. If you keep -quiet, your arm keeps quiet and your biceps keeps quiet. But your heart -never keeps quiet. Whether you are awake or whether you are asleep, -whether you are running about or lying down quite still, whatever you -are doing or not doing, as long as you are alive your heart keeps on -steadily at work. Every second, or rather oftener, there comes a short -sharp squeeze from the auricles, from both exactly at the same time, and -just as the auricles have finished their squeeze, there comes a great -hug from the ventricles, from both at the same time, but a much stronger -hug from the left than from the right; and then for a brief space there -is perfect quiet. But before the second has quite passed away, the -auricles have begun again, and after them the ventricles once more, and -thus the contracting and relaxing of the walls of the heart’s cavities, -this beat of the heart as it is called, this short snap of the two -auricles, this longer, steadier pull of the two ventricles, have gone on -in your own body since before you were born, and will go on until the -moment comes when friends gathering round your bedside will say that you -are “gone.” - -=34. But how does this beat of the heart make the blood move?= Let us see. - -Remember that you have, or when you are grown up will have, bottled up -in the closed blood-vessels of your body about 12 lbs. of blood. You -have seen that the heart and the blood-vessels form a system of closed -tubes; the walls are in some places, in the capillaries for instance, -very thin, but they are sound and whole--and though the road is quite -open from the capillaries through the veins, heart, and arteries to the -capillaries again, there is no way out of the tubes except by making a -breach somewhere in the walls. - -This closed system of heart and tubes is pretty well filled by the 12 -lbs. of blood. - -What then must happen each time the heart contracts? - -Let us begin with the right ventricle. Suppose it is full of blood. It -contracts. The blood in it, squeezed on all sides, tries to go back into -the right auricle, but the tricuspid flaps have been driven back and -block the way. The more the blood presses on them, the tighter they -become, and the more completely they shut out all possibility of getting -into the auricle. - -The way into the pulmonary artery is open, the blood can go there. But -stay, the artery is already full of blood, and so are the capillaries -and veins in the lung. Yes, but the artery will stretch ever so much. -Take a piece of pulmonary artery, and having tied one end, pump or pour -water into the other; you will see how much it will stretch. Into the -pulmonary artery, then, goes the blood, stretching it in order to find -room. As the ventricle squeezes and squeezes, until its walls meet in -the middle, all the blood that was in it finds its way out into the -artery. But the beat of the ventricle soon ceases, the squeeze is over -and gone, and back tumbles the blood into the ventricle, or would -tumble, only the first few drops that shoot backwards are caught by the -watch-pocket semilunar valves. Back fly these valves with a sharp click -(for the things of which we are speaking happen in a fraction of a -second), and all further return is cut off. The blood has been squeezed -out of the ventricle, and is safely lodged in the pulmonary artery. - -But the pulmonary artery is ever so much on the stretch. It was fairly -full before it received this fresh lot of blood; now it is over-full--at -least that part of it which is nearest to the heart is over-full. What -happens next? What happens when you stretch a piece of india-rubber and -then let it go? It returns to its former size. The ventricle has -stretched the piece of pulmonary artery near it, beyond the natural -size, and then (when it ceased to contract) has let it go. Accordingly -the piece of pulmonary artery tries to return to its former size, and -since it cannot send the blood back to the ventricle, squeezes it on to -the next piece of the artery nearer the capillaries, stretching that in -turn. - -This again in turn sends it on the next piece--and so on right to the -capillaries. The over-full pulmonary artery, stretched to hold more than -it fairly can, empties itself through the capillaries into the pulmonary -veins until it is not more than comfortably full. But the pulmonary -veins also are already full,--what are they to do? To empty the surplus -into the left auricle. Oftener than every second there will come a time -when they can do so. - -For at the same time that the right ventricle pumped a quantity of blood -into the pulmonary artery and safely lodged it there, the left ventricle -pumped a like quantity into the aorta, safely lodged it there, and was -left empty itself. But just at that moment the left auricle began to -contract and to squeeze the blood that was in it. - -Where could that blood go? It could not go back into the pulmonary -veins, for they were already full, and the blood in them was being -pressed behind by the over-full pulmonary arteries. But it could pass -easily into the empty ventricle--and in it tumbled, the mitral flaps -readily flying back and opening up a wide way. And so the auricle -emptied itself into the ventricle. But now the auricle ceases to -contract--its walls no longer squeeze--it is empty and wants filling, -and so comes the moment when the pulmonary veins can pour into it the -blood which has been driven into them by the over-full pulmonary artery. - -Thus the right ventricle drives the blood into the over-full pulmonary -artery, the pulmonary artery overflows into the pulmonary veins, the -pulmonary veins carry the surplus to the empty left auricle, the left -auricle presses it into the empty left ventricle, the left ventricle -pumps it into the aorta--(the stretching of the aorta and of its -branches is what we call the pulse)--the over-full aorta overflows just -as did the pulmonary artery, through the capillaries of the body into -the great venæ cavæ--through these the blood falls into the empty right -auricle, the right auricle drives it into the empty right ventricle, -and the full right ventricle is the point at which we began. - -=Thus the alternate contractions of auricles and ventricles, thanks to -the valves in the heart and in the veins, pump the blood, stroke by -stroke, through the wide system of tubes; and thus in every capillary -all over the body we find blood pressed upon behind by over-full -arteries, with a way open to it in front, thanks to the auricles, which -are, once a second or oftener, empty and ready to take up a fresh supply -from the veins.= Thus it comes to pass that every little fragment of your -body is bathed by blood, which a few moments ago was in your heart, and -a few moments before that was in some other part of your frame. Thus it -is that no part of your body can keep itself to itself; the blood makes -all things common as it flies from spot to spot. The red corpuscle that -a minute ago was in your brain, is now perhaps in your liver, and in -another minute may be in a muscle of your arm or in a bone of your leg: -wherever it goes it has something to bring, and something to fetch. A -restless heart is for ever driving a busy blood, which wherever it goes -buys and sells, making perhaps an occasional bargain as it shoots along -the great arteries and great veins, but busiest of all as it lingers in -the narrow pathways of the capillaries. - -=35.= When you look down upon a great city from a high place, as upon -London from St. Paul’s, you see stretching below you a network of -streets, the meshes of which are filled with blocks of houses. You can -watch the crowds of men and carts jostling through the streets, but the -work within the houses is hidden from your view. Yet you know that, busy -as seems the street, the turmoil and press which you see there are but -tokens of the real business which is being carried on in the house. - -So is it with any piece of the body upon which you look through the -microscope. You can watch the red blood jostling through the network of -capillary streets. But each mesh bounded by red lines is filled with -living flesh, is a block of tiny houses, built of muscle, or of skin, or -of brain, as the case may be. You cannot _see_ much going on there, -however strong your microscope; yet that is where the chief work goes -on. In the city the raw material is carried through the street to the -factory, and the manufactured article may be brought out again into the -street, but the din of the labour is within the factory gates. In the -body the blood within the capillary is a stream of raw material about to -be made muscle, or bone, or brain, and of stuff which, having been -muscle, or bone, or brain, is no longer of any use, and is on its way to -be cast out. The actual making of muscle, or of bone, or of brain, is -carried on, and the work of each is done, outside the blood, in the -little plots of tissue into which no red corpuscle comes. - -The capillaries are closed tubes; they keep the red corpuscles in their -place. But their walls are so thin and delicate that they let the watery -plasma of the blood, the colourless fluid in which the corpuscles float, -soak through them into the parts inside the mesh. You probably know that -many things will pass through thin skins and membranes in which no holes -can be found even after the most careful search. If you put peas into a -bladder and tie the neck, the peas will not get out until the bladder is -untied or torn. But if you were to put a solution of sugar or of salt -into the bladder, and place the bladder with its neck tied ever so -tightly in a basin of pure water, you would find that very soon the -water in the basin would begin to taste of sugar or salt--and that -without your being able to discover any hole, however small, in the -bladder. By putting various substances in the bladder, you will find -that solid particles and things which will not dissolve in water keep -inside the bladder, whereas sugar and salt, and many other things which -dissolve in water, will make their way through the bladder into the -water outside, and will keep on passing until the water in the basin is -as strong of sugar or salt as the water in the bladder. This property -which membranes such as a bladder have of letting certain substances -pass through them is called =osmosis=. You will at once see how important -a part it plays in your own body. It is by osmosis chiefly that the raw -nourishing material in the blood gets into the little islets of flesh -lying, as we have seen, in the meshwork of the capillaries. It is by -osmosis chiefly that the worn-out stuff from the same islets gets back -into the blood. It is by osmosis chiefly that food gets out of the -stomach into the blood. It is by osmosis chiefly that the waste, -worn-out matters are drained away from the blood, and so cast out of the -body altogether. By osmosis the blood nourishes and purifies the flesh. -By osmosis the blood is itself nourished and kept pure. - -There are two chief things by which the blood, and through the blood -the body, is nourished. These are food and air. The air we have always -with us, we have no need to buy it or toil for it; hence we take it as -we want it, a little at a time, and often. We gather up no store of it; -and cannot bear the lack of it for more than a few moments. - -For our food we have to labour; we store it up in our bodies from time -to time, at intervals of hours, in what we call meals, and can go hours -or even days without a fresh supply. - -Let us first of all see how the blood, and, through the blood, the body, -is nourished by air. - - - - -HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI. - - -=36.= I have already said, perhaps more than once, that our muscles burn, -burn in a wet way without giving light. And when I say our muscles, I -might say our whole body, some parts burning more fiercely than others. - -You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens -when a candle is placed in a closed jar of pure air. The oxygen gets -less, carbonic acid comes in its place, and after a while the candle -goes out for want of oxygen to carry on that oxidation which is the -essence of burning. You also know that exactly the same thing would -happen if you were (only you need not do it) to put a bird or a mouse in -the jar instead of a candle. The oxygen would go, carbonic acid would -come, and the little flame of life in the mouse would flicker and go -out, and after a while its body would be cold. - -But suppose you were to put a fish or a snail in a jar of pure fresh -water, and cork the jar tight. There seems at first sight to be no air -in the jar. But there is. If you were to take that fresh water, and put -it under an air-pump, you could pump bubbles of air out of it; and if -you were to examine these bubbles you would find them to contain oxygen -and nitrogen, with very little carbonic acid. =The water contains -dissolved air.= After the fish or the snail had been some time in the -jar, you would see its flame of life flicker and die out, just like that -of the bird in air; and if you then pumped the air out of the water you -would find that the oxygen was nearly gone and that carbonic acid had -come in its place. - -You see, then, that air can be breathed, as we call it, even when it is -dissolved in water. - -Now to return to our muscle. When you were watching the circulation in -the frog’s foot, you could tell the artery from the vein, because in the -artery the blood was flowing _to_ the capillaries, and in the vein -_from_ them. Both artery and vein were rather red, and of about the same -tint of colour. But if you could see in your own body a large artery -going to your biceps muscle, and a large vein coming away from it, you -would be struck at once with the difference of colour between them. The -artery would look bright scarlet, the vein a dark purple; and if you -were to prick both, the blood would gush from the artery in a bright -scarlet jet, and bubble from the vein in a dark purple stream. And -wherever you found an artery and a vein (with a great exception of which -I shall have to speak directly), the blood in the artery would be bright -scarlet, and that in the vein dark purple. Hence we call the bright -scarlet blood which is found in the arteries =arterial blood=, and the -dark purple blood which is found in the veins =venous blood=. - -What is the difference between the two? If you were to pump away at some -arterial blood, as you did at the water in which you put your fish, you -would be able to obtain from it some air, or, more correctly, some gas; -a great deal more gas, in fact, than you did from the water. A pint of -blood would yield you half a pint of gas. This gas you would find on -examination not to be air, _i.e._ not made up of a great deal of -nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be -very little nitrogen, but a good deal of oxygen, and still more carbonic -acid. - -If you were to pump away at some venous blood you would get about as -much gas, but it would be very different in composition. The little -nitrogen would remain about the same, but the oxygen would be about half -gone, while the carbonic acid would be much increased. - -This, then, is one great difference (for there are others) between -venous and arterial blood, that while =both contain, dissolved in them, -oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen -and more carbonic acid than arterial blood=. - -=37. In passing through the capillaries on its way to the vein, the blood -in the artery has lost oxygen and gained carbonic acid.= Where has the -oxygen gone to? Whence comes the carbonic acid? To and from the islets -of flesh between the capillaries, to the bloodless muscular fibre or bit -of nerve or skin which the blood-holding capillaries wrap round. The -oxygen has passed from the blood within the capillaries to the flesh -outside; from the flesh outside the carbonic acid has passed to the -blood within the capillaries. And this goes on all over the body. -Everywhere the flesh is breathing blood, is breathing gas dissolved in -the blood, just as a fish breathes water, _i.e._ breathes the air -dissolved in the water. - -Goes on everywhere with one great exception. There is one great artery, -with its branches, in which blood is not bright, scarlet, arterial, but -dark, purple, venous. There are certain great veins in which the blood -is not dark, purple, venous, but bright, scarlet, arterial. You know -which they are. The pulmonary artery and the pulmonary veins. The blood -in the pulmonary veins contains more oxygen and less carbonic acid than -the blood in the pulmonary artery. =It has lost carbonic acid and gained -oxygen, as it passed through the capillaries of the lungs.= - -=38.= What are the lungs? As you saw them in the rabbit, or as you may see -them in the sheep, they are shrunk and collapsed. We shall presently -learn why. But if you blow into them through the windpipe, which divides -into branches, one for either lung, you can blow them out ever so much -bigger. They are in reality bladders which can be filled with air, but -which, left to themselves, at once empty themselves again. - -They are bladders of a peculiar construction. Imagine a thick short bush -or tree crowded with leaves; imagine the trunk and the branches, small -and great, down to the veriest twigs, all hollow; imagine further that -the leaves themselves were little hollow bladders, stuck on to the -smallest hollow twigs, and made of some delicate, but strong and -exceedingly elastic, substance. If you blew down the trunk you might -stretch and swell out all the hollow leaves; when you left off blowing -they would all fall together, and shrink up again. - -Around such a framework of hollow branches called =bronchial-tubes=, and -hollow elastic bladders called =air-cells=, is wrapped the intricate -network of pulmonary arteries, veins, and capillaries, in such a way -that each air-cell, each little bladder, is covered by the finest and -most close-set network of capillaries, very much as a child’s -india-rubber ball is covered round with a network of string. Very thin -are the walls of the air-cell, so thin that the blood in the capillary -is separated from the air in the air-cell by the thinnest possible sheet -of finest membrane. As the dark purple blood rushes through the crowded -network, its carbonic acid escapes through this thin membrane, from the -blood into the air, and oxygen slips from the air into the blood. - -Thus the dark purple venous blood coming along the pulmonary artery, as -it glides in the pulmonary capillaries along the outside of the inflated -air-cells, by loss of carbonic acid and gain of oxygen is changed into -the bright scarlet blood of the pulmonary veins. - -This then is the mystery of our constant need of air. =The flesh of the -body of whatever kind, everywhere all over the body, breathes blood, -making pure arterial blood venous and impure, all over the body except -in the lungs, where the blood itself breathes air, and changes from -impure and venous to pure and arterial.= - -=39.= Through the capillaries of the muscle a stream of blood is ever -flowing so long as life lasts and the heart has power to beat; every -instant a fresh supply of bright, pure, arterial blood comes to take the -place of that which has become dark, venous, and impure. Without this -constant renewal of its blood the muscle would be choked, and its vital -flame would flicker and die out. - -In the lungs, the air filling the air-cells would if left to itself soon -lose all its oxygen and become loaded with carbonic acid; and the blood -in the capillaries of the lungs would no longer be changed from venous -to arterial, but would travel on to the pulmonary vein as dark and -impure as in the pulmonary artery. =Just as the blood in the muscle must -be constantly renewed, so must the air in the lungs be continually -changed.= - -How is this renewal of the air in the lungs brought about? - -In the dead rabbit you saw the lungs, shrunk, collapsed, emptied of much -of their air, and lying almost hidden at the back of the chest (Fig. 1, -_G.G._) The cavity of the chest seemed to be a great empty space, hardly -half filled by the lungs and heart. But this is quite an unnatural -condition of the lungs. Take another rabbit, and before you touch the -chest at all, open the abdomen and remove all its contents--stomach, -liver, intestines, &c. You will then get a capital view of the -=diaphragm=, which as you already know forms a complete partition between -the chest and the belly. You will notice that it is arched up towards -the chest, so that the under surface at which you are looking is quite -hollow. If you hold the rabbit up by its hind legs with its head hanging -down, and pour some water into the abdomen, quite a little pool will -gather in the shallow cup of the diaphragm. - -In the rabbit the diaphragm is very transparent; you can see right -through it into the chest, and you will have no difficulty in -recognizing the pink lungs shining through it. =You will notice that they -cover almost all the diaphragm--in fact they fill up the whole of the -cavity of the chest that is not occupied by the heart.= - -If you seize the diaphragm carefully in the middle with a pair of -forceps, and pull it down towards the abdomen, you will find that you -cannot create a space between the lungs and the diaphragm, but that the -lungs follow the diaphragm, and are quite as close to it when it is -pulled down as when it is drawn up. - -=In other words, when the diaphragm is arched up as you find it on -opening the abdomen, the lungs quite fill the chest; and when the -diaphragm is drawn down and the cavity of the chest made bigger, the -lungs swell out so that they still fill up the chest.= - -=40.= How do they swell out? By drawing air in through the windpipe. If -you listen, you will perhaps hear the air rush in as you pull the -diaphragm down--and if you tie the windpipe, or quite close up the nose -and mouth, you will find it much harder to pull down the diaphragm, -because no fresh air can get into the lungs. - -Now prick a hole through the diaphragm into the cavity of the chest, -without wounding the lungs. You will hear a sudden rush of air, and the -lungs will shrink up almost out of sight. They are no longer close -against the diaphragm as they were before; and if you open the chest you -will find that they have shrunk to the back of the thorax as you saw -them in the first rabbit. The rush of air is partly a rush of air out of -the lungs, and partly a rush of air into the chest between the chest -walls and the outside of the lungs. - -But before you lay open the chest, pull the diaphragm up and down as you -did before you made the hole in the diaphragm. You will find that you -have no effect whatever on the lungs. They remain perfectly quiet, and -do not swell up at all. By working the diaphragm up and down, you only -drive air through the hole you have made, =in and out of the cavity of -the chest=, not =in and out of the lungs= as you did before. - -We see then that the =chest is an air-tight chamber=, and that the lungs, -when the chest walls are whole, =are always on the stretch=, are on the -stretch even when the diaphragm is arched up as high as it can go. - -Why is it that the lungs are thus always on the stretch? Because the -chest is air-tight, so that no air can get in between the outside of the -lungs and the inside of the chest wall. You know from your Physics -Primer (Art. 29, p. 34) that the atmosphere is always pressing on -everything. It is pressing on all parts of the rabbit; it presses on the -inside of the windpipe and on the inside of the lungs. It presses on -the outside of the abdomen, and so presses on the under surface of the -diaphragm, and drives it up into the chest as far as it will go. But it -will not go very far, because its edges are fastened to the firm walls -of the chest. The air also presses on the outside of the chest, but -cannot squeeze that much, because its walls are stout. - -If the walls of the chest were soft and flabby, the atmosphere would -squeeze them right up, and so through them press on the outside of the -lungs; since they are firm it cannot. The chest walls keep the pressure -of the atmosphere off the outside of the lungs. - -The lungs then are pressed by the atmosphere on their insides and not on -their outsides; and it is this inside pressure which keeps them on the -stretch or expanded. When you blow into a bladder, you put it on the -stretch and expand it because the pressure of your breath inside the -bladder is greater than the pressure of the atmosphere outside the -bladder. If, instead of making the pressure inside _greater_ than that -outside, you were to make the pressure outside _less_ than that inside, -as by putting the bladder under an air-pump, you would get just the same -effect; you would expand the bladder. That is just what the chest walls -do; they keep the pressure outside the lungs less than that inside the -lungs, and that is why the lungs, as long as the chest walls are sound, -are always expanded and on the stretch. - -When you make a hole into the chest, and let the air in between the -outside of the lungs and the chest wall, the pressure of the atmosphere -gets at the outside of the lungs; there is then the same atmospheric -pressure outside as inside the lungs; there is nothing to keep them on -the stretch, and so they shrink up to their natural size, just as does -the bladder when you leave off blowing into it, or when you take it out -of the air-pump. - -When before you made the hole in the diaphragm you pulled the diaphragm -down, you =still further lessened the pressure on the outside of the -lungs=; hence the pressure inside the lungs caused them to swell up and -follow the diaphragm. But this put the lungs still more on the stretch, -so that when you let go the diaphragm and ceased to pull on it, the -lungs went back again to their former size, emptying themselves of part -of their air and pulling the diaphragm up with them. When there is a -hole in the chest wall, pulling the diaphragm down does not make any -difference to the pressure outside the lungs. They are then always -pressed upon by the same atmospheric pressure inside and outside, and so -remain perfectly quiet. - -=When in an air-tight chest the diaphragm is pulled down, the pressure of -the atmosphere drives air into the lungs through the windpipe and swells -them up. When the diaphragm is let go, the stretched lungs return to -their former size, emptying themselves of the extra quantity of air -which they had received.= - -Suppose now the diaphragm were pulled down and let go again regularly -every few seconds: what would happen? Why, every time the diaphragm went -down a certain quantity of air would enter into the lungs, and every -time it was let go that quantity of air would come out of the lungs -again. - -[Illustration: FIG. 12.--_The Diaphragm of a Dog viewed from the Lower -or Abdominal Side._ - - _V.C.I._ the vena cava inferior; _O._ the œsophagus; _Ao._ the - aorta; the broad white tendinous middle (_B_) is easily - distinguished from the radiating muscular fibres (_A_) which pass - down to the ribs and into the pillars (_C_ _D_) in front of the - vertebræ. -] - -This is what does take place in breathing or respiration. Every few -seconds, about seventeen times a minute, the diaphragm does descend, and -a quantity of air rushes into the lungs through the windpipe. This is -called =inspiration=. As soon as that has taken place, the diaphragm -ceases to pull downwards, the stretched lungs return to their former -size, carrying the diaphragm up with them, and squeeze out the extra -quantity of air. This is called =expiration=. - -As the diaphragm descends it presses down on the abdomen; when it ceases -to descend, the contents of the abdomen help to press it up. If you -place your hand on your stomach, you can feel the abdomen bulging out -each time the diaphragm descends in inspiration, and going in again each -time the diaphragm returns to its place in expiration. - -=41.= But what causes the diaphragm to descend? - -If you look at the diaphragm of the rabbit (or of any other animal) a -little carefully, you will see that it is in reality a flat thin muscle, -rather curiously arranged; for the red fleshy muscular fibres are on the -outside all round the edge (Fig. 12, _A_ and _C_), while the centre _B_ -is composed of a whitish transparent tendon. These muscular fibres, like -all other muscular fibres, have the power of contracting. What must -happen when they contract and become shortened? - -When these muscular fibres are at rest, as in the dead rabbit, the whole -diaphragm is arched up, as we have seen, towards the thorax, somewhat as -is shown in Fig. 13, _B_. It is partly pushed up by all the contents of -the abdomen (for the cavity of the abdomen, you will remember, is quite -filled by the liver, stomach, intestines, and other organs), partly -pulled up by the lungs, which, as we know, are always on the stretch. -When the muscular fibres contract, they pull at the central tendon (just -as the biceps pulls at its lower tendon), =and pull the diaphragm flat=; -and some of the fibres, such as those at _C_, Fig. 12, also pull it -=down=. =The diaphragm during its contraction is flattened and descends=, -somewhat as is shown in Fig. 13, _A_. - -[Illustration: FIG. 13.--_Diagrammatic Sections of the Body in_ - - _A._ inspiration; _B._ expiration. _Tr._ trachea; _St._ sternum; - _D._ diaphragm; _Ab._ abdominal walls. The shading roughly - indicates the stationary air. The unshaded portion at the top of - _A_ is the tidal air. -] - -=The descent of the diaphragm in inspiration is caused by a contraction -of its muscular fibres. During expiration the diaphragm is at rest; its -muscular fibres relax; and it goes up because it is partly drawn up by -the lungs, partly pushed up by the contents of the abdomen.= - -=42.= Other structures besides the diaphragm assist in pumping air in and -out of the lungs. By the action of the diaphragm the chest is -alternately lengthened and shortened. But if you watch anyone, and -especially a woman, breathing, you will notice that with every breath -the chest rises and falls; the front of the chest, the sternum, as you -have learnt to call it, comes forward and goes back; and a little -attention will convince you that it comes forward during inspiration, -_i.e._ while the diaphragm is descending, and falls back during -expiration. But this coming forward of the sternum means a widening of -the chest from back to front, and the falling back of the sternum means -a corresponding narrowing. So that while the chest is being lengthened -by the descent of the diaphragm, it is also being widened by the coming -forward of the sternum. In inspiration the lungs are expanded not only -downwards, by the movement of the diaphragm, but also outwards, by the -movement of the walls of the chest. - -What thrusts forward the sternum? If you were to watch closely the sides -of the chest of a very thin person, you would be able to notice that at -every breathing in, at every inspiration, the ribs are pulled up a -little way. Now, each rib is connected with the backbone behind by a -joint, and is firmly fastened to the sternum in front by cartilage (see -Frontispiece). If you were to fasten a piece of string to the middle of -one of the ribs and to pull it, you would find you were working on a -lever, with the fulcrum at the backbone, with the weight acting at the -sternum, and the power at the point where your string was tied. Every -time you pulled the string the rib would move on its fulcrum at the -backbone, in such a way that the front end of the rib would rise up, and -the sternum would be thrust out a little. When you left off pulling, the -sternum, which in being thrust forward had been put on the stretch, -would sink back, and the rib would fall down to its previous position. - -[Illustration: FIG. 14.--_View of Four Ribs of the Dog with the -Intercostal Muscles._ - - _a._ The bony rib; _b_, the cartilage; _c_, the junction of bone - and cartilage; _d_, unossified; _e_, ossified, portions of the - sternum. _A._ External intercostal muscle. _B._ Internal - intercostal muscle. In the middle interspace, the external - intercostal has been removed to show the internal intercostal - beneath it. -] - -Between the ribs are certain muscles called =intercostal muscles= (Fig. -14). The exact action of these you will learn at some future time. -Meanwhile it will be enough to say that they act like the piece of -string we are speaking of. =When they contract, they pull up the ribs and -thrust out the sternum; when they leave off contracting, the ribs and -sternum fall back to their previous position.= - -There are many other muscles which help in breathing, especially in hard -or deep breathing, but it will be sufficient for you to remember that in -ordinary breathing there are two chief movements taking place exactly at -the same time, by means of which air is drawn into the chest, both -movements being caused by the contraction of muscles. First, the -diaphragm contracts and flattens itself, making the chest deeper or -longer; secondly, at the same time the ribs are raised and the sternum -thrust out by the contraction of the intercostal muscles, making the -chest wider. But as the chest becomes wider and longer, the lungs become -wider and longer too. In order to fill up the extra room thus made in -the lungs, air enters into them through the windpipe. This is -=inspiration=. But soon the diaphragm and the intercostal muscles cease to -contract; the diaphragm returns to its arched condition, the ribs sink -down, the sternum falls back, and the extra air rushes back again out of -the lungs through the windpipe. This is =expiration=. An inspiration and -an expiration make up a whole breath; and thus we breathe some seventeen -times in every minute of our lives. - -=43.= But what makes the diaphragm and intercostal muscles contract and -rest in so beautifully regular a fashion? The biceps of the arm, we saw, -was made to contract by our will. It is not our will, however, which -makes us breathe. We breathe often without knowing it; we breathe in our -sleep when our will is dead; we breathe whether we will or no, because -we cannot help it. We can quicken our breathing, we can take a short or -deep breath as we please, we can change our breathing by the force of -our will; but the breathing itself goes on without, and in spite of, our -will. It is =an involuntary act.= - -Though breathing is not an effort of the will, it is an effort of the -brain; an effort, too, of one particular part of the brain, that part -where the brain joins on to the spinal cord. Nerves run from the -diaphragm and the intercostal and other muscles through the spinal cord, -to this part of the brain. And seventeen times a minute a message comes -down along these nerves, from the brain, bidding them contract; they -obey, and you breathe. Why and how that message comes, you will learn at -some future time. When your head is cut off, or when that part of the -brain which joins on to the spinal cord is injured by accident or made -powerless by disease, the message ceases to be sent, and you cease to -breathe. - -=44.= At every breath, then, a certain quantity of air goes in and out of -the chest; but only a small quantity. =You must not think the lungs are -quite emptied and quite filled at each breath.= On the contrary, you only -take in each time a mere handful of air, which reaches about as far as -the large branches of the windpipe, and does not itself go into the -air-cells at all. This is often called =tidal= air; and the rest of the -air in the lungs, which does not move, is often called the =stationary= -air (see Fig. 13). - -How then does the carbonic acid at the bottom of the lungs get out? How -do the capillaries in the air-cells get their fresh oxygen? - -The stationary air mingles with the tidal air at every breath. If you -want to ventilate a room, you are not obliged to take a pair of bellows -and drive out every bit of the old air in the room, and supply its place -with new air: it will be enough if you open a window or a door and let -in a draught of pure air across one corner, say, of the room. That -current of pure air flowing across the corner will mingle with all the -rest of the air until the whole air in the room becomes pure; and the -mingling will take place very quickly. So it is in the lungs. The tidal -air comes in with each inspiration as pure air from without; but before -it comes out at the next expiration it gives up some of its oxygen to -the stationary air, and robs the stationary air of some of its carbonic -acid. For each breath of tidal air the stationary air is so much the -better, having lost some of its carbonic acid and gained some fresh -oxygen. The tidal air rapidly purifies the stationary air, and the -stationary air purifies the blood. - -Thus it comes to pass that the tidal air, which at each pull of the -diaphragm and push of the sternum goes into the chest as pure air with -twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred -parts, comes out, when the diaphragm goes up and the sternum falls back, -as impure air with only sixteen parts oxygen, but with five parts -carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried -through the stationary air to the blood in the capillaries, and the -gained carbonic acid came through the stationary air from the blood in -the capillaries. So each breath helps to purify the blood, and the -pumping of air in and out of the chest changes the impure, hurtful, -venous, to pure, refreshing, arterial blood; the blood breathes air in -the lungs, that all the body may in turn breathe blood. - - - - -HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII. - - -=45.= The blood is not only purified by air, it is also renewed and made -good by food. The food we eat becomes blood. But our food, though -frequently moist, is for the most part solid. We cut it into small -pieces on the plate, and with our teeth we crush and tear it into still -smaller morsels in our mouth. Still, however well chewed, a great deal -of it, most of it in fact, is swallowed solid. In order to become blood -it must first be dissolved. It is dissolved in the alimentary canal, and -we call the dissolving =digestion=. Let us see how digestion is carried -on. - -Your skin, though sometimes quite moist with perspiration, is as -frequently quite dry. The inside of your mouth is always moist--very -frequently quite filled with fluid; and even when you speak of it as -being dry, it is still very moist. Why is this? The inside of your mouth -is also very much redder than your skin. The redness and the moisture go -together. - -In speaking of the capillaries, I said that almost all parts of the body -were completely riddled with them, but =not quite all=. A certain part of -the skin, for instance, has no capillaries or blood-vessels at all. You -know that where your skin is thick, you can shave off pieces of skin -without “fetching blood;” if your - -[Illustration: FIG. 15.--_Section of Skin, highly magnified._ - - _a_, horny epidermis; _b_, softer layer; _c_, dermis; _d_, - lowermost vertical layer of epidermic cells; _e_, cells lining the - sweat duct continuous with epidermic cells; _h_, corkscrew canal of - sweat duct. To the right of the sweat duct the dermis is raised - into a papilla, in which the small artery, _f_, breaks up into - capillaries, ultimately forming the veins, _g_. -] - -knife were very sharp and you very skilful, you might do the same in -every part of your skin. If you were to put some of the skin you had -thus cut off under the microscope, you would find that it was made up of -little scales. And if you were to take a very thin upright slice running -through the whole thickness of the skin, and examine that under a high -power of the microscope, you would find that the skin was made up of two -quite different parts or layers, as shown in Fig. 15. The upper layer, -_a_, _b_, is nothing but a mass of little bodies packed closely -together. At the top they are pressed flat into scales, but lower down -they are round or oval, and at the same time soft. They are called -=cells=. As you advance in your study of Physiology you will hear more and -more about cells. This layer of cells, either soft and round, or -flattened and dried into scales, is called the =epidermis=. No -blood-vessel is ever found in the epidermis, and hence, when you cut it, -it never bleeds. As long as you live it is always growing. The top -scales are always being rubbed off. Whenever you wash your hands, -especially with soap, you wash off some of the top scales; and you would -soon wash your skin away, were it not that new round cells are always -being formed at the bottom of the epidermis, along the line at _d_ (Fig. -15), and always moving up to the top, where they become dried into -scales. Thus the skin, or more strictly the epidermis, is always being -renewed. Sometimes, as after scarlet fever, the new skin grows quickly, -and the old skin comes away in great flakes or patches. - -The lower layer below the epidermis is what is called the =dermis=, or -=true skin=. This is full of capillaries and blood-vessels, and when the -knife or razor gets down to this, you bleed. It is not made up of cells -like the epidermis, but of that fibrous substance which you early learnt -to call connective tissue (see p. 9). Its top is rarely level, but -generally raised into little hillocks, called =papillæ=, as in the figure; -the epidermis forming a thick cap over each papillæ, and filling up the -hollows between them. Most of the papillæ are full of blood-vessels. - -Now, then, I think you will understand why your skin is not red, but -flesh-coloured, and why it is generally dry. The true skin under the -epidermis is always moist, because of the blood-vessels there; the waste -and fluid parts of the blood pass readily through the walls of the -capillaries, as you have learnt, by osmosis, and so keep everything -round them moist. But this moisture is not enough to soak through the -thick coating of epidermis, and so the top part of the epidermis remains -dry and scaly. - -The true skin underneath the epidermis is always red; you know that if -you shave off the surface of your skin anywhere, it gets redder and -redder the deeper you go down, even though you do not fetch blood. It is -red because of the immense number of capillaries, all full of red blood, -which are crowded into it. When you look at these capillaries through a -great thickness of epidermis, the redness is partly hidden from you, as -when you put a sheet of thin white paper over a red cloth, and the skin -seems pink or flesh-coloured; and where the epidermis is very thick, as -at the heel, the skin is not even pink, but white or yellow, more or -less dirty according to circumstances. - -=46.= But if the moist true skin is thus everywhere covered by a thick -coat of epidermis, which keeps the moisture in, how is it that the skin -is nevertheless sometimes quite moist, as when we perspire? - -[Illustration: FIG. 16.--_Coiled end of a Sweat Gland, Epithelium not -shown._ - -_a_, the coil; _b_, the duct; _c_, network of capillaries, inside which -the duct gland lies.] - -If you look at Fig. 15, you will see that the epidermis is at one point -pierced by a canal (_h_) running right through it. You will notice that -this canal is not closed at the bottom of the epidermis, but runs right -into the dermis or true skin, where the canal becomes a tube, with just -one layer (_e_) of cells, like the cells of the epidermis, for its -walls. There is no room in Fig. 15 to show what becomes of this tube, -but it runs some way down under the skin all among the blood-vessels, -and then twisting itself up into a knot, ends blindly, as is shown in -Fig. 16, where _b_ is a continuation on a smaller scale of the same -tube which is seen in Fig. 15. This knot is covered by a close network -of capillaries, which at _c_ are supposed to be unravelled and taken -away from the knotted tube in order to show them. The capillaries, you -will understand, though inside the knot, are always outside the tube. If -you were to drop a very diminutive marble in at _h_ (Fig. 15), it would -rattle down the corkscrew passage through the thick epidermis, shoot -down the straight tube _b_ (Fig. 16), and roll through the knot _a_, -until it came to rest at the blind end of the tube. Along its whole -course it would touch nothing but cells, like the cells of the -epidermis, a single layer of which forms the walls of the tube where it -runs below the epidermis. If it got lodged at _h_ (Fig. 15), or got -lodged in the knot at _a_ (Fig. 16), it would in both cases be touching -epidermic cells. But there would be this great difference. At _h_ it -would be ever so far removed from any blood capillary; at _a_ it would -only have to make its way through a thin layer of single cells, and it -would be touching a capillary directly. At _h_ it might remain dry for -some time; at _a_ it would get wet directly, for there is nothing to -prevent the fluid parts of the blood oozing out through the thin wall of -the capillaries, and so through the thin wall of the tube into the canal -of the tube, on to the marble. - -In fact, the inside of the knot is always moist and filled with fluid. -When the capillaries round the knot get over-full of blood, as they -often do, a great deal of colourless watery fluid passes from them into -the tube. The tube gets full, the fluid wells up right into the -corkscrew portion in the thickness of the epidermis, and at last -overflows at the mouth of the tube over the skin. We call this fluid -sweat or =perspiration=. We call the tube with its knotted end =a gland=; -and we call the act by which the colourless fluid passes out of the -blood capillaries into the canal of the tube, =secretion=. We speak of the -=sweat gland secreting sweat out of the blood brought by the capillaries -which are wrapped round the gland=. - -=47.= Now we can understand why the inside of the mouth is red and moist. -The mouth has a skin just like the skin of the hand. There is an outside -epidermis, made up of cells and free from capillaries, and beneath that -a dermis or true skin crowded with capillaries. Only the epidermis of -the mouth is ever so much thinner than that of the hand. The red -capillaries easily shine through it, and their moisture can make its way -through. Hence the mouth is red and moist. Besides there are many glands -in it, something like the sweat gland, but differing in shape; these -especially help to keep it moist. - -Because it is always red and moist and soft, the skin of the inside of -the mouth is generally not called a skin at all, but =mucous membrane=, -and the upper layer is not called epidermis, but =epithelium=. You will -remember, however, that a mucous membrane is in reality a skin in which -the epidermis is thin and soft, and is called epithelium. - -The mouth is the beginning of the alimentary canal. Throughout its whole -length the alimentary canal is lined by a skin or mucous membrane like -that of the mouth, only over the greater part of it the epithelium is -still thinner than in the mouth, and indeed is made up of a single -layer only of cells. The whole of the inside of the canal is therefore -red and moist, and whatever lies in the canal is separated by a very -thin partition only from the blood in the capillaries, which are found -in immense numbers in the walls of the canal. The alimentary canal is, -as you know, a long tube, wide at the stomach but narrow elsewhere. In -all parts of its length the tube is made up of mucous membrane on the -inside, and on the outside of muscles, differing somewhat from the -muscles of the body and of the heart, but having the same power of -contracting, and by contracting of squeezing the contents of the tube, -just as the muscles of the heart squeeze the blood in its cavities. The -muscles, and especially the mucous membrane, are crowded with -blood-vessels. - -Though the epithelium of the mucous membrane is very thin, the mucous -membrane itself is thick, in some places quite as thick as the skin of -the body. This thickness is caused by its being =crowded with glands=. In -the skin the sweat glands are generally some little distance apart, but -in the mucous membrane of the stomach and of the intestines they are -packed so close together, that the membrane seems to be wholly made up -of glands. - -These glands vary in shape in different parts. Nowhere are they exactly -like the sweat glands, because none of them are long thin tubes coiled -up at the end in a knot, and none of them have a great thickness of -epidermis to pass through. Most of them are short, rather wide tubes; -some of them are branched at the deep end. They all, however, resemble -the sweat glands in being tubes or pouches closed at the bottom but open -at top, lined by a single layer of cells, and wrapped round with blood -capillaries. From these capillaries, a watery fluid passes into the -tubes, and from the tubes into the alimentary canal. This watery fluid -is, however, of a different nature from sweat, and is not the same in -all parts of the canal. The fluid which is, as we say, secreted by the -glands in the walls of the stomach is an =acid fluid=, and is called -=gastric juice=; that by the glands in the walls of the intestines is =an -alkaline fluid=, and is called =intestinal juice=. - -=48.= But besides these glands in the mucous membrane of the mouth, the -stomach, and the intestines, there are other glands, which seem at first -sight to have nothing to do with the mucous membrane. - -Beneath the skin, underneath each ear, just behind the jaw, is a soft -body, which ordinarily you cannot feel, but which, when inflamed by what -is called “the mumps,” swells up into a great lump. In a sheep’s head -you would find just the same body, and if you were to examine it you -would notice fastened to it a fleshy cord running underneath the skin -across the cheek towards the mouth. By cutting the cord across you would -discover that what seemed a cord was in reality a narrow tube coming -from the soft body we are speaking of and opening into the mouth. Just -close to the soft body this tube divides into two smaller tubes, these -divide again into still smaller ones, or give off small branches; all -these once more divide and branch like the boughs of a tiny tree; and so -they go on branching and dividing, getting smaller and smaller, until -they end in fine tubes with blind swollen ends. All the tubes, great and -small, are lined with epithelium and wrapped round with blood-vessels, -and being packed close together with connective tissue, make up the soft -body we are speaking of. This body is in fact a =gland=, and is called a -=salivary gland=; as you see it is not a simple gland like a sweat gland, -but is made up of a host of tube-like glands all joined together, and -hence is called a =compound gland=. Being placed far away from the mouth, -it has to be connected with the cavity of the mouth by a long tube, -which is called its =duct=. You cannot fail to notice how like such a -gland is, in its structure, to a lung. The lung is in fact a gland -secreting carbonic acid: and the duct of the two lungs is called the -trachea. The salivary gland beneath the ear is called the =parotid gland=; -there is another very similar one underneath the corner of the jaw on -either side, called the =submaxillary gland=. By each of them a watery -fluid is secreted, which, flowing along their ducts into the mouth and -being there mixed with the moisture secreted by the other glands in the -mouth, is called =saliva=. - -In the cavity of the abdomen lying just below the stomach is a much -larger but altogether similar compound gland called =the pancreas=, which -pours its secretion called =pancreatic juice= into the alimentary canal -just where the small intestine begins (Fig. 17, _g._) - -That large organ the liver, though the plan of its construction is not -quite the same as that of the pancreas or salivary glands, as you will -by and by learn, is nevertheless a huge gland, secreting from the blood -capillaries into which the portal vein (see p. 62) breaks up, a fluid -called =bile= or =gall=, which by a duct, the =gall duct=, is poured into the -top of the intestine (Fig. 17, _e_). When bile is not wanted, as when -we are fasting, it turns off by a side passage from the duct into the -gall-bladder (Fig. 17, _f_), to be stored up there till needed. - -=49.= What are the uses of all these juices and secretions? To dissolve -the food we eat. - -[Illustration: FIG. 17.--_The Stomach laid open behind._ - - _a_, the œsophagus or gullet; _b_, one end of the stomach; _d_, - the other end joining the intestine; _e_, gall duct; _f_, the - gall-bladder; _g_, the pancreatic duct; _h_, _i_, the small - intestine. -] - -We eat all manner of dishes, but in all of them that are worth eating we -find the same kind of things, which we call =food-stuffs=. - -We eat various kinds of meat; but all meats are made up chiefly of two -things: the substance of the muscular fibre, which you have already -learnt is a =proteid= matter containing nitrogen, and the =fat= which wraps -round the lean muscular flesh. Now, proteids are, when cooked, insoluble -in water (see p. 49); and fat, you know, will not mingle with water. -Both these parts of meat, both these food-stuffs, must be acted upon -before they can pass from the inside of the alimentary canal, through -the epithelium of the mucous membrane, into the blood capillaries. - -Besides meat we eat bread. Bread is chiefly composed of =starch=; but -besides starch we find in it a substance containing nitrogen, -exceedingly like the proteid matter of muscle or of blood. - -Potatoes contain a very great deal of starch with a very small quantity -of proteid matter; and nearly all the vegetables we eat contain starch, -with more or less proteid matter. - -Then we generally eat more or less sugar, either as such or in the form -of sweet fruits. We also take salt with our meals, and in almost -everything we eat, animal or vegetable, meat, bread, potatoes or fruit, -we swallow a quantity of mineral substances, that is, various kinds of -salts, such as potash, lime, magnesia, iron, with sulphuric, -hydrochloric, phosphoric, and other acids. - -=In everything on which we live we find one or more of the following -food-stuffs:--Proteid matter, starch or sugar, and fat, together with -certain minerals and water. It is on these we live: any article which -contains either proteid matter, or starch, or fat, is useful for food. -Any article which contains none of them is useless for food, unless it -be for the sake of the minerals or water it holds.= - -We are not obliged to eat all these food-stuffs. Proteid matter we must -have always. It is the only food-stuff which contains nitrogen. It is -the only substance which can renew the nitrogenous proteid matter of the -blood and so the nitrogenous proteid matter of the body. - -We might indeed manage to live on proteid matter alone, for it contains -not only nitrogen but also carbon and hydrogen, and out of it, with the -help of a few minerals, we might renew the whole blood and build up any -and every part of the body. But, as you will learn hereafter, it would -be uneconomical and unwise to do so. Starch, sugar, and fats, contain -carbon and hydrogen without nitrogen; and hence, if we are to live on -these we must add some proteid matter to them. - -=50.= Of these food-stuffs, putting on one side the minerals, sugar (of -which, as you know, there are several kinds, cane sugar, grape sugar, -and the like) is the only one which is really soluble, and will pass -readily by osmosis through thin membranes (see p. 84). If you take a -quantity of white of egg, or blood serum, or meat, or fibrin, or a -quantity of starch boiled or unboiled, or a quantity of oil or fat, -place it in a bladder, and immerse the bladder in pure water, you will -find that none of it passes through the bladder into the water outside, -as sugar or salt would do. In the same way a quantity of meat, or of -starch, or of fat, placed in your alimentary canal, would never get -through the membrane which separates the inside of the canal from the -inside of the capillaries, and so would remain perfectly useless as food -unless something were done to it. While the food is simply inside the -alimentary canal, it is really outside your body. It can only be said -to be inside your body when it gets into your blood. - -In the things we eat, moreover, these food-stuffs are mixed up with a -great many things that are not food-stuffs at all; they are packed away -in all manner of little cases, which are for the most part no more good -for eating than the boxes or paper in which the sweetmeats you buy are -wrapped up. The food-stuffs have to be dissolved out of these boxes and -packing. - -=The juices secreted by the glands= of which we have been speaking, -dissolve the food-stuffs out of their wrappings, act upon them so as to -make them fit to pass into the blood, and leave all the wrappings as -useless stuff which passes out of the alimentary canal without entering -into the blood, and therefore without really forming part of the body at -all. - -This preparation and dissolving of food-stuffs is called digestion. - -Different food-stuffs are acted upon in different parts of the -alimentary canal. - -The saliva of the mouth has a wonderful power of =changing starch into -sugar=. If you take a mouthful of boiled starch, which is thick, sticky, -pasty, and tasteless, and hold it in your mouth for a few moments, it -will become thin and watery, and will taste quite sweet, because the -starch has been changed into sugar. Now sugar, as you know, will readily -pass through membranes, though starch will not. - -The gastric juice in the stomach does not act much on starch, but it -=rapidly dissolves all proteid matters=. - -If you take a piece of boiled meat, put it in some gastric juice and -keep the mixture warm, in a very short time the meat will gradually -disappear. All the proteid matter will be dissolved, and only the -wrappings of the muscular fibre and the fat be left. You will have a -solution of meat--a solution, moreover, which, strange to say, will -easily pass through membranes, and is therefore ready to get into the -blood. - -The pancreatic juice and the juice secreted by the intestine act both on -starch as saliva does, and on proteids very much as gastric juice does. - -=51.= The bile and the pancreatic juice together act upon all fats in a -very curious way. - -You know that if you shake up oil and water together, though by violent -shaking you may mix them a good deal, directly you leave off they -separate again, and all the oil is seen floating on the top of the -water. If, however, you shake up oil with pancreatic juice and bile, the -oil does not separate. You get a sort of creamy mixture, and will have -to wait a very long time before the oil floats to the top. Milk, you -know, contains fat, the fat which is generally called butter. If you -examine milk under the microscope, you will find that the fat is all -separated into the tiniest possible drops. So also, when you shake up -oil or butter, or any other fat, with bile and pancreatic juice, you -will find on examination that the fat or oil is all separated into the -tiniest possible drops. What is the purpose of this? - -If you look at the inside of the small intestine of any animal, you will -find that it is not smooth and shiny like the outside of the intestine, -but shaggy, or, rather, velvety. This is because the mucous membrane is -crowded all over with little tags, like very little tongues, hanging -down into the inside of - -[Illustration: FIG. 18.--_Semi-diagrammatic View of Two Villi of the -Small Intestines._ (Magnified about 50 diameters.) - - _a_, substance of the villus; _b_, its epithelium, of which some - cells are seen detached at _b_^{2}; _c d_, the artery and vein, - with their connecting capillary network, which envelopes and hides - _e_, the lacteal which occupies the centre of the villus and opens - into a network of lacteal vessels at its base. -] - -the intestine. These are called =villi=; they are not unlike the papillæ -of the skin (Fig. 15), if you suppose all the epidermis stripped except -the bottom row of cells (_d_), and the papilla itself pulled out a good -deal. Fig. 18 is a sketch to illustrate the structure of a =villus=. The -epithelium (_b_), you see, is made up of a single row of cells. Beneath -the epithelium, just as in the papilla of the skin, is a network of -blood capillaries, shown, for convenience, in the right-hand villus -only. But besides the blood capillaries, there is in each villus, what -there is not in a papilla of the skin, another capillary (shown, for -convenience, in the left-hand villus only) which does not contain blood, -which is not connected with any artery or with any vein, but which -begins in the villus. This is a =lacteal=. I have said nothing of these at -present. In most parts of the body we find, besides blood capillaries, -fine passages very much like capillaries, except that they contain a -colourless fluid instead of blood, and do not branch off from any larger -vessels like arteries. They seem to start out of the part in which they -are found, like the roots of a plant in the soil. But though unlike -blood capillaries in not branching off from larger trunks, they resemble -capillaries in joining together to form larger trunks corresponding to -veins, and the colourless fluid flows from the fine capillary channels -towards these larger trunks. This colourless fluid is called =lymph=; it -is very much like blood without the red corpuscles, and the channels in -which it flows are called =lymphatics=. - -The lymphatics from nearly all parts of the body join at last into a -great trunk called the =thoracic duct=, which empties itself into the -great veins of the neck, as is shown in the diagram, Fig. 6, _Lct._, -_Ly._, _Th. D._ - -Now, many of the lymphatics start from the innumerable villi of the -intestine, and are there called =lacteals= (Fig. 6, _Lct._); so that -lacteals may be said to be those lymphatics which have their roots in -the villi of the intestine. - -But what has all this to do with the digestion of fat? Lacteal means -=milky=, and the lymphatics coming from the villi are called lacteals -because, when digestion is going on, the fluid in them, instead of being -transparent as in the rest of the lymphatics, =is white and milky=. Why is -it thus white and milky? Because it is crowded with minute particles of -fat, and those minute particles of fat come from the inside of the -intestine. They are the same minute particles into which the bile and -pancreatic juice have divided the fat taken as food. We know this -because when no fat is eaten the lacteals do not get milky; and when for -any reason bile and pancreatic juice are prevented from getting into the -intestine, though ever so much fat be eaten, it does not get into the -lacteals at all, it remains in the intestine in great pieces, and is -finally cast out as useless. - -=52.= This, then, is what becomes of the food-stuffs:-- - -The fats are broken up by the bile and pancreatic juice into minute -particles. These minute particles, we do not exactly know how, pass -through the epithelium of the villus into the lacteal vessels, from the -lacteals into the thoracic duct, and from the thoracic duct into the -vena cava. Thus the fats we eat get into the blood. - -The starch is changed into sugar in the mouth by saliva, and in the -intestine by the pancreatic juice; but sugar passes readily through -membranes, and so slips into the blood capillaries of the walls of the -alimentary canal. Thus all the sugar we eat, and all the goodness of the -starch we eat, pass into the blood. - -The proteids are dissolved in the stomach by the gastric juice, and what -passes the stomach is dissolved in the intestine, dissolved in such a -way that it can pass through membranes; and thus proteids pass into the -blood. - -Probably some of the sugar and proteids pass into the lacteals as well. - -The minerals are dissolved either in the mouth, or in the stomach, or in -the intestine, and pass into the blood. - -And water passes into the blood everywhere along the whole length of the -canal. - -When we eat a piece of bread, while we are chewing it in our mouth it is -getting moistened and mixed with saliva. Part of its starch is thereby -changed into sugar, and all of it is softened and loosened. Passing into -the stomach, some of the proteids are dissolved out by the gastric -juice, and pass into the blood, and all the rest of the bread breaks up -into a pulpy mass. Passing then into the intestine, what is left of the -starch is changed by the pancreatic juice into sugar, and is at once -drained off either into the lacteals or straight into the blood. In the -intestine what remains of the proteids is dissolved, till nothing is -left but the shells of the tiny chambers in which the starch and -proteids were stored up by the wheat-plant as it grew. - -When we eat a piece of meat, it is torn into morsels by the teeth and -well moistened by saliva, but suffers else little change in the mouth. -In the stomach, however, the proteids rapidly vanish under the action of -the gastric juice. The morsels soften, the fibres of the muscle break -short off and come asunder; the fat is set free from the chambers in -which it was stored up by the living ox or sheep, and, melted by the -warmth of the stomach, floats in great drops on the top of the softened -pulpy mass of the half-digested food. Rolled about in the stomach for -some time by the contraction of the muscles which help to form the -stomach walls, losing much of its proteids all the while to the hungry -blood, the much-changed meat is squeezed into the intestine. Here the -bile and the pancreatic juice, breaking up the fat into tiny particles, -mix fat, and broken meat, and empty wrappings, and salts, and water, all -together into a thick, dirty, yellowish cream. Squeezed along the -intestine by the contraction of the muscular walls, the goodness of this -cream is little by little sucked up. The fat goes drop by drop, particle -by particle, into the lacteals, and so away into the blood. The -proteids, more and more dissolved the further they travel along the -canal, soak away into blood-vessel or into lacteal. The salts and the -water go the same way, until at last the digested meat, with all its -goodness gone, with nothing left but indigestible wrappings, or perhaps -as well some broken bits of fibre or of fat, is cast aside as no longer -of any use. - -=Thus all food-stuffs, not much altered, with all their goodness -unchanged, pass either at once into the blood, or first into the -lacteals and then into the blood, and the useless wrappings of the -food-stuffs are cast away.= - -While we are digesting, the blood is for ever rushing along the branches -of the aorta, through the small arteries and capillaries of the stomach -and intestine, along the branches of the portal vein, and so through the -liver back to the heart; and during the few seconds it tarries in the -intestine, it loads itself with food-stuffs from the alimentary canal, -becoming richer and richer at every round. While we are digesting, the -thoracic duct is pouring, drop by drop, into the great veins of the neck -the rich milky fluid brought to it by the lacteals from the intestine, -and as the blood sweeps by the opening of the thoracic duct on its way -down from the neck to the heart, it carries that rich milky fluid with -it, and the heart scatters it again all over the body. - -=Thus the blood feeds on the food we eat, and the body feeds on the -blood.= - - - - -HOW THE BLOOD GETS RID OF WASTE MATTERS. § VIII. - - -=53.= But if the blood is thus continually being made rich by things, it -must also as continually be getting rid of things. The things with which -it parts are not, however, the same as those which it takes. The blood, -as we have said, is fuel for the muscles, for the brain, and for other -parts of the body. These burn the blood, burn it with heat but without -light. But, as you have learnt from your Chemistry Primer, Art. 4, -burning is only change, not destruction; in burning nothing is lost. If -the muscle burns blood, it burns it into something; that something, -being already burnt, cannot be burnt again, and must be got rid of. - -Into what things does the body burn itself while it is alive? - -I have already said that if you were to take a piece of meat or some -blood, and dry it and burn it, you would find that it was turned into -four things--water, carbonic acid, ammonia, and ashes. The body is made -up of nitrogen, carbon, hydrogen, and oxygen, with sulphur, phosphorus, -and some other elements. The nitrogen and hydrogen go to form ammonia; -the hydrogen, with the oxygen of combustion, forms water; the carbon, -carbonic acid; the phosphorus, sulphur, and other elements go to form -phosphates, sulphates, and other salts. - -=In whatever way the body be oxidized, whether it be rapidly burnt in a -furnace, whether it be slowly oxidized after death, as when it moulders -away either above ground or in the soil, whether it be quickly oxidized -by living arterial blood while still alive--in all these several ways -the things into which it is burnt, into which it is oxidized, are the -same. Whatever be the steps, the end is always water, carbonic acid, -ammonia, and salts.= - -These are the things which are always being formed in the blood through -the oxidation of the body, these are the things of which the body has -always to be getting rid. - -In addition to the water which comes from the oxidation of the solids of -the body, we are always taking in an immense quantity of water; partly -because it is absolutely necessary that our bodies within should be kept -continually moist, partly because food cannot pass into the blood except -when dissolved in water, and partly because we need washing inside quite -as much as outside; if we had not, so to speak, a stream of water -continually passing through our bodies to wash away all impurities, we -should soon be choked, just as an engine is choked with soot and ashes -if it be not properly cleaned. We have, then, to get rid daily of a -large quantity of washing water over and above that which comes from the -burning of the hydrogen of our food. - -We have already seen that a great deal of the carbonic acid goes out by -the lungs at the same time that the oxygen comes in. A large quantity of -water escapes by the same channel. You very well know that however dry -the air you breathe, it comes out of your body quite wet with water. - -We have also already seen how the blood secretes sweat into the -sweat-glands, and so on to the skin. Perspiration is little more than -water with a little salt in it. The skin, therefore, helps to purify the -blood through the sweat-glands, by getting rid of water with a little -salt. You must remember that a great deal of water passes away from your -skin without your knowing it. Instead of settling on the skin in drops -of sweat, it passes off at once as vapour or steam. Some carbonic acid -also makes its way from the blood through the skin. - -=54.= It only remains for us to inquire, In what way does the blood get -rid of the ammonia and the rest of the saline matters that do not pass -through the skin? - -These are secreted from the blood by the kidney, dissolved in a large -quantity of water in the form of =urine=. - -What is the kidney? You will learn more about this organ by and by. -Meanwhile it will for our present purpose be sufficient to say that a -kidney is a bundle of long tubular glands, not so very unlike -sweat-glands, all bound together into the rounded mass whose appearance -is familiar to you. =Into these glands the blood secretes urine just as -it secretes sweat into the sweat-glands=. The glands themselves unite -into a common tube or duct which carries the urine into the receptacle -called the urinary bladder, from whence it is cast out when required. - -What is urine? Urine is in reality water holding in solution several -salts, and in particular containing a quantity of ammonia. The ammonia -in urine is generally in a particular condition, being combined with a -little carbonic acid, in the form of what is called =urea=. If urea is not -actually ammonia, it is at least next door to it. - -The three great channels, then, by which the blood purifies itself, by -which it gets rid of its waste, are the lungs, the kidneys, and the -skin. Through the lungs, carbonic acid and water escape; through the -kidneys, water, ammonia in the shape of urea, and various salts; through -the skin, water and a few salts. As the blood passes through lung, -kidney, and skin, it throws off little by little the impurities which -clog it, one at one place, another at another, and returns from each -purer and fresher. The need to get rid of carbonic acid and to gain a -fresh supply of oxygen is more pressing than the need to get rid of -either ammonia or salts. Hence, while all the blood which leaves the -left ventricle has to pass through the lungs before it returns to the -left ventricle again, only a small part of it passes through the -kidneys, just enough to fill at each stroke the small arteries leading -to those organs. The blood craves for great draughts of oxygen, and -breathes out great mouthfuls of carbonic acid, but is quite content to -part with its ammonia and salts in little driblets, bit by bit. - -The three channels manage between them to keep the blood pure and fresh, -working hard and clearing off much when much food or water is taken or -much work is done, and taking their ease and working slow when little -food is eaten or when the body is at rest. - - - - -THE WHOLE STORY SHORTLY TOLD. § IX. - - -=55.= And now you ought to be able to understand how it is that we live on -the food we eat. - -Food, inasmuch as it can be burnt, is a source of power. In burning it -gives forth heat, and heat is power. If we so pleased, we might burn in -a furnace the things which we eat as food, and with them drive a -locomotive or work a mill; if we so pleased, we might convert them into -gunpowder, and with them fire cannon or blast rocks. Instead of doing -so, we burn them in our own bodies, and use their power in ourselves. - -Food passing into the alimentary canal is there digested; the nourishing -food-stuffs are with very little change dissolved out from the -innutritious refuse; they pass into and become part and parcel of the -blood. - -The blood, driven by the unresting stroke of the heart’s pump, courses -throughout the whole body, and in the narrow capillaries bathes every -smallest bit of almost every part. Kept continually rich in combustible -material by frequent supplies of food, the blood as well at every round -sucks up oxygen from the air of the lungs; and thus arterial blood is -ever carrying to all parts of the body, to muscle, brain, bone, nerve, -skin, and gland, stuff to burn and oxygen to burn it with. - -Everywhere oxidation, burning, is going on, in some spots or at some -times fiercely, in other spots or at other times faintly, changing the -arterial blood rich in oxygen to venous blood poor in oxygen. From most -places where oxidation is going on, the venous blood goes away hotter -than the arterial which came; and all the hot blood mingling together -and rushing over the whole body keeps the whole body warm. Sweeping as -it continually does through innumerable little furnaces, the blood must -needs be warm. This is why =we= are warm. But from some places, as from -the skin, the venous blood goes away cooler than the arterial which -came, because while journeying through the capillaries of the skin it -has given up much of its heat to whatever is touching the skin, and has -also lost much heat in turning liquid perspiration into vapour. This is -why so long as we are in health we never get hotter than a certain -degree of temperature, the so-called blood-heat, 98° Fahr., and why we -make warm the clothes which we wear and the bed in which we sleep. - -Everywhere oxidation is going on, oxidation either of the blood itself -or of the structures which it bathes, and whose losses it has to make -good. Everywhere change is going on. Little by little, bit by bit, every -part of the body, here quickly, there slowly, is continually mouldering -away and as continually being made anew by the blood. Made anew -according to its own nature. Though it is the same blood which is -rushing through all the capillaries, it makes different things in -different parts. In the muscle it makes muscle; in the nerve, nerve; in -the bone, bone; in the glands, juice. Though it is the same blood, it -gives different qualities to different parts: out of it one gland makes -saliva, another gastric juice: out of it the bone gets strength, the -brain power to feel, the muscle power to contract. - -When the biceps muscle contracts and raises the arm, it does work. The -power to do that work, the muscle got from the blood, and the blood from -the food. All the work of which we are capable comes, then, from our -food, from the oxidation of our food, just as the power of the -steam-engine comes from the oxidation of its fuel. But you know that in -the steam-engine only a very small part of the power, or energy, as it -is called, of the fuel goes to move the wheel. By far the greater part -is lost in heat. So it is with our bodies: all the force we can exert -with our bodies is but a small part of the power of our food; all the -rest goes to keep us warm. - -Visiting all parts of the body, rebuilding and refreshing every spot it -touches, the blood current also carries away from each organ the waste -matters of which that organ has no longer any use. Just as each part or -organ has different properties and different work, so also is the waste -of each not exactly the same, though all are alike inasmuch as they are -all the results of oxidation. The waste of the muscle is not exactly the -same as the waste of the brain or of the liver. Possibly the waste -things which the blood bears from one organ may be useful to another, -and so be made to do double work, just as the tar which the gasworks -throw away makes the fortune of the colour manufacturer. - -Be this as it may, the waste products of all parts, travelling hither -and thither in the body, come at last to be brought down to very simple -things, with all their virtue gone out of them, with all, or all but -all, their power of burning lost, fit for nothing but to be cast away, -come at last to be urea or ammonia, carbonic acid, and salts. In this -shape, the food, after a longer or shorter sojourn in the body, having -done its work, having built up this or that part, having helped the -muscle to contract or the liver to secrete, having by its burning given -rise to work or to heat, goes back powerless to the earth and air from -which it came. And so the tale is told. - - - - -HOW WE FEEL AND WILL. § X. - - -=56.= One other matter we have to note before we have given the full -answer to the question why we move. - -We have seen that we move by reason of our muscles contracting, and that -in a general way a muscle contracts because a something started in the -brain by our will passes down from the brain through more or less of the -spinal cord, along certain nerves till it reaches the muscle. It is this -something, which we may call a =nervous impulse=, which causes the muscle -to contract. - -But what leads us to exercise our wills? What starts the nervous -impulse? - -All the nerves in the body do not end in muscles. Many of them end, for -instance, in the skin, in those papillæ of which I spoke a little while -ago. These nerves cannot be used for carrying nervous impulses from the -brain to the skin. By an effort of the will you can make your muscles -contract; but try as much as you can, you cannot produce any change in -your skin. - -What purpose do these nerves serve, then? If you prick or touch your -finger, you feel the prick or touch; you say you have =sensation= in your -finger. Suppose you were to cut across the nerves which lead from the -skin of your finger along your arm up to your brain. What would happen? -If you pricked or touched your finger, you would not feel either prick -or touch. You would say you had lost all sensation in your finger. These -nerves ending in the finger then, have a different use from those ending -in the muscle. =The latter carry impulses from the brain to the muscle, -and so, being instruments for causing movements, are called motor -nerves. The former, carrying impulses from the skin to the brain, and -being instruments for bringing about sensations, are called sensory -nerves.= All parts of the skin are provided with these sensory nerves, -but not to the same extent. The parts where they abound, as the fingers, -are said to be very sensitive; the parts where they are scanty, as the -back of the trunk, are said to be less sensitive. Other parts besides -the skin have also sensory nerves. - -Motor nerves are of one kind only; they all have one kind of work to -do--to make a muscle contract. But there are several kinds of sensory -nerves, each kind having a special work to do. The several works which -these different kinds of sensory nerves have to do are called =the -senses=. - -The work of the nerves of the skin, all over the body, is called the -=sense of touch=. By touch you can learn whether a body is rough or -smooth, wet or dry, hot or cold, and so on. - -You cannot, however, by touch distinguish between salt and sugar. Yet -directly you place either salt or sugar on your tongue you can recognize -it, because you then employ sensory nerves of another kind, the nerves -which give us the =sense of taste=. So also we have nerves of =smell=, -nerves of =hearing=, and nerves of =sight=. - -The nerves of touch, where they end, or rather where they begin in the -skin, sometimes have and sometimes have not, little peculiar structures -attached to them, little =organs of touch=. So also the nerves of taste, -and smell, end or rather begin in a peculiar way. When we come to the -nerves of hearing and of seeing, we find these beginning in most -elaborate and complicated organs, the ear and the eye. - -Of all these =organs of the senses= you will learn more hereafter; -meanwhile, I want you to understand that by means of these various -sensory nerves, we are, so long as we are alive and awake, receiving -impressions from the external world, sensations of touch, sensations of -roughness and smoothness, of heat and cold, sensations of good and bad -odours, sensations of tastes of various kinds, sensations of all manner -of sounds, sensations of the colours and forms of things. - -By our skin, by our nose, by our tongue and palate, by our ears, and -above all by our eyes, impressions caused by the external world are for -ever travelling up sensory nerves to the brain; thither come also -impressions from within ourselves, telling us where our limbs are and -what our muscles are doing. Within the brain these impressions become -sensations. They stir the brain to action; and the brain, working on -them and by them, through ways we know not of, governs the body as a -conscious intelligent will. - - * * * * * - - NICHOLSON’S GEOLOGY. - - _Text-Book of Geology, for Schools and Colleges._ - - By H. ALLEYNE NICHOLSON, M. D., D. SC., M. A., PH. D., - F. R. S. E., F. G. S., etc., Professor of Natural History - and Botany in University College, Toronto. - - _12mo. 266 pages. Price, $1.50._ - -This work is thoroughly adapted for the use of beginners. At the same -time the subject is treated with such fulness as to render the work -suitable for advanced classes, while it is intended to serve as an -introduction to a larger work which is in course of preparation by the -author. - - * * * * * - - NICHOLSON’S ZOOLOGY. - - _Text-Book of Zoology, for Schools and Colleges._ - - BY SAME AUTHOR AS ABOVE. - - _12mo. 353 pages. Price, $1.75._ - -In this volume much more space has been devoted, comparatively speaking, -to the Invertebrate Animals, than has usually been the case in works of -this nature: upon the belief that all teachings of Zoology should, where -possible, be accompanied by practical work, while the young student is -much more likely to busy himself practically with shells, insects, -corals, and the like, than with the larger and less attainable -Vertebrate Animals. - -Considerable space has been devoted to the discussion of the principles -of Zoological classification, and the body of the work is prefaced by a -synoptical view of the chief divisions of the animal kingdom. - -⁂ A copy of any of the above works, for examination, will be sent by -mail, post-paid, to any Teacher or School-Officer remitting one-half its -price. - - - D. APPLETON & CO., PUBLISHERS, - 549 & 551 BROADWAY, NEW YORK. - - -FOOTNOTES: - - [1] It is unusual for muscles to have two tendons at the same end. - Hence the name =biceps=, or “two-headed.” - - [2] From _pulmo_, =lung=; the artery of the lung. - - [3] From _hepar_, =liver=; the vein of the liver. - - -Typographical errors corrected by the etext transcriber: - -would be unvailing=> would be unavailing {pg 34} - -cordæ tendineæ=> chordæ tendineæ {pg 70} - -the triscuspid valve between=> the tricuspid valve between {pg 72} - -the body may may=> the body may {pg 103} - - - - - - - - - - - - - - - - - - - -End of the Project Gutenberg EBook of Physiology, by M. 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Foster - -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 www.gutenberg.org/license - - -Title: Physiology - -Author: M. Foster - -Release Date: October 22, 2016 [EBook #53347] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK PHYSIOLOGY *** - - - - -Produced by Chuck Greif, deaurider and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - -</pre> - -<hr class="full" /> - -<p class="figcenter"> -<img src="images/cover.jpg" width="308" height="500" alt="Image unavailable: cover" /> -</p> - -<table border="0" cellpadding="0" cellspacing="0" summary="" -style="border: 2px black solid;margin: 1em auto 2em auto;max-width:50%; -padding:1%;"> -<tr><td> - -<p class="c"><a href="#TABLE_OF_CONTENTS">Table of Contents.</a></p> -<p class="c">Some typographical errors have been corrected; -<a href="#transcrib">a list follows the text</a>.</p> - -<p class="c"><span class="nonvis">(In certain versions of this etext [in certain browsers] -clicking on the image -will bring up a larger version.)</span></p> - -<p class="c">(etext transcriber's note)</p></td></tr> -</table> - -<p><span class="pagenum"><a name="page_i" id="page_i"></a>{i}</span></p> - -<p class="c"> -<big>LOCKYER’S ASTRONOMY</big><br /> -<br /> -<i>ELEMENTS OF ASTRONOMY</i>:<br /> -<br /> -Accompanied with numerous Illustrations, a Colored Representation<br /> -of the Solar, Stellar, and Nebular Spectra,<br /> -and Celestial Charts of the Northern<br /> -and the Southern Hemisphere.<br /> -<br /> -By <span class="smcap">J. Norman Lockyer</span>.<br /> -<br /> -<i>American edition</i>, revised and specially adapted to the Schools<br /> -of the United States.<br /> -<br /> -<i>12mo. 312 pages. Price, $1.75.</i><br /> -</p> - -<p>The volume is as practical as possible. To aid the student in -identifying the stars and constellations, the fine Celestial Charts of -Arago, which answer all the purposes of a costly Atlas of the Heavens, -are appended to the work—this being the only text-book, as far as the -Publishers are aware, that possesses this great advantage. Directions -are given for finding the most interesting objects in the heavens at -certain hours on different evenings throughout the year. Every device is -used to make the study interesting; and the Publishers feel assured that -teachers who once try this book will be unwilling to exchange it for any -other.</p> - -<p class="r"> -<span style="margin-right: 10%;">D. APPLETON & CO., <span class="smcap">Publishers</span>,</span><br /> -549 & 551 <span class="smcap">Broadway, New York</span>.<br /> -</p> - -<p><span class="pagenum"><a name="page_ii" id="page_ii"></a>{ii}</span></p> - -<p><span class="pagenum"><a name="page_iii" id="page_iii"></a>{iii}</span></p> - -<p class="hang"> -<big><big>SCIENCE PRIMERS, </big></big><i>edited by</i><br /> -<span class="smcap"><i>Professors</i> Huxley</span>, <span class="smcap">Roscoe</span>, <i>and</i><br /> -<span class="smcap">Balfour Stewart</span>.</p> - -<p class="c"> -VI.<br /> -<i>PHYSIOLOGY.</i><br /> -</p> - -<p><span class="pagenum"><a name="page_v" id="page_v"></a>{v}</span> </p> - -<p><span class="pagenum"><a name="page_vi" id="page_vi"></a>{vi}</span> -<a name="page_iv" id="page_iv"></a> <a name="front" id="front"></a></p> - -<div class="figcenter"> -<a href="images/i_f06_lg.jpg"> -<br /> -<img src="images/i_f06_sml.jpg" alt="Image unavailable: EXPLANATION OF THE PLATE. -Fig. I.—The Human Skeleton in Profile." /></a> -<br /> -<span class="caption">EXPLANATION OF THE PLATE. -<br /> -<span class="smcap">Fig. I.—The Human Skeleton in Profile.</span></span> - -</div> - -<table border="0" cellpadding="0" cellspacing="2" summary="" -style="font-size:70%;"> -<tr><td><i>Mn.</i></td><td>The Mandible or Lower Jaw.</td></tr> -<tr><td><i>St.</i></td><td>The Sternum.</td><td rowspan="3" valign="middle" class="bl">—In the Thorax.</td></tr> -<tr><td><i>R.</i></td><td>The Ribs.</td></tr> -<tr><td><i>R´.</i></td><td>The Cartilages of the Ribs.</td></tr> -<tr><td><i>Scp.</i></td><td>The Scapula, or Shoulder Blade.</td></tr> -<tr><td><i>Cl.</i></td><td>The Clavicle, or Collar Bone.</td></tr> -<tr><td><i>H.</i></td><td>The Humerus.</td><td rowspan="3" valign="middle" class="bl">—In the Arm.</td></tr> -<tr><td><i>Ra.</i></td><td>The Radius.</td></tr> -<tr><td><i>U.</i></td><td>The Ulna.</td></tr> -<tr><td><i>F.</i></td><td>The Femur.</td><td rowspan="3" valign="middle" class="bl">—In the Leg.</td></tr> -<tr><td><i>Tb.</i></td><td>The Tibia.</td></tr> -<tr><td><i>Fb.</i></td><td>The Fibula.</td></tr> -</table> - -<div class="figcenter"> -<span class="caption"><span class="smcap">Fig. II.</span> -<br />A front view of the Sternum, <i>St.</i>, with the Cartilages of the Ribs, -<i>R´.</i>, and part of the Ribs themselves <i>R.</i></span></div> - -<p><span class="pagenum"><a name="page_vii" id="page_vii"></a>{vii}</span></p> - -<p class="eng">Science Primers.</p> - -<h1 class="spc">PHYSIOLOGY.</h1> - -<p class="c"><small>BY</small><br /> -M. FOSTER, M.A., M.D., F.R.S.,<br /> -<small>FELLOW OF TRINITY COLLEGE, CAMBRIDGE.</small><br /> -<br /> -WITH ILLUSTRATIONS.<br /> -<br /> -NEW YORK:<br /> -D. APPLETON AND COMPANY,<br /> -<span class="smcap">549 and 551 Broadway</span>.<br /> -1877.<br /> -</p> - -<p><span class="pagenum"><a name="page_viii" id="page_viii"></a>{viii}</span></p> - -<p><span class="pagenum"><a name="page_ix" id="page_ix"></a>{ix}</span></p> - -<h2><a name="PREFACE" id="PREFACE"></a>PREFACE.</h2> - -<p class="nind"><span class="smcap">This</span> Primer is an attempt to explain in the most simple manner possible -some of the most important and most general facts of Physiology, and may -be looked upon as an introduction to the Elementary Lessons of Professor -Huxley.</p> - -<p>In my descriptions and explanations I have supposed the reader to be -willing to handle and examine such things as a dead rabbit and a sheep’s -heart; and written accordingly, I have done this purposely, from an -increasing conviction that actual observation of structures is as -necessary for the sound learning of even elementary physiology, as are -actual experiments for chemistry. At the same time I have tried to make -my text intelligible to those who think reading verbal descriptions less -tiresome than observing things for themselves.</p> - -<p>It seemed more desirable in so elementary a work to insist, even with -repetition, on some few fundamental truths, than to attempt to skim over -the whole wide field of Physiology. I have therefore omitted all that -relates to the Senses and to the functions of the Nervous System, merely -just referring to them in the concluding article. These the reader must -study in the “Elementary Lessons.”</p> - -<p class="r"> -<span class="smcap">M. Foster.</span><br /> -</p> - -<p><span class="pagenum"><a name="page_x" id="page_x"></a>{x}</span></p> - -<h2><a name="TABLE_OF_CONTENTS" id="TABLE_OF_CONTENTS"></a>TABLE OF CONTENTS.</h2> - -<table border="0" cellpadding="1" cellspacing="0" summary=""> - -<tr><td><small>ART</small>.</td><td><small>SECT</small>.</td></tr> - -<tr valign="top"><td> </td><td class="c">I.</td><td class="c"><a href="#INTRODUCTION_I"> INTRODUCTION.</a></td></tr> - -<tr><td> </td><td> </td><td class="rt"><small>PAGE</small></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_1">1.</a></td><td class="c" valign="top">“</td><td valign="top">What Physiology is </td><td class="rt" valign="bottom"><a href="#page_001">1</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_2">2.</a></td><td class="c" valign="top">“</td><td valign="top">Animals move of their own accord</td><td class="rt" valign="bottom"><a href="#page_001">1</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_3">3.</a></td><td class="c" valign="top">“</td><td valign="top">Animals are warm</td><td class="rt" valign="bottom"><a href="#page_003">3</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_4">4.</a></td><td class="c" valign="top">“</td><td valign="top">Why animals are warm and move about—they burn</td><td class="rt" valign="bottom"><a href="#page_004">4</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_5">5.</a></td><td class="c" valign="top">“</td><td valign="top">The need of Oxygen</td><td class="rt" valign="bottom"><a href="#page_005">5</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_6">6.</a></td><td class="c" valign="top">“</td><td valign="top">The waste matters</td><td class="rt" valign="bottom"><a href="#page_005">5</a></td></tr> - -<tr valign="top"><td> </td><td class="c">II.</td><td class="c"> <a href="#THE_PARTS_OF_WHICH_THE_BODY_IS_MADE_UP_II">THE PARTS OF WHICH THE BODY IS MADE UP.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_7">7.</a></td><td class="c" valign="top">“</td><td valign="top">The Tissues</td><td class="rt" valign="bottom"><a href="#page_007">7</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_8">8.</a></td><td class="c" valign="top">“</td><td valign="top">The cavities of the Thorax and Abdomen</td><td class="rt" valign="bottom"><a href="#page_009">9</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_9">9.</a></td><td class="c" valign="top">“</td><td valign="top">The Vertebral Column</td><td class="rt" valign="bottom"><a href="#page_012">12</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_10">10.</a></td><td class="c" valign="top">“</td><td valign="top"> Head and Neck</td><td class="rt" valign="bottom"><a href="#page_015">15</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_11">11.</a></td><td class="c" valign="top">“</td><td valign="top"> Nerves</td><td class="rt" valign="bottom"><a href="#page_018">18</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_12">12.</a></td><td class="c" valign="top">“</td><td valign="top"> General arrangement of all these parts</td><td class="rt" valign="bottom"><a href="#page_019">19</a></td></tr> - -<tr valign="top"><td> </td><td class="c">III.</td><td class="c"> <a href="#WHAT_TAKES_PLACE_WHEN_WE_MOVE_III">WHAT TAKES PLACE WHEN WE MOVE.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_13">13.</a></td><td class="c" valign="top">“</td><td valign="top"> The Bones of the Arm</td><td class="rt" valign="bottom"><a href="#page_021">21</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_14">14.</a></td><td class="c" valign="top">“</td><td valign="top"> The structure of the Elbow Joint</td><td class="rt" valign="bottom"><a href="#page_023">23</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_15">15.</a></td><td class="c" valign="top">“</td><td valign="top"> Other joints in the body</td><td class="rt" valign="bottom"><a href="#page_025">25</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_16">16.</a></td><td class="c" valign="top">“</td><td valign="top"> The arm is bent by the contraction of the Biceps Muscle</td><td class="rt" valign="bottom"><a href="#page_026">26</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_17">17.</a></td><td class="c" valign="top">“</td><td valign="top"> How the will makes the Biceps Muscle contract</td><td class="rt" valign="bottom"><a href="#page_032">32</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_18">18.</a></td><td class="c" valign="top">“</td><td valign="top"> The power of a muscle to contract depends on its being supplied with blood<span class="pagenum"><a name="page_xi" id="page_xi"></a>{xi}</span></td><td class="rt" valign="bottom"><a href="#page_035">35</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_19">19.</a></td><td class="c" valign="top">“</td><td valign="top"> It is the food in the blood which gives the muscle strength</td><td class="rt" valign="bottom"><a href="#page_037">37</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_20">20.</a></td><td class="c" valign="top">“</td><td valign="top"> The continual need of food</td><td class="rt" valign="bottom"><a href="#page_038">38</a></td></tr> - -<tr valign="top"><td> </td><td class="c">IV.</td><td class="c"> <a href="#THE_NATURE_OF_BLOOD_IV">THE NATURE OF BLOOD.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_21">21.</a></td><td class="c" valign="top">“</td><td valign="top"> The Blood in the Capillaries</td><td class="rt" valign="bottom"><a href="#page_040">40</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_22">22.</a></td><td class="c" valign="top">“</td><td valign="top"> The Corpuscles of the Blood</td><td class="rt" valign="bottom"><a href="#page_042">42</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_23">23.</a></td><td class="c" valign="top">“</td><td valign="top"> The clotting of Blood</td><td class="rt" valign="bottom"><a href="#page_045">45</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_24">24.</a></td><td class="c" valign="top">“</td><td valign="top"> The substances present in Serum</td><td class="rt" valign="bottom"><a href="#page_048">48</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_25">25.</a></td><td class="c" valign="top">“</td><td valign="top"> The minerals in Blood</td><td class="rt" valign="bottom"><a href="#page_050">50</a></td></tr> - -<tr valign="top"><td> </td><td class="c">V.</td><td class="c"> <a href="#HOW_THE_BLOOD_MOVES_V">HOW THE BLOOD MOVES.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_26">26.</a></td><td class="c" valign="top">“</td><td valign="top"> The Arteries, Capillaries, and Veins</td><td class="rt" valign="bottom"><a href="#page_051">51</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_27">27.</a></td><td class="c" valign="top">“</td><td valign="top"> The Sheep’s Heart</td><td class="rt" valign="bottom"><a href="#page_055">55</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_28">28.</a></td><td class="c" valign="top">“</td><td valign="top"> The Course of the Circulation</td><td class="rt" valign="bottom"><a href="#page_058">58</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_29">29.</a></td><td class="c" valign="top">“</td><td valign="top"> Why the blood moves in one direction only; the Valves of the Veins</td><td class="rt" valign="bottom"><a href="#page_064">64</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_30">30.</a></td><td class="c" valign="top">“</td><td valign="top"> The Tricuspid Valves of the Heart</td><td class="rt" valign="bottom"><a href="#page_066">66</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_31">31.</a></td><td class="c" valign="top">“</td><td valign="top"> The pulmonary Semilunar Valves</td><td class="rt" valign="bottom"><a href="#page_071">71</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_32">32.</a></td><td class="c" valign="top">“</td><td valign="top"> The left side of the Heart</td><td class="rt" valign="bottom"><a href="#page_072">72</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_33">33.</a></td><td class="c" valign="top">“</td><td valign="top"> What makes the blood move at all: The beat of the Heart</td><td class="rt" valign="bottom"><a href="#page_076">76</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_34">34.</a></td><td class="c" valign="top">“</td><td valign="top"> The action of the Heart as a whole</td><td class="rt" valign="bottom"><a href="#page_079">79</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_35">35.</a></td><td class="c" valign="top">“</td><td valign="top"> The Capillaries and the Tissues</td><td class="rt" valign="bottom"><a href="#page_082">82</a></td></tr> - -<tr valign="top"><td> </td><td class="c">VI.</td><td class="c"> <a href="#HOW_THE_BLOOD_IS_CHANGED_BY_AIR_BREATHING_VI">HOW THE BLOOD IS CHANGED BY AIR: BREATHING.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_36">36.</a></td><td class="c" valign="top">“</td><td valign="top"> Venous and Arterial Blood</td><td class="rt" valign="bottom"><a href="#page_085">85</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_37">37.</a></td><td class="c" valign="top">“</td><td valign="top"> The change from Arterial to Venous, and from Venous to Arterial Blood</td><td class="rt" valign="bottom"><a href="#page_087">87</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_38">38.</a></td><td class="c" valign="top">“</td><td valign="top"> The Lungs</td><td class="rt" valign="bottom"><a href="#page_088">88</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_39">39.</a></td><td class="c" valign="top">“</td><td valign="top"> The renewal of Air in the Lungs. How the descent of the Diaphragm expands the Lungs<span class="pagenum"><a name="page_xii" id="page_xii"></a>{xii}</span></td><td class="rt" valign="bottom"><a href="#page_090">90</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_40">40.</a></td><td class="c" valign="top">“</td><td valign="top"> The natural distension of the Lungs. Inspiration. Expiration</td><td class="rt" valign="bottom"><a href="#page_091">91</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_41">41.</a></td><td class="c" valign="top">“</td><td valign="top"> How the Diaphragm descends</td><td class="rt" valign="bottom"><a href="#page_096">96</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_42">42.</a></td><td class="c" valign="top">“</td><td valign="top"> The Chest is also enlarged by the movements of the Ribs and Sternum</td><td class="rt" valign="bottom"><a href="#page_098">98</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_43">43.</a></td><td class="c" valign="top">“</td><td valign="top"> Breathing an involuntary act</td><td class="rt" valign="bottom"><a href="#page_100">100</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_44">44.</a></td><td class="c" valign="top">“</td><td valign="top"> Tidal air; stationary air</td><td class="rt" valign="bottom"><a href="#page_101">101</a></td></tr> - -<tr valign="top"><td> </td><td class="c">VII.</td><td class="c"> <a href="#HOW_THE_BLOOD_IS_CHANGED_BY_FOOD_DIGESTION_VII">HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_45">45.</a></td><td class="c" valign="top">“</td><td valign="top"> Why the inside of the mouth is always red and moist</td><td class="rt" valign="bottom"><a href="#page_103">103</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_46">46.</a></td><td class="c" valign="top">“</td><td valign="top"> Why the Skin is sometimes moist. Sweat Glands</td><td class="rt" valign="bottom"><a href="#page_106">106</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_47">47.</a></td><td class="c" valign="top">“</td><td valign="top"> The Mucous Membrane of the Alimentary Canal and its Glands</td><td class="rt" valign="bottom"><a href="#page_109">109</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_48">48.</a></td><td class="c" valign="top">“</td><td valign="top"> The Salivary Glands, Pancreas and Liver</td><td class="rt" valign="bottom"><a href="#page_111">111</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_49">49.</a></td><td class="c" valign="top">“</td><td valign="top"> Food-stuffs</td><td class="rt" valign="bottom"><a href="#page_113">113</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_50">50.</a></td><td class="c" valign="top">“</td><td valign="top"> How proteids and starch are changed</td><td class="rt" valign="bottom"><a href="#page_115">115</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_51">51.</a></td><td class="c" valign="top">“</td><td valign="top"> Lacteals and Lymphatics</td><td class="rt" valign="bottom"><a href="#page_117">117</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_52">52.</a></td><td class="c" valign="top">“</td><td valign="top"> What becomes of the Food-stuffs</td><td class="rt" valign="bottom"><a href="#page_120">120</a></td></tr> - -<tr valign="top"><td> </td><td class="c">VIII.</td><td class="c"><a href="#HOW_THE_BLOOD_GETS_RID_OF_WASTE_MATTERS_VIII">HOW THE BLOOD GETS RID OF WASTE MATTERS.</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_53">53.</a></td><td class="c" valign="top">“</td><td valign="top"> The need of getting rid of Waste Matters</td><td class="rt" valign="bottom"><a href="#page_123">123</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_54">54.</a></td><td class="c" valign="top">“</td><td valign="top"> The Kidneys get rid of Ammonia in the form of Urea</td><td class="rt" valign="bottom"><a href="#page_125">125</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_55">55.</a></td><td valign="top"> -IX.</td><td valign="top" class="c"> <a href="#THE_WHOLE_STORY_SHORTLY_TOLD_IX">THE WHOLE STORY SHORTLY TOLD.</a></td><td class="rt" valign="bottom"><a href="#page_127">127</a></td></tr> - -<tr><td class="rt" valign="top"><a href="#sect_56">56.</a></td><td valign="top"> -X.</td><td valign="top" class="c"> <a href="#HOW_WE_FEEL_AND_WILL_X">HOW WE FEEL AND WILL.</a></td><td class="rt" valign="bottom"><a href="#page_130">130</a></td></tr> -</table> - -<p><span class="pagenum"><a name="page_001" id="page_001"></a>{1}</span></p> - -<h1><span class="spc">SCIENCE PRIMERS.</span><br /><br /> -<i>PHYSIOLOGY.</i></h1> - -<h2><a name="INTRODUCTION_I" id="INTRODUCTION_I"></a>INTRODUCTION. § I.</h2> - -<p><b><a name="sect_1" id="sect_1">1.</a></b> Did you ever on a winter’s day, when the ground was as hard as a -stone, the ponds all frozen, and everything cold and still, stop for a -moment, as you were running in play along the road or skating over the -ice, to wonder at yourself and ask these two questions:—“Why am I so -warm when all things around me, the ground, the trees, the water, and -the air, are so cold? How is it that I am moving about, running, -walking, jumping, when nothing else that I can see is stirring at all, -except perhaps a stray bird seeking in vain for food?”</p> - -<p>These two questions neither you nor anyone else can answer fully; but we -may answer them in part, and the knowledge which helps us to the answer -is called <b>Physiology</b>.</p> - -<p><b><a name="sect_2" id="sect_2">2.</a> You can move of your own accord.</b> You do not need to wait, like the -boughs or the leaves, till the wind blows upon you, or, like the stones, -till somebody stirs you. The bird, too, can move of its<span class="pagenum"><a name="page_002" id="page_002"></a>{2}</span> own accord, so -can a dog, so can any animal as long as it is alive. If you leave a -stone in any particular spot, you expect to find the stone there when -you come to it again a long time afterwards; if you do not, you say -somebody or something has moved it. But if you put a sparrow or mouse on -the grass plot, you know that directly your back is turned it will be -off.</p> - -<p>All animals move of themselves. But only so long as they are alive. When -you find the body of a snake on the road, the first thing you do is to -stir it with a stick. If it moves only as you move it, and as far as you -move it, just as a bit of rope might do, you say it is dead. But if, -when you touch it, it stirs of itself, wriggles about, and perhaps at -last glides away, you know it is alive. Every living animal, of whatever -kind, from yourself down to the tiniest creature that swims about in a -little pool of water and cannot be seen without a microscope, moves of -itself. Left to itself, it moves and rests, rests and moves; stirred by -anything, away it goes, running, flying, creeping, crawling, or -swimming.</p> - -<p>Something of the kind sometimes happens with lifeless things. When a -stone is carefully balanced on the top of a high wall, a mere touch will -send it toppling down to the ground. But when it has reached the ground -it stops there, and if you want to repeat the trick you must carry the -stone up to the top of the wall again. You know the toy made like a -mouse, which, when you touch it in a particular place, runs away -apparently of its own accord, as if it were alive. But it soon stops, -and when it has stopped you may touch it again and again without<span class="pagenum"><a name="page_003" id="page_003"></a>{3}</span> making -it go on. Not until you have wound it up will it go on again as it did -before. And every time you want it to run you must wind it up afresh. -Living animals move again and again, and yet need no winding up, for -they are always winding themselves up. Indeed, as we go on in our -studies we shall come to look upon our own bodies and those of all -animals as pieces of delicate machinery with all manner of springs, -which are always running down but always winding themselves up again.</p> - -<p><b><a name="sect_3" id="sect_3">3.</a> You are warm</b>; beautifully warm, even on the coldest winter day, if -you have been running hard; very warm if you are well wrapped up with -clothing, which, as you say, keeps the cold out, but really keeps the -warmth in. The bed you go to at night may be cold, but it is warm when -you leave it in the morning. Your body is as good as a fire, warming -itself and everything near it.</p> - -<p>The bird too is warm, so is the dog and the horse, and every four-footed -beast you know. Some animals however, such as reptiles, frogs, fish, -snails, insects, and the like do not seem warm when you touch them. Yet -really they are always a little warm, and some times they get quite -warm. If you were to put a thermometer into a hive of bees when they are -busy you would find that they are very warm indeed. All animals are more -or less warm as long as they are alive, some of them, such as birds and -four-footed beasts, being very warm. But only so long as they are alive; -after death they quickly become cold. When you find a bird lying on the -grass quite still, not stirring when it is touched, to make quite sure -of its being dead you feel it. If it is quite<span class="pagenum"><a name="page_004" id="page_004"></a>{4}</span> cold, you say it has been -dead some time; if it is still warm, you say it is only just -dead—perhaps hardly dead, and may yet revive.</p> - -<p><b><a name="sect_4" id="sect_4">4.</a> You are warm, and you move about of yourself. You are able to move -because you are warm; you are warm in order that you may move. How does -this come about?</b> Just think for a moment of something which is not an -animal, but which is warm and moves about, which only moves when it is -warm, and which is warm in order that it may move. I mean a locomotive -steam-engine. What makes the engine move? The burning coke or coal, -whose heat turns the water into steam, and so works the piston, while at -the same time the whole engine becomes warm. You know that for the -engine to do so much work, to run so many miles, so much coal must be -burnt; to keep it working it must be “stoked” with fresh coal, and all -the while it is working it is warm: when its stock of coal is burnt out -it stops, and, like a dead animal, grows cold.</p> - -<p>Well, your body too, just like the steam-engine, moves about and is -warm, because a fire is always burning in your body. That fire, like the -furnace of the engine, needs fresh fuel from time to time, only your -fuel is not coal, but food. In three points your body differs from the -steam-engine. In the first place, you do not use your fire to change -water into steam, but in quite a different way, as we shall see further -on. Secondly, your fire is a burning not of dry coal, but of wet food, a -burning which although an oxidation (Chemistry Primer, Art. 5) takes -place in the midst of water, and goes on without any<span class="pagenum"><a name="page_005" id="page_005"></a>{5}</span> light being given -out. Thirdly, the food you take is not burnt in a separate part of your -body, in a furnace like that of the engine set apart for the purpose. -The food becomes part and parcel of your body, and it is your whole body -which is burnt, bit by bit.</p> - -<p>Thus it is the food burning or being oxidized within your body, or as -part of your body, which enables you to move and keeps you warm. If you -try to do without food, you grow chilly and cold, feeble, faint, and too -weak to move. If you take the right quantity of proper food, you will be -able to get the best work out of the engine, your body; and if you work -your body aright, you can keep yourself warm on the coldest winter day, -without any need of artificial fire.</p> - -<p><b><a name="sect_5" id="sect_5">5.</a></b> But if this be so, in order to oxidize your food, you <b>have need of -oxygen</b>. The fire of the engine goes out if it is not fed with air as -well as fuel. So will your fire too. If you were shut up in an air-tight -room, the oxygen in the room would get less and less, from the moment -you entered the room, being used up by you; the oxidation of your body -would after a while flag, and you would soon die for want of fresh -oxygen (see Chemistry Primer, p. 14).</p> - -<p>You have, throughout your whole life, a need of fresh oxygen, you must -always be breathing fresh air to carry on in your body the oxidation -which gives you strength and warmth.</p> - -<p><b><a name="sect_6" id="sect_6">6.</a></b> When a candle is burnt (Chemistry Primer, p. 6) it turns into -carbonic acid, and water. When wood or coal is burnt, we get ashes as -well. If you were to take all your daily food and dry it, it too would -burn<span class="pagenum"><a name="page_006" id="page_006"></a>{6}</span> into ashes, carbonic acid, and water (with one or two other things -of which we shall speak afterwards).</p> - -<p>Your body is always giving out carbonic acid (Chemistry Primer, Exp. 7). -Your body is always giving out water by the lungs, as seen when you -breathe on a glass, by the skin, and by the kidneys; and we shall see -that we always give out more water than we take in as food or drink. -Your body too is daily giving out by the kidneys and bowels, matters -which are not exactly ashes, but very like them. We do not oxidize our -food quite into ashes, but very nearly; we burn it into substances which -are no longer useful for oxidation in the body, and which, being -useless, are cast out of the body as waste matters.</p> - -<p>The tale then is complete. By the help of the oxygen of the air which -you take in as you breathe, you oxidize the food which is in your body. -You get rid of the water, the carbonic acid, and other waste matters -which are left after the oxidation, and out of the oxidation you get the -heat which keeps you warm and the power which enables you to move.</p> - -<p>Thus all your life long you are in constant need of oxygen and food. The -oxygen you take in at every breath, the food at every meal. How you get -rid of the waste matters we shall see further on.</p> - -<p>If you were to live, as one philosopher of old did, in a large pair of -delicate scales, you would find that the scale in which you were would -sink down at every meal, and gradually rise between as you got lighter -and hungry. If the food you took were more than you wanted, so that it -could not all be oxidized, it would remain in your body as part of your -flesh, and<span class="pagenum"><a name="page_007" id="page_007"></a>{7}</span> you would grow heavier and stouter from day to day; if it -were less, you would grow thinner and lighter; if it were just as much -as and no more than you needed, you would remain day after day of -exactly the same weight, the scale in which you sat rising as much -between meals as it sank at the meal time.</p> - -<p>What we have to learn in this Primer is—How the food becomes part and -parcel of your body; how it gets oxidized; how the oxidation gives you -power to move; how it is that you are able to move in all manner of -ways, when you like, how you like, and as much as you like.</p> - -<p>First of all we must learn something about the build of your body, of -what parts it is made, and how the parts are put together.</p> - -<h2><a name="THE_PARTS_OF_WHICH_THE_BODY_IS_MADE_UP_II" id="THE_PARTS_OF_WHICH_THE_BODY_IS_MADE_UP_II"></a>THE PARTS OF WHICH THE BODY IS MADE UP. § II.</h2> - -<p><b><a name="sect_7" id="sect_7">7.</a></b> When you want to make a snow man, you take one great roll of snow to -make the body or trunk. This you rest on two thinner rolls which serve -as legs. Near the top of the trunk you stick in another thin roll on -either side—these you call the two arms: and lastly, on quite the top -of the trunk you place a round ball for a head. Head, trunk, and limbs, -<i>i.e.</i> legs and arms—these together make up a complete body.</p> - -<p>In your snow man these are all alike, all balls of snow differing only -in size and form; but in your own body, head, trunk, and limbs are quite -unlike, as you might easily tell on taking them to pieces. Now you -cannot very well take your own body to pieces, but you easily can that -of a dead rabbit. Suppose you take one of the limbs, say a leg, to begin -with.<span class="pagenum"><a name="page_008" id="page_008"></a>{8}</span></p> - -<p>First of all there is the <b>skin</b> with the hair on the outside. If you -carefully cut this through with a knife or pair of scissors and strip it -off, you will find it smooth and shiny inside. Underneath the skin you -see what you call <b>flesh</b>, rather paler, not so red as the flesh of beef -or mutton, but still quite like it. Covering the flesh there may be a -little <b>fat</b>. In a sheep’s leg as you see it at the butcher’s there is a -good deal of fat, in the rabbit’s there is very little.</p> - -<p>This reddish flesh you must henceforward learn to speak of as <b>muscle</b>. If -you pull it about a little, you will find that you can separate it -easily into parcels or slips running lengthways down the leg, each slip -being fastened tight at either end, but loose between. Each slip is what -is called <b>a muscle</b>. You will notice that many of these muscles are -joined, sometimes at one end only, sometimes at both, to white or bluish -white glistening cords or bands; made evidently of different material -from the muscle itself. They are not soft and fleshy like the muscle, -but firm and stiff. These are <b>tendons</b>. Sometimes they are broad and -short, sometimes thin and long.</p> - -<p>As you are separating these muscles from each other you will see -(running down the leg between them) little white soft threads, very -often branching out and getting too small to be seen. These are <b>nerves</b>. -Between the muscles too are other little cords, red, or reddish black, -and if you prick them, a drop or several drops of blood will ooze out. -These are <b>veins</b>, and are not really cords or threads, but hollow tubes, -filled with blood. Lying alongside the veins are similar small tubes, -containing very little blood, or none at all. These are <b>arteries</b>. The -<b>veins</b> and<span class="pagenum"><a name="page_009" id="page_009"></a>{9}</span> <b>arteries</b> together are called <b>blood-vessels</b>, and it will be -easy for you to make out that the larger ones you see are really hollow -tubes. Lastly, if you separate the muscles still more, you will come -upon the hard <b>bone</b> in the middle of the leg, and if you look closely you -will find that many of the muscles are fastened to this bone.</p> - -<p>Now try to put back everything in its place, and you will find that -though you have neither cut nor torn nor broken either muscle or -blood-vessel or bone, you cannot get things back into their place again. -Everything looks “messy.” This is partly because, though you have torn -neither muscle nor blood-vessel, you have torn something which binds -skin and muscle and fat and blood-vessels and bone all together; and if -you look again you will see that between them there is a delicate -stringy substance which binds and packs them all together, just as -cotton-wool is used to pack up delicate toys and instruments. This -stringy packing material which you have torn and spoilt is called -<b>connective</b> because it connects all the parts together.</p> - -<p>Well, then, in the leg (and it is just the same in the arm) we have -skin, fat, muscle, tendons, blood-vessels, nerves, and bone all packed -together with connective and covered with skin. These together form the -solid leg. We may speak of them as <b>the tissues</b> of the leg.</p> - -<p><b><a name="sect_8" id="sect_8">8.</a></b> If now you turn to the trunk and cut through the skin of the belly, -you will first of all see muscles again, with nerves and blood-vessels -as before. But when you carefully cut through the muscles (for you -cannot easily separate them from each other here), you come upon -something which you did not find in<span class="pagenum"><a name="page_010" id="page_010"></a>{10}</span> </p> - -<p><a name="fig_1" id="fig_1"></a><span class="pagenum"><a name="page_011" id="page_011"></a>{11}</span> </p> - -<div class="figcenter"> -<a href="images/i_p010_lg.jpg"> -<br /> -<img src="images/i_p010_sml.jpg" width="446" height="692" alt="Image unavailable: Fig. 1.—The Viscera of a Rabbit as seen upon simply -opening the Cavities of the Thorax and Abdomen without any further -Dissection. - -A, cavity of the thorax, pleural cavity of either side; B, -diaphragm; C, ventricles of the heart; D, auricles; E, -pulmonary artery; F, aorta; G, lungs, collapsed, and occupying -only the back part of the chest; H, lateral portions of pleural -membranes; I, cartilage at the end of sternum; K, portion of -the wall of body left between thorax and abdomen; a, cut ends of -the ribs; L, the liver, in this case lying more to the left than -the right of the body; M, the stomach; N, duodenum; O, small -intestine; P, the cæcum, so largely developed in this and other -herbivorous animals; Q, the large intestine." /></a> -<br /> -<span class="caption">Fig. 1.—The Viscera of a Rabbit as seen upon simply -opening the Cavities of the Thorax and Abdomen without any further -Dissection.</span> -<p class="caption"> -A, cavity of the thorax, pleural cavity of either side; B, -diaphragm; C, ventricles of the heart; D, auricles; E, -pulmonary artery; F, aorta; G, lungs, collapsed, and occupying -only the back part of the chest; H, lateral portions of pleural -membranes; I, cartilage at the end of sternum; K, portion of -the wall of body left between thorax and abdomen; a, cut ends of -the ribs; L, the liver, in this case lying more to the left than -the right of the body; M, the stomach; N, duodenum; O, small -intestine; P, the cæcum, so largely developed in this and other -herbivorous animals; Q, the large intestine.</p> -</div> - -<p class="nind">the leg, a <b>great cavity</b>. This is something quite new—there is nothing -like it in the leg—a great cavity, quite filled with something, but -still a great cavity; and if you slit the rabbit right up the front of -its trunk and turn down or cut away the sides as has been done in <a href="#fig_1">Fig. -1</a>, you will see that the whole trunk is <b>hollow</b> from top to bottom, from -the neck to the legs.</p> - -<p>If you look carefully you will see that the cavity is divided into two -by a cross partition (<a href="#fig_1">Fig. 1</a>, <i>B</i>) called the <b>diaphragm</b>. The part <b>below</b> -the diaphragm is the larger of the two, and is called the <b>abdomen</b> or -belly; in it you will see a large dark red mass, which is the <b>liver</b> -(<i>L</i>). Near the liver is the smooth pale <b>stomach</b> (<i>M</i>), and filling up -the rest of the abdomen you will see the coils of the <b>intestine</b> or -bowel, very narrow in some parts (<i>O</i>), very broad (<i>P</i> <i>Q</i>), broader -even than the stomach, in others. If you pull the bowels on one side as -you easily can do, you will find lying underneath them two small -brownish red lumps, one on each side. These are the <b>kidneys</b>.</p> - -<p>In the smaller cavity <b>above</b> the diaphragm, called the <b>thorax</b> or chest, -you will see in the middle the <b>heart</b> (<i>C</i>), and on each side of the -heart two pink bodies, which when you squeeze them feel spongy. These -are the two <b>lungs</b> (<i>G</i>). You will notice that the heart and lungs do not -fill up the cavity of the chest nearly so much as the liver, stomach, -bowels, &c. fill up the cavity of the belly. In fact,<span class="pagenum"><a name="page_012" id="page_012"></a>{12}</span> in the chest -there seems to be a large empty space. But as we shall see further on, -the lungs did quite fill the chest before you opened it, but shrank up -very much directly you cut into it, and so left the great space you see.</p> - -<p><b><a name="sect_9" id="sect_9">9.</a></b> The trunk then is really a great chamber containing what are called -the <b>viscera</b>, and divided into an upper and lower half, the upper half -being filled with the heart and lungs, the lower with the liver, -stomach, bowels, and some other organs. In front the abdomen is covered -by skin and muscle only. But if all the sides of the trunk were made of -such soft material it would be then a mere bag which could never keep -its shape unless it were stuffed quite full. Some part of it must be -strengthened and stiffened. And indeed the trunk is not a bag with soft -yielding sides, but a box with walls which are in part firm and hard. -You noticed that when you were cutting through the front of the chest -you had to cut through several hard places. These were the <b>ribs</b> (<a href="#fig_1">Fig. 1</a>, -<i>a</i>), made either of hard bone or of a softer gristly substance called -<b>cartilage</b>. And if you take away all the viscera from the cavity of the -trunk and pass your finger along the back of the cavity, you will feel -all the way down from the neck to the legs a hard part. This is the -<b>backbone</b> or <b>vertebral column</b>. When you want to make a straw man stand -upright you run a pole right through him to give him support. Such a -support is the backbone to your own body, keeping the trunk from falling -together.</p> - -<p>In the abdomen nothing more is wanted than this backbone, the sides and -front of the cavity being covered in with skin and muscle only. In the -chest<span class="pagenum"><a name="page_013" id="page_013"></a>{13}</span> the sides are strengthened by the ribs, long thin hoops of bone -which are fastened to the backbone behind and meet in front in a firm -hard part, partly bone, partly cartilage, called the <b>sternum</b>.</p> - -<p>But this backbone is not made of one long straight piece of bone. If it -were you would never be able to bend your body. To enable you to do this -it is made up of ever so many little flat round pieces of bone, laid one -a-top of the other, with their flat sides carefully joined together, -like so many bungs stuck together. Each of these little round flat -pieces of the backbone is called a <b>vertebra</b>, and is of a very peculiar -shape. Suppose you took a bung of bone, and fastened on to one side of -its edge a ring of bone. That would represent a vertebra. The solid bung -is what is called the <b>body</b>, and the hollow ring is what is called the -<b>arch</b> of the vertebra. Now if you put a number of these bodies together -one upon the top of the other, so that the bodies all came together and -the rings all came together, you would have something very like the -vertebral column (see <a href="#front">Frontispiece</a>, also <a href="#fig_2">Fig. 2</a>). The bungs or bodies -would make a solid jointed pillar, and the rings or arches would make -together a tunnel or canal. And that is really what you have in the -backbone. Only each vertebra is not exactly shaped like a bung and a -ring; the body is very like a bung, but the arch is rough and jagged, -and the bodies are joined together in a particular way. Still we have -all the bodies of the vertebræ forming together a solid pillar which -gives support to the trunk; and the arches forming together a tunnel or -canal which is called the <b>spinal canal</b>, (<a href="#fig_2">Fig. 2</a>, <i>C.S.</i>) the use of -which we shall see<span class="pagenum"><a name="page_014" id="page_014"></a>{14}</span></p> - -<p><a name="fig_2" id="fig_2"></a></p> - -<div class="figcenter"> -<a href="images/i_p014_lg.jpg"> -<br /> -<img src="images/i_p014_sml.jpg" width="387" height="601" alt="Image unavailable: Fig. 2. - -A, a diagrammatic view of the human body cut in half lengthways. -C.S., the cavity of the brain and spinal cord; N, that of the -nose; M, that of the mouth; Al. Al., the alimentary canal -represented as a simple straight tube; H, the heart; D, the -diaphragm. - -B, a transverse vertical section of the head taken along the line -a b; letters as before. - -C, a transverse section taken along the line c d; letters as -before." /></a> -<br /> -<span class="caption"><span class="smcap">Fig</span>. 2. -</span><p class="caption"> - -A, a diagrammatic view of the human body cut in half lengthways. -C.S., the cavity of the brain and spinal cord; N, that of the -nose; M, that of the mouth; Al. Al., the alimentary canal -represented as a simple straight tube; H, the heart; D, the -diaphragm. -</p><p class="caption"> -B, a transverse vertical section of the head taken along the line -a b; letters as before. -</p><p class="caption"> -C, a transverse section taken along the line c d; letters as -before.</p> -</div> - -<p class="nind">directly. The round flat body of each vertebra is<span class="pagenum"><a name="page_015" id="page_015"></a>{15}</span> turned to the front -towards the cavity of the trunk, and it is the row of vertebral bodies -which you feel as a hard ridge when you pass your fingers down the back -of the abdomen. The arches are at the back of the bodies, so you cannot -feel them in the abdomen; but if you turn the rabbit on its belly and -pass your finger down its back, you will feel through the skin (and you -can feel the same on your own body) a sharp edge, formed by what are -called the spines, <i>i.e.</i> the uneven tips of the arches of the vertebræ -(<a href="#fig_2">Fig. 2</a>) all the way down the back.</p> - -<p>So that what we really have in the trunk is this. In front a large -cavity, containing the viscera, and surrounded in the upper part or -thorax by hoops of bone, but not (or only slightly) in the lower part or -abdomen; behind, a much smaller long narrow cavity or canal formed by -the arches of the vertebræ, and therefore surrounded by bone all the way -along, and containing we shall presently see what; and between these two -cavities, separating the one from the other, a solid pillar formed by -the bodies of the vertebræ. So that if you were to take a cross slice, -or transverse section as it is called, of the rabbit across the chest, -you would get something like what is represented in <a href="#fig_2">Fig. 2</a>, C, where -<i>C.S.</i> is the narrow canal of the arches and where the broad cavity of -the chest containing the heart <i>H</i> is enclosed in the ribs reaching from -the vertebra behind to the sternum in front. Both cavities are covered -up on the outside with muscles, blood-vessels, nerves, connective, and -skin, just as in the leg.</p> - -<p><b><a name="sect_10" id="sect_10">10.</a></b> We have now to consider the <b>head and neck</b>.<span class="pagenum"><a name="page_016" id="page_016"></a>{16}</span> If you cut through the -skin of the neck of the rabbit, you will see, first of all, muscles and -nerves, and several large blood-vessels; but you will find no large -cavity like that in the trunk. So far the neck is just like the leg. But -if you look carefully you will see two tubes which are not -blood-vessels, and the like of which you saw nowhere in the leg. One of -these tubes is firm, with hardish rings in it; it is the windpipe or -<b>trachea</b>; the other is soft, and its sides fall flat together; this is -the gullet or <b>œsophagus</b>, leading from the mouth to the stomach. -Behind these and the muscles in which they run you will find, just as in -the trunk, a vertebral column, without ribs, but composed of bodies, and -behind the bodies there is a vertebral canal. This vertebral column and -vertebral canal in the neck are simply continuations of the vertebral -column and canal of the trunk.</p> - -<p><b>The neck, then, differs from the leg in having a vertebral column and -canal with a trachea and œsophagus, and differs from the trunk in -having no cavity and no ribs.</b></p> - -<p>The head, again, is unlike all these. Indeed, you will not understand -how the head is made unless you take a rabbit’s skull and place it side -by side with the rabbit’s head. If you do this, you will at once see how -the mouth and throat are formed. <b>You will notice that the skull is all -in one piece</b>, except a bone which you will at once recognize as the -jawbone, or, to speak more correctly, the lower <b>jawbone</b>; for there are -two jawbones. Both these carry teeth, but the upper one is simply part -of the skull, and does not move; the lower one does move; it can be made -to shut close on the upper jaw, or can be separated a<span class="pagenum"><a name="page_017" id="page_017"></a>{17}</span> good way from it. -The opening between the two jaws is the gap or gape of the mouth, which -as you know can be opened or shut at pleasure. If you try it on yourself -you will find that, as in the rabbit, it is the lower jaw which moves -when you open or shut your mouth. The upper jaw does not move at all -except when your whole head moves. Underneath the skull at the top of -the neck the mouth narrows into the throat, into the upper part of which -the cavity of the nose opens. So that there are two ways into the -throat, one through the mouth and the other through the nose (<a href="#fig_2">Fig. 2</a>).</p> - -<p>At the back of the skull you will see a rounded opening, and if you put -a bodkin through this opening you will find it leads into a large hollow -space in the inside of the skull. In the living rabbit this hollow space -is filled up with the <b>brain</b>. The skull, in fact, is a box of bone to -hold the brain, a bony brain-case. This bony case fits on to the top of -the vertebræ of the neck in such a way that the rounded opening we spoke -of just now is placed exactly over the top of the tunnel or canal formed -by the rings or arches of the vertebræ. If you were to put a wire -through the arch of the lowest vertebra, you might push it up through -the canal formed by the arches of all the vertebræ, right into the brain -cavity. In fact the brain-case and the row of arches of the vertebræ -form together one canal, which is a narrow tube in the back and in the -neck, but swells out in the head into a wide rounded space (<a href="#fig_2">Fig. 2</a>, A -and B, <i>C.S.</i>) During life this canal is filled with a peculiar white -delicate material, which is called <b>nervous matter</b>. The rounded mass of -this material which fills up the cavity<span class="pagenum"><a name="page_018" id="page_018"></a>{18}</span> of the skull is called the -<b>brain</b>; the narrower, rod-like, or band-like mass which runs down the -vertebral canal in the neck and back is called the <b>spinal cord</b>. They -have separate names, but they are quite joined together, and the rounded -brain tapers off into the band-like cord in such a way that it is -difficult to say where the one begins and the other ends.</p> - -<p><b><a name="sect_11" id="sect_11">11.</a></b> In the skull, besides the larger openings we have spoken of, you -will find several small holes leading from the outside of the skull into -the inside of the brain-case. Some of these holes are filled up during -life by blood-vessels, but in others run those delicate white threads or -cords which you have already learnt to call nerves. <b>Nerves are in fact -branches of nervous material running out from the brain or spinal cord.</b> -Those from the brain pass through holes in the skull, and at first sight -seem to spread out very irregularly. Those which branch off from the -spinal cord are far more regular. A nerve runs out on each side between -every two vertebræ, little rounded gaps being left for that purpose -where the vertebræ fit together, so that when you look at a spinal cord -with portions of the nerves still connected with it, it seems not unlike -a double comb with a row of teeth on either side. The nerves which -spring in this way from the spinal cord are called <b>spinal nerves</b>, and -soon after they leave the vertebral canal they divide into branches, and -so are spread nearly all over the body. In any piece of skin or flesh -you examine, never mind in what part of the body, you will find nerves -and blood-vessels. If you trace the nerves out in one direction, you -will find them joining together to form larger nerves, and these again -joining others, till at last all end in either the<span class="pagenum"><a name="page_019" id="page_019"></a>{19}</span> spinal cord or the -brain. If you try to trace the same nerves in the other direction, you -will find them branching into smaller and smaller nerves, until they -become too small to be seen. If you take a microscope you will find they -get still smaller and smaller until they become the very finest possible -threads.</p> - -<p>The blood-vessels in a similar way join together into larger and larger -tubes, which last all end, as we shall see, in the heart. <b>Every part of -the body, with some few exceptions, is crowded with nerves and -blood-vessels. The nerves all come from the brain or spinal cord—the -vessels from the heart. So that every part of the body is governed by -two centres, the heart, and the brain or spinal cord.</b> You will see how -important it is to remember this when we get on a little further in our -studies.</p> - -<p><b><a name="sect_12" id="sect_12">12.</a></b> Well, then, the body is made up in this way. First there is the -head. In this is the skull covered with skin and flesh, and containing -the brain. The skull rests on the top of the backbone, where the head -joins the neck. In the upper part of the neck, the throat divides into -two pipes or tubes—one the windpipe, the other the gullet. These -running down the neck in front of the vertebral column, covered up by -many muscles, when they get about as far down as the level of the -shoulders, pass into the great cavity of the body, and first into the -upper part of it, or chest.</p> - -<p>Here the windpipe ends in the lungs, but the gullet runs straight -through the chest, lying close at the back on the backbone, and passes -through a hole in the diaphragm into the abdomen, where it swells out<span class="pagenum"><a name="page_020" id="page_020"></a>{20}</span> -into the stomach. Then it narrows again into the intestine, and after -winding about inside the cavity of the abdomen a good deal, finally -leaves it.</p> - -<p>You see the <b>alimentary canal</b> (for that is the name given to this long -tube made up of gullet, stomach, intestine, &c.) <b>goes right through the -cavity of the body without opening into it</b>—very much as the tall narrow -glass of a lamp passes through the large globe glass. You might pour -anything down the narrow glass without its going into the globe glass, -and you might fill the globe glass and yet leave the narrow glass quite -empty. If you imagine both glasses soft and flexible instead of hard and -stiff, and suppose the narrow glass to be very long and twisted about so -as to all but fill the globe, you will have a very fair idea of how the -alimentary canal is placed in the cavity of the body.</p> - -<p>Besides the alimentary canal, there is in the chest, in addition to the -windpipe and lungs, the heart with its great tubes, and in the abdomen -there are the liver, the kidneys, and other organs.</p> - -<p>These two great cavities, with all that is inside them, together with -wrappings of flesh and skin which make up the walls of the cavities, -form the trunk, and on to the trunk are fastened the jointed legs and -arms. These have no large cavities, and the alimentary canal goes -nowhere near them.</p> - -<p>One more thing you have to note. There is only one alimentary canal, one -liver, one heart—but there are two kidneys and two lungs, the one on -one side, the other on the other, and the one very much like the other. -There are two arms and two legs, the one almost exactly like the other. -There is only one head, but<span class="pagenum"><a name="page_021" id="page_021"></a>{21}</span> one side of the head is almost exactly like -the other. One side of the vertebral column is exactly like the -other—as are also the two halves of the brain and the two halves of the -spinal cord.</p> - -<p>In fact, if you were to cut your rabbit in half from his nose to his -tail, you would find that except for his alimentary canal, his heart, -and his liver, one half was almost exactly the counterpart of the other.</p> - -<p>Such is the structure of a rabbit, and your own body, in all the points -I have mentioned, is made up exactly in the same way.</p> - -<h2><a name="WHAT_TAKES_PLACE_WHEN_WE_MOVE_III" id="WHAT_TAKES_PLACE_WHEN_WE_MOVE_III"></a>WHAT TAKES PLACE WHEN WE MOVE. § III.</h2> - -<p><b><a name="sect_13" id="sect_13">13.</a></b> Let us now go back to the question. <b>How is it that we can move about -as we do?</b> And first of all let us take one particular movement and see -if we can understand that.</p> - -<p>For instance, you can bend your arm. You know that when your arm is -lying flat on the table, you can, if you like, bend the lower part of -your arm (the fore-arm as it is called, reaching from the elbow to the -hand) on the upper arm until your fingers touch your shoulder. How do -you manage to do that?</p> - -<p>Look at the bones of the arm in a skeleton. (<a href="#front">Frontispiece</a>; also <a href="#fig_3">Fig. 3</a>.) -You will see that in the upper arm there is one rather large bone (<i>H</i>) -reaching from the shoulder to the elbow, while in the fore-arm there are -two, one (<i>U</i>) being wider and stouter than the other (<i>Ra</i>) at the -elbow, but smaller and more slender at the wrist. The bone in the upper -arm is called the <b>humerus</b>; the bone in the fore-arm, which is stoutest -at the elbow, is called the <b>ulna</b>; the one<span class="pagenum"><a name="page_022" id="page_022"></a>{22}</span> which is stoutest at the -wrist is called the <b>radius</b>. If you look carefully you will see that the -end of the humerus at the elbow is curiously rounded, and the end of the -ulna at the elbow curiously scooped out, in such a way that the one fits -loosely into the other.</p> - -<p><a name="fig_3" id="fig_3"></a></p> - -<div class="figcenter"> -<a href="images/i_p022_lg.jpg"> -<br /> -<img src="images/i_p022_sml.jpg" width="373" height="267" alt="Image unavailable: Fig 3.—The Bones of the Upper Extremity with the Biceps -Muscle. - -The two tendons by which this muscle is attached to the scapula, or -shoulder-blade, are seen at a. P indicates the attachment of the -muscle to the radius, and hence the point of action of the power; -F, the fulcrum, the lower end of the humerus on which the upper end -of the radius (together with the ulna) moves; W, the weight (of the -hand)." /></a> -<br /> -<span class="caption">Fig 3.—The Bones of the Upper Extremity with the Biceps -Muscle. -</span><p class="caption"> -The two tendons by which this muscle is attached to the scapula, or -shoulder-blade, are seen at a. P indicates the attachment of the -muscle to the radius, and hence the point of action of the power; -F, the fulcrum, the lower end of the humerus on which the upper end -of the radius (together with the ulna) moves; W, the weight (of the -hand).</p> -</div> - -<p>If you try to move them about one on the other, you will find that you -can easily double the ulna very closely on the humerus without their -ends coming apart, and if you notice you will see that as you move the -ulna up and down, its end and the end of the humerus slide over each -other. But they will only slide one way, what we may call up and down. -If you try to slide them from side to side, you will find that they get -locked. They have only one movement,<span class="pagenum"><a name="page_023" id="page_023"></a>{23}</span> like that of a door on its hinge, -and that movement is of such a kind as to double the ulna on the -humerus.</p> - -<p>Moreover, if you look a little more carefully you will find that, though -you can easily double the ulna on the front of the humerus, and then -pull it back again until the two are in a straight line, you cannot bend -the ulna on the back of the humerus. On examining the end of the ulna -you will find at the back of it a beak-like projection (<a href="#fig_3">Fig. 3</a>, also -Frontispiece), which when the bones are straightened out locks into the -end of the humerus, and so prevents the ulna being bent any further -back. This is the reason why you can only bend your arm one way. As you -very well know, you can bend your arm so as to touch the top of your -shoulder with your fingers, but you can’t bend it the other way so as to -touch the back of your shoulder; you can’t bring it any further back -than the straight line.</p> - -<p><b><a name="sect_14" id="sect_14">14.</a></b> Well, then, at the elbow the two bones, the humerus and ulna, are so -shaped and so fit into each other that the arm may be straightened or -bent. In the skeleton the two bones are quite separate, <i>i.e.</i> they have -to be fastened together by something, else they would fall apart. Most -probably in the skeleton you have been examining they are fastened -together by wires or slips of brass. But they would hold together if you -took away the wire or brass slips and bound some tape round the two -ends, tight enough to keep them touching each other, but loose enough to -allow them to move on each other. You might easily manage it if you took -short slips of tape, or, better still, of india-rubber, and placed them -all round the elbow, back, front, and sides, fastening one end of each -slip to the humerus and the other to the ulna. If you<span class="pagenum"><a name="page_024" id="page_024"></a>{24}</span> did this you -would be imitating very closely the manner in which the bones at the -elbow are kept together in your own arm. Only the slips are not made of -india-rubber, but are flat bands of that stringy, or as we may now call -it fibrous stuff, which in the preceding lessons you learnt to call -connective tissue. These flat bands have a special name, and are called -<b>ligaments</b>.</p> - -<p>At the elbow the two ends of the ulna and humerus are kept in place by -ligaments or flat bands of connective tissue.</p> - -<p>In the skeleton, the surfaces of the two bones at the elbow where they -rub against each other, though somewhat smooth, are dry. If you ever -looked at the knuckle of a leg of mutton before it was cooked, you will -have noticed that you have there two bones slipping over each other -somewhat as they do at the elbow, and will remember that where the bones -meet they are wonderfully smooth, and very moist, so as to be quite -slippery. It is just the same in your own elbow; the end of the ulna and -the end of the humerus are beautifully smooth and quite moist, so that -they slip over each other as easily as possible. You know that your eye -is always moist. It is kept moist by tears, though you don’t speak of -tears until your eyes overflow with moisture; but in reality you are -always crying a little. Well, there are, so to speak, tears always being -shed inside the wrapping of ligaments around the elbow, and they keep -the two surfaces of the bones continually moist.</p> - -<p>The ends of bones where they touch each other are also smooth, because -they are coated over with what is called gristle or cartilage. Bone is -very hard<span class="pagenum"><a name="page_025" id="page_025"></a>{25}</span> and very solid; there is not much water in it. Bones dry up -very little. Cartilage is not nearly so hard as bone; there is very much -more water in it. When it is quite fresh it is very smooth, but because -it has a good deal of water in it, it shrinks very much when it dries -up, and when dried is not nearly so smooth as when it is fresh. You can -see the dried-up cartilage on the ends of the bones in the skeleton—it -is somewhat smooth still, but you can form no idea of how smooth it is -in the living body by simply seeing it on the dried skeleton.</p> - -<p><b>At the elbow, then, we have the ends of two bones fitting into each -other, so that they will move in a certain direction; these ends are -smoothed with cartilage, kept moist with a fluid, and held in place by -ligaments. All this is a called a joint.</b></p> - -<p><b><a name="sect_15" id="sect_15">15.</a></b> There are a great many other joints in the body besides the -elbow-joint: there is the shoulder-joint, the knee-joint, the hip-joint, -and so on. These differ from the elbow-joint in the shape of the ends of -the bone, in the way the bones move on each other, and in several other -particulars, but we must not go into these differences now. They are all -like the elbow, since in each case one bone fits into another, the -surfaces are coated with cartilage, are kept moist with fluid (what the -grooms call joint-oil, though it is not an oil at all), and are held in -place by ligaments.</p> - -<p>I dare say you will have noticed that though I have been speaking only -of the humerus and ulna at the elbow, the other bone of the -fore-arm—the radius—has something to do with the elbow too. I left it -out in order to simplify matters, but it is nevertheless quite<span class="pagenum"><a name="page_026" id="page_026"></a>{26}</span> true -that the end of the humerus moves over the end of the radius as well as -over the end of the ulna, and that the end of the radius is also coated -with cartilage and is included in the wrapping of the ligaments. I might -add that the radius also moves independently on the ulna, but I don’t -want to trouble you with this just now. What I wanted to show you was -that the elbow is a joint, a joint so constructed that it allows the -fore-arm to be bent on the upper arm.</p> - -<p><b><a name="sect_16" id="sect_16">16.</a> In order that the arm may be bent, some force must be used.</b> The ulna -or radius—for the two move together—must be pushed or pulled towards -the humerus, or the humerus must be pushed or pulled towards the radius -and ulna. How is this done in your own arm?</p> - -<p>Take the bones of the arm; fix the top end of the humerus; tie it to -something so that it cannot move. Fasten a piece of string to either the -radius or ulna (it doesn’t matter which), rather near the elbow. Bore a -hole through the top of the humerus and pass the string through it. Your -string must be long enough to let the arm be quite straight without any -strain on the string. Now, taking hold of the string where it comes out -through the humerus, pull it. The fore-arm will be bent on the arm. Why? -<b>Because you have been working a lever of the third order.</b></p> - -<p>The radius and ulna form the lever; its fulcrum is the end of the -humerus in the elbow (<a href="#fig_3">Fig. 3</a>, F); the weight to be moved is the weight -of the radius and ulna (with that of the bones of the hand if present), -and this may be represented by a weight applied at about the middle of -the fore-arm; the power is the pull<span class="pagenum"><a name="page_027" id="page_027"></a>{27}</span> you give the string, and that is -brought to bear on your lever at the point where the string is fastened -to the radius, <i>i.e.</i> nearer the fulcrum than the point where the weight -is applied; and you know that when you have the fulcrum at one end and -the power between the fulcrum and the weight, you have a lever of the -third order.</p> - -<p>Now, in order to make the thing a little more like what takes place in -your own arm, instead of boring a hole through the humerus, let the -string glide in a groove which you will see at the top of the humerus, -and fasten the end of it to the shoulder-blade or anything you like -above the humerus, and let the string be just long enough to let the arm -be quite straightened out, but no longer, so that when the arm is -straight the string is just about tight, or at least not loose.</p> - -<p>Now shorten the string by pinching it up into a loop. Whenever you do -this you will bend the fore-arm on the arm. Suppose you used a string -which you had not to pinch up, but which, when you pleased, you could -make to shorten itself. <b>Every time it shortened itself it would pull the -fore-arm up and would bend the arm—and every time it slackened again, -the arm would fall back into the straight position.</b></p> - -<p>In your arm there is not a string, but a body, placed very much as our -string is placed, and which has the power of shortening itself when -required. Every time it shortens itself it bends the arm, and when it -has done shortening and lengthens again, the arm falls back into its -straight position. This body which thus can shorten and lengthen itself -is called a muscle.<span class="pagenum"><a name="page_028" id="page_028"></a>{28}</span></p> - -<p>If you put one hand on the front of your other upper arm, about half-way -between your shoulder and elbow, and then bend that arm, you will feel -something rising up under your hand. This is the muscle, which bends the -arm, shortening, or, as we shall learn to call it, <b>contracting</b>.</p> - -<p>In your own arm, as in the limb of the rabbit which you studied in your -last lesson, the flesh is arranged in masses or bundles of various sizes -and shapes, and each mass or bundle is called a muscle. There are -several muscles in the arm, but there is in particular a large one -occupying the front of the arm, called the <b>biceps</b>. It is a rounded mass -of red flesh, considerably longer than it is broad or thick, and -tapering away at either end. It is represented in <a href="#fig_3">Fig. 3</a>.</p> - -<p>You may remember that while examining the leg of the rabbit you noticed -that in many of the muscles, the soft flesh, which made up the greater -part of the muscle, at one or both ends of the muscle suddenly left off, -and changed into much firmer material which was white and glistening. -This firmer white part you were told was called the tendon of the -muscle. The rest of the muscle, generally called “the belly,” is made up -of what you are accustomed to call flesh, or lean meat, but which you -must now learn to speak of as <b>muscular substance</b>. Every muscle, in fact, -consists in the first place of a mass of muscular substance. <b>This -muscular substance is made up of an immense number of soft strings or -fibres, all running in one direction and done up into large and small -bundles.</b> At either end of the muscle these soft muscular fibres are -joined on to firmer but thinner fibres of connective or fibrous<span class="pagenum"><a name="page_029" id="page_029"></a>{29}</span> tissue. -And these thinner but firmer fibres make up the cord or band of tendon -with which the muscle finishes off at either end.</p> - -<p><b>It is by these tendons that the soft muscles are joined on to the hard -bones, or to some of the other firm textures of the body.</b> The tendons -are sometimes round and cord-like, sometimes flat and spread out. -Sometimes they are very long, sometimes very short, so as to be scarcely -visible. But always you have some amount of the firmer fibres of -connective tissue joining the soft muscular fibres on to the bones, and -generally the tendons are not only firmer but much thinner and more -slender than the belly of the muscle.</p> - -<p>The muscular belly of the biceps is placed in the front of the upper -arm. Some little way above the elbow-joint it ends in a small round -strong tendon which slips over the front of the elbow and is fastened -to, <i>i.e.</i> grows on to, the radius at some little distance below the -joint (<a href="#fig_3">Fig. 3</a>, P). The upper part of the muscular belly ends a little -below the shoulder, not in one tendon but in two<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> tendons (<a href="#fig_3">Fig. 3</a>, -<i>a</i>), which gliding over the end of the humerus are fastened to the -shoulder-blade (or <b>scapula</b> as it is called), into which the humerus fits -with a joint.</p> - -<p>We have then in the biceps a thick fleshy muscular belly placed in the -front of the arm and fastened by tendons, at one end to the -shoulder-blade, and at the other to the fore-arm. What would happen if -when the arm is straight and the shoulder-blade fixed, the biceps were -suddenly to grow very much shorter than<span class="pagenum"><a name="page_030" id="page_030"></a>{30}</span> it was? Evidently the same -thing that happened when you pinched up and shortened the string which, -if you look back you will see, we supposed to be placed very much as the -biceps with its tendons is placed. <b>The radius and ulna would be pulled -up, the fore-arm would be bent on the arm.</b></p> - -<p>Now tendons have no power of shortening themselves, but muscular -substance does possess this remarkable power of suddenly shortening -itself. Under certain circumstances each soft muscular fibre of which -the muscle is made will suddenly become shorter, and thus the whole -muscle becomes shorter, and so pulls its two tendinous ends closer -together, and if one end be fastened to something fixed, and the other -to something moveable, the moveable thing will be moved.</p> - -<p>This way that a muscle or a muscular fibre has of suddenly shortening -itself is called a <b>muscular contraction</b>. <b>All muscles, all muscular -fibres, have the power of contracting.</b> Now a mass of substance like the -biceps might grow shorter in two ways. It might squeeze itself together -and become smaller altogether, it might squeeze itself as you would -squeeze a sponge into a smaller bulk. Or it might change its form and -not its bulk, becoming thicker as it became shorter, just as you might -by pressing the two ends together squeeze a long thin roll of soft wax -into a short thick one. It might get shorter in either of these two -ways, but it does actually do so in the latter way; it gets thicker at -the same time that it gets shorter, and gets nearly as much thicker as -it gets shorter. And that is why, when you put your hand on the arm -which is being bent, you feel<span class="pagenum"><a name="page_031" id="page_031"></a>{31}</span> something rise up. <b>You feel the biceps -getting thicker as it is getting shorter in order to bend the arm.</b></p> - -<p>The shortening does not last for ever. Sooner or later the muscle -lengthens again, getting thinner once more, and so returns to its former -state. The lengthened condition of the muscle is the natural condition, -the condition of rest. The shortening or contraction is an effort which -can only be continued for a certain time. The contraction bends the arm, -and as long as the muscle remains shortened the arm keeps bent; but as -the muscle lengthens, the weight of the hand and fore-arm, if there is -nothing to prevent, straightens the arm out again.</p> - -<p><b>It is in the muscle alone, in the belly made up of muscular fibres, that -the shortening takes place.</b> The tendons do not shorten at all. On the -contrary, if anything they lengthen a little, but only a very little, -when the muscle pulls upon them. Their purpose is to convey to the bone -the pull of the muscle. They are not necessary, only convenient. It -would be possible but awkward to do without them. Suppose the fleshy -fibres of the biceps reached from the shoulder-blade to the fore-arm: -you could bend your arm as before, but it would be very tiresome to have -the muscle swelling up in the inside of the elbow, or on the top of the -shoulder; in either place it would be very much in the way. By keeping -the fleshy, the real contracting muscle, in the arm, and carrying the -thin tendons to the arm and to the shoulder, you are enabled to do the -work much more easily and conveniently.</p> - -<p>Well, then, we have got thus far in understanding<span class="pagenum"><a name="page_032" id="page_032"></a>{32}</span> how the arm is bent. -The biceps muscle contracts and shortens, tries to bring its two -tendinous ends together. The upper tendons, being fastened to the fixed -shoulder-blade, cannot move; but the lower tendon is fixed to the -radius; the radius, with the ulna to which it is fastened, readily moves -up and down on the elbow-joint—the shape of bones in the joint and all -the arrangements of the joint, as we have seen, readily permitting this. -When the muscle, then, pulls on its lower tendon, its pulls on the -radius at the point where the tendon is fastened on to the bone. The -radius thus pulled on forms with the ulna a lever of the third order, -working on the end of the humerus as a fulcrum; and thus as the tendon -is pulled the fore-arm is bent.</p> - -<p><b><a name="sect_17" id="sect_17">17.</a></b> But now comes the question. What makes the muscle shorten or -contract? You willed to move your arm, and moved it, as we have seen, by -making the biceps contract; but <b>how did your will make the biceps -contract</b>?</p> - -<p>If you could examine your arm as you did the leg of the rabbit, you -would find running into your biceps muscle, one or more of those soft -white threads or cords, which you have already learnt to recognize as -<b>nerves</b>.</p> - -<p>These nerves seem to grow into and be lost in the biceps muscle. We need -not follow them any further in that direction, but if we were to trace -them in the other direction, up the arm, we should find that they soon -meet with other similar nerves, and that the several nerves joining -together form stouter and thicker nerve-cords. These again join others, -and so we should proceed until we came to quite stoutish<span class="pagenum"><a name="page_033" id="page_033"></a>{33}</span> white -nerve-trunks as they are called, which we should find passed at last -between the vertebræ, somewhere in the neck, into the inside of the -vertebral canal, where they became mixed up with the mass of nervous -material we have already spoken of as the spinal cord.</p> - -<p>What have these nerves to do with the bending of the arm? Why simply -this. Suppose you were able without much trouble to cut across the -delicate nerves going to your biceps, and did so: what would happen? You -would find that you had lost all power of bending your arm; however much -you willed it, there would be no swelling rise up in your arm. Your -biceps would remain perfectly quiet, and would not shorten at all, would -not contract in obedience to your will.</p> - -<p>What does this show? It proves that when you will to bend your arm, -something passes along the nerves going to the biceps muscle, which -something causes that muscle to contract? The nerve, then, is a bridge -between your will and the muscle—so that when the bridge is broken or -cut away, the will cannot get to the muscle.</p> - -<p>If anywhere between the muscle and the spinal cord you cut the nerve -which goes, or branches from which go, to the muscle, you destroy the -communication between the will and the muscle.</p> - -<p>The spinal cord, as we have seen, is a mass of nervous substance -continuous with the brain; from the spinal cord nearly all the nerves of -the body are given off; those nerves whose branches go to the biceps -muscle in the arm leave the spinal cord somewhere in the neck.</p> - -<p>If you had the misfortune to have your spinal<span class="pagenum"><a name="page_034" id="page_034"></a>{34}</span> cord cut across or -injured in your neck, you might still live, but you would be paralysed. -You might will to bend the arm, but you could not do it. You would know -you were willing, you would feel you were making an effort, but the -effort would be unavailing. The spinal cord is part of the bridge -between the will and the muscle.</p> - -<p>When you bend your arm, then, this is what takes place. <b>By the exercise -of your will a something is started in your brain. That something—we -will not stop now to ask what that something is—passes from your brain -to the spinal cord, leaves the spinal cord and travels along certain -nerves, picking its way among the intricate bundles of delicate nervous -threads which run from the upper part of the spinal cord to the arm -until it reaches the biceps muscle. The muscle, directly that -“something” comes to it along its nerves, contracts, shortens, and grows -thick; it rises up in the arm; its lower tendon pulls at the radius; the -radius with the ulna moves on the fulcrum of the humerus at the -elbow-joint, and the arm is bent.</b></p> - -<p>You wish to leave off bending the arm. Your will ceases to act. The -something to which your will had given rise dies away in the brain, dies -away in the spinal cord, dies away in the nerves, even in the finest -twigs. The muscle, no longer excited by that something, ceases to -contract, ceases to swell up, ceases to pull at the radius, and the -fore-arm by its own weight falls into its former straightness, -stretching, as it falls, the muscle to its natural length.<span class="pagenum"><a name="page_035" id="page_035"></a>{35}</span></p> - -<p><b><a name="sect_18" id="sect_18">18.</a></b> So far I hope you have followed me, but we are still very far from -being at the bottom of the matter. Why does the muscle contract when -that something reaches it through the nerves? We must content ourselves -by saying that it is the property of the muscle to do so. Does the -muscle always possess this property? No, not always.</p> - -<p>Suppose you were to tie a cord very tightly round the top of your arm, -close to the shoulder. What would happen? If you tied it tight enough (I -don’t ask you to do it, for you might hurt yourself) the arm would -become pale, and very soon would begin to grow cold. It would get -numbed, and would gradually seem to grow very heavy and clumsy; your -feeling in it would be blunted, and after a while be altogether lost. -When you tried to bend your arm you would find great difficulty in doing -so. Though you tried ever so much, you could not easily make the biceps -contract, and at last you would not be able to do so at all. You would -discover that you had lost all power of bending your arm. And then if -you undid the cord you would find that after some very uncomfortable -sensations, little by little the power would come back to you; the arm -would grow warm again, the heaviness and clumsiness would pass away, the -feeling in it would return, you would be able to bend it, and at last -all would be as it was before.</p> - -<p>What did you do when you tied the cord tight? The chief thing you did -was to press on the blood-vessels in the arm and so stop the blood from -moving in them. If instead of tying the cord round the whole arm you had -tied a finer thread round the blood-vessels<span class="pagenum"><a name="page_036" id="page_036"></a>{36}</span> only, you would have -brought about very nearly the same effect. We saw in the last lesson how -all parts of the body are supplied with blood-vessels, with veins, and -arteries. In the arm there is a very large artery, branches from which -go all over the arm. Some of these branches go to the biceps muscle. -What would happen if you tied these branches only, tying them so tight -as to stop all the blood in them, but not interfering with the -blood-vessels in the rest of your arm? The arm as a whole would grow -neither pale nor cold, it would not become clumsy or heavy, you would -not lose your feeling in it, but nevertheless if you tried to bend your -arm you would find you could not do it. You could not make the biceps -contract, though all the rest of the arm might seem to be quite right.</p> - -<p>What does this teach us? <b>It teaches us that the power which a muscle has -of contracting when called upon to do so, may be lost and regained, and -that it is lost when the blood is prevented from getting to it.</b> When a -cord is tied round the whole arm, the power of the whole arm is lost. -This loss of power is the beginning of death, and indeed if the cord -were not unloosed the arm would quite die—would mortify, as it is said. -When only those blood-vessels which go to the biceps are tied, the -biceps alone begins to die, all the rest of the arm remaining alive, and -the first sign of death in the biceps is the loss of the power to -contract when called upon to do so.</p> - -<p>In order that you may bend your arm, then, you must not only have a -biceps muscle with its nerves, its tendons, and all its arrangements of -bones<span class="pagenum"><a name="page_037" id="page_037"></a>{37}</span> and joints, but the muscle must be supplied with blood.</p> - -<p><b><a name="sect_19" id="sect_19">19.</a></b> We can now go a step further and ask the question, <b>What is there in -the blood that thus gives to the muscle the power of contracting, that -in other words keeps the muscle alive?</b> The answer is very easily found. -What is the name commonly given to this power of a muscle to contract? -We generally call it strength. Lay your arm straight out on the table, -put a heavy weight in your hand, and try to bend your arm. If you could -do it, one would say you were strong; if you could not, one would say -you were weak—all the stronger or weaker, the heavier or lighter the -weight. In the one case your biceps had great power of contracting; in -the other, little power. Try and find out the heaviest weight you can -raise in this way by bending your arm, some morning, not too long after -breakfast, when you are fresh and in good condition. Go without any -dinner, and in the afternoon or evening, when you are tired and hungry, -try to raise the same weight in the same way. You will not be able to do -it. Your biceps will have lost some of its power of contracting, will be -weaker than it was in the morning. What makes it weak? The want of food. -But how can the food affect the muscle? You do not place the food in the -muscle; you put it into your mouth, and from thence it goes into your -stomach and into the rest of your alimentary canal, and there seems to -disappear. How does the food get at the muscle? By means of the blood. -The food becomes blood. <b>The things which you eat as food become changed -into other things which form part<span class="pagenum"><a name="page_038" id="page_038"></a>{38}</span> of the blood. Those things going to -the muscle give it strength and enable it to contract.</b> And that is why -food makes you strong.</p> - -<p><b><a name="sect_20" id="sect_20">20.</a></b> But you are always wanting food day by day, from time to time. Why -is that? Because the muscle in getting strength out of the food changes -it, uses it up, and so is always wanting fresh blood and new food. We -have seen in Art. 1 that food is fuel. We have also seen that muscle -(and other parts of the body do the same) is always burning, burning -without flame but with heat, burning slowly but burning all the same, -and doing the more work the more it burns. The fuel it burns is not dry -wood or coal, but wet, watery blood, a special kind of fuel prepared for -its private use, in the workshop of the stomach or elsewhere, out of the -food eaten by the mouth. This it is always using up; of this it must -always have a proper supply, if it is to go on working. Hence there must -always be fresh blood preparing; hence there must from time to time be -fresh supplies of food out of which to manufacture fresh blood.</p> - -<p>To understand then fully what happens when you bend your arm, we have to -learn not only what we have learnt about the bones and the joint and the -muscle and the nerves, about the machinery and the engine, we have to -study also how the food is changed into blood, how the blood is brought -to the muscle, what it is in the blood on which the muscle lives, what -it is which the muscle burns, and how the things which result from the -burning, the ashes and the smoke or carbonic acid and the rest of them, -are carried away from the muscle and out of the body.</p> - -<p>Meanwhile let me remind you that for the sake of<span class="pagenum"><a name="page_039" id="page_039"></a>{39}</span> being simple I have -been all this while speaking of one muscle only, the biceps in the arm. -But there are a multitude of muscles in the body besides the biceps, as -there are many bones besides those of the arm, and many joints besides -the elbow. But what I have said of the one is in a general way true of -all the rest. The muscles have various forms, they pull upon the bones -in various ways, they work on levers of various kinds. The joints differ -much in the way in which they work. All manner of movements are produced -by muscles pulling sometimes with and sometimes against each other. But -you will find when you come to examine them that all the movements of -which your body is capable depend at bottom on this—<b>that certain -muscular fibres, in obedience to a something reaching them through their -nerves, contract, shorten, and grow thick, and so pull their one end -towards the other, and that to do this they must be continually supplied -with pure blood</b>.</p> - -<p>Moreover, what I have said of the relations of muscle to blood is also -true of all other parts of the body. Just as the muscle cannot work -without a due supply of blood, so also the brain and the spinal cord and -the nerves have even a more pressing need of pure blood. The weakness -and faintness which we feel from want of food is quite as much a -weakness of the brain and of the nerves as of the muscles,—perhaps -rather more so. And other parts of the body of which we shall have to -speak later on need blood too.</p> - -<p>The whole history of our daily life is shortly this. The food we eat -becomes blood, the blood is carried<span class="pagenum"><a name="page_040" id="page_040"></a>{40}</span> all over the body, round and round -in the torrent of the circulation; as it sweeps past them, or rather -through them, the muscle, the brain, the nerve, the skin pick out new -food for their work and give back the things they have used or no longer -want. As they all have different works, some use up what others have -thrown away. There are, besides, scavengers and cleaners to pick up -things no longer wanted anywhere and to throw them out of the body. Thus -the blood is kept pure as well as fresh. Through the blood thus ever -brought to them, each part does its work: the muscle contracts, the -brain feels and wills, the nerves carry the feeling and the willing, and -the other organs of the body do their work too, and thus the whole body -is kept alive and well.</p> - -<h2><a name="THE_NATURE_OF_BLOOD_IV" id="THE_NATURE_OF_BLOOD_IV"></a>THE NATURE OF BLOOD. § IV.</h2> - -<p><b><a name="sect_21" id="sect_21">21.</a> What, then, is this blood which does so much?</b></p> - -<p>Did you ever look through a good microscope at the thin transparent web -of a frog’s foot, and watch the red blood coursing along its narrow -channels? If not, go and look at it at once; you will never understand -any physiology till you have done so. There you will see a network of -delicate passages far finer than any of your own hairs, and through -those passages a tumbling crowd of tiny oval yellow globules hurrying -and jostling along. Some of the passages are wider than others, and -through some of the wider ones you will see a thick stream of globules -rushing onwards towards the smaller channels, and spreading out among -them. The globules which<span class="pagenum"><a name="page_041" id="page_041"></a>{41}</span> you see are floating in a fluid so clear that -you cannot see it. Some of the smaller channels are so narrow that only -one globule or <b>corpuscle</b>, as we may call it, can pass through at a time, -and very frequently you may see them passing in single file. Watching -them as they glide along these narrow paths, you will note that at last -they tumble again into wider passages, somewhat like those from which -they came, except that the stream runs away from instead of towards the -narrower channels; and in the stream the corpuscle you are watching -shoots out of sight. The finest passages are called <b>capillaries</b>; they -are guarded by delicate walls which you can hardly see; they seem to you -passages only, and how fine and small they are will come home to you -when you recollect that all you are looking at is going on in the depths -of a skin which is so thin that perhaps you would be inclined to say it -has no thickness at all.</p> - -<p>The larger channels which are bringing the blood down to the capillaries -are the ends of vessels like those which in the rabbit you learnt to -call arteries, and the other larger channels through which the blood is -rushing away from the capillaries are the beginnings of veins.</p> - -<p>When you have watched this frog’s foot for some little time, turn away -and reflect that in almost every part of your own body, in every square -inch, in almost every square line, something very similar might be seen -could the microscope be brought to bear upon it, only the corpuscles are -smaller and round, the capillaries narrower and for the most part more -thick-set, and the race a swifter one. In<span class="pagenum"><a name="page_042" id="page_042"></a>{42}</span> the muscle of which we were -speaking in the last lesson, each of the soft long fibres of which the -muscle is composed is wrapped round with a close network of these tiny -capillaries, through which, as long as life lasts, for ever rushes a -swift stream of blood, reddened by countless numbers of tiny corpuscles.</p> - -<p>In every part of your flesh, in your brain and spinal cord, in your -skin, your bones, your lungs, in all organs and in nearly every part of -your body, there is the same hurrying rush through narrow tubes of red -corpuscles and of the clear fluid in which these swim.</p> - -<p>If you prick your finger it bleeds. Almost any part of your body would -bleed were you to prick it. So thick-set are the little blood-vessels, -that wherever you thrust a needle, be it as fine a needle as you please, -you will be sure to pierce and tear some little blood channel, either -artery or capillary or vein, and out will come the ruddy drop.</p> - -<p><b><a name="sect_22" id="sect_22">22.</a> What is blood?</b> It is a fluid; it runs about like water: yet it is -thicker than water, thicker for two reasons. In the first place, water, -that is pure water, is all one substance. If you were to look at it with -ever so powerful a microscope, you would see nothing in it. It is -exceedingly transparent—you can see very well through ever such a -thickness of clean water. But if you were to try and look through even a -very thin sheet of blood spread out between two glass plates, you would -find that you could see very little; <b>blood is very opaque</b>. If again you -examine a drop of your blood with a microscope, what do you see? <b>A -number of little</b><span class="pagenum"><a name="page_043" id="page_043"></a>{43}</span></p> - -<p><a name="fig_4" id="fig_4"></a></p> - -<div class="figcenter"> -<a href="images/i_p043_lg.jpg"> -<br /> -<br /> -<img src="images/i_p043_sml.jpg" width="401" height="426" alt="Image unavailable: Fig. 4.—Red and White Corpuscles of the Blood -magnified. - -A. Moderately magnified. The red corpuscles are seen lying in -rows like rolls of coins; at a and a are seen two white -corpuscles. - -B. Red corpuscles much more highly magnified, seen in face; C. -ditto, seen in profile; D. ditto, in rows, rather more highly -magnified; E. a red corpuscle swollen into a sphere by imbibition -of water. - -F. A white corpuscle magnified same as B.; G. ditto, throwing -out some blunt processes; K. ditto, treated with acetic acid, and -showing nucleus, magnified same as D. - -H. Red corpuscles puckered or crenate all over. - -I. Ditto, at the edge only." /></a> -<br /> -<span class="caption">Fig. 4.—Red and White Corpuscles of the Blood -magnified. -</span> -<p class="caption"> -A. Moderately magnified. The red corpuscles are seen lying in -rows like rolls of coins; at a and a are seen two white -corpuscles.</p> -<p class="caption"> - -B. Red corpuscles much more highly magnified, seen in face; C. -ditto, seen in profile; D. ditto, in rows, rather more highly -magnified; E. a red corpuscle swollen into a sphere by imbibition -of water. -</p><p class="caption"> -F. A white corpuscle magnified same as B.; G. ditto, throwing -out some blunt processes; K. ditto, treated with acetic acid, and -showing nucleus, magnified same as D. -</p><p class="caption"> -H. Red corpuscles puckered or crenate all over. -</p><p class="caption"> -I. Ditto, at the edge only.</p> -</div> - -<p><b>round bodies, the blood discs or blood corpuscles</b> (<a href="#fig_4">Fig. 4</a>, <i>A</i>). If you -look carefully you will notice that most of them are round, as <i>B</i>; but -every now and then you see something like <i>C</i>. That is one of the round -ones seen sideways; for they are not round or spherical like a ball, but -circular and<span class="pagenum"><a name="page_044" id="page_044"></a>{44}</span> dimpled in the middle, something like certain kinds of -biscuit. When you see one by itself it looks a little yellow in colour, -that is all; but when you see them in a lump, the lump is clearly red. -Remember how small they are: three thousand of them put flat in a line, -edge to edge, like a row of draughts, would just about stretch across -one inch. All the redness there is in blood belongs to them. When you -see one of them, you see so little of the redness that it seems yellow. -If you were to put a drop of blood into a tumbler of water, the water -would not be stained red, but only just turned of a yellowish tint, so -little redness would be given to it by the drop of blood. In the same -way a very very thin slice of currant jelly would look yellowish, not -red.</p> - -<p>These red corpuscles are not hard solid things, but delicate and soft, -very tender, very easily broken to pieces, more like the tiniest lumps -of red jelly than anything else, and yet made so as to bear all the -squeezing which they get as they are driven round and round the body.</p> - -<p>Besides these red corpuscles, you may see if you look attentively <b>other -little bodies, just a little bigger than the red corpuscles, not -coloured at all, and not circular and flat, but quite round like a ball</b> -(<a href="#fig_4">Fig. 4</a>, <i>a</i>, <i>F</i>, <i>G</i>). That is to say, these are very often quite -round, only they have a curious trick of changing their form. Imagine -you were looking at a suet dumpling so small that about two thousand -five hundred of them could be placed side by side in the length of one -inch—and suppose the round dumpling while you were looking at it -gradually changed into the shape of a three-cornered tart, and then -into<span class="pagenum"><a name="page_045" id="page_045"></a>{45}</span> a rounded square, and then into the shape of a pear, and then into -a thing that had no shape at all, and then back again into a round ball, -and kept doing this apparently all of its own accord while you were -looking at it—wouldn’t you think it very curious? Well, one of these -little bodies in the blood of which we are speaking, and which are -called white corpuscles, may be seen, when a drop of blood is watched -under the microscope, to go on in this way, continually changing its -shape. But of these <b>white corpuscles</b> of the blood, and of their -wonderful movements, you will learn more as you go on in your -physiological studies.</p> - -<p><b><a name="sect_23" id="sect_23"></a>23.</b> Besides these red and white corpuscles there is nothing else very -important in the blood that you can <i>see</i> with the microscope; but their -being in the blood is one reason why blood is thicker than water.</p> - -<p>Did you ever see a pig or sheep killed? If so, you would be sure to -notice that the blood ran quite fluid from the blood-vessels in the -neck, ran and was spilt like so much water—but that very soon the blood -caught in the pail or spilt on the stones became quite solid, so that -you could pick it up in lumps. Whenever blood is shed from the living -body, within a short time it becomes solid. This becoming solid is -called the <b>clotting</b> or <b>coagulation of blood</b>.</p> - -<p>What makes it clot? Suppose while the blood was running from the pig’s -neck into the butcher’s pail, and while it was still quite fluid, you -were to take a bunch of twigs and keep slowly stirring the blood round -and round in the pail. You would naturally expect that the blood would -soon begin to clot, would get thicker and thicker and more and more -difficult to stir. But<span class="pagenum"><a name="page_046" id="page_046"></a>{46}</span> it does not; and if you keep on stirring long -enough you will find that it never clots at all. <b>By continually stirring -it you will prevent its clotting.</b> Now take out your bundle of twigs: you -will find it covered all over with a thick reddish mass of some soft -sticky substance; and if you pump on the red mass you will be able to -wash away all its red colour, and will have nothing left but a quantity -of white, soft, sticky, stringy material, all entangled and matted -together among the twigs of your bundle. This stringy material is in -reality made up of a number of fine, delicate, soft, elastic threads or -fibres, and is called <b>fibrin</b>.</p> - -<p><b>You see, by stirring, or, as it is frequently called, whipping the blood -with the bundle of twigs, you have taken the fibrin out of the blood, -and so prevented its clotting.</b></p> - -<p>If you were to take one of the clotted lumps of blood that were spilt on -the ground or a bit of the clot from a pail in which the blood had not -been whipped, and wash it long enough, you would find at last that all -the colour went away from the lump, and you had nothing left but a small -quantity of white stringy substance. This white stringy substance is -fibrin—exactly the same thing you got on your bundle of twigs.</p> - -<p>If the blood is carefully caught in a pail, and afterwards not disturbed -at all, it clots into a solid mass. The whole of the blood seems to have -changed into a complete jelly; and if you turn it out of the pail, as -you may do, it keeps its shape, and gives you quite a mould of the pail, -a great trembling red jelly just the shape of the inside of the pail.</p> - -<p>But if you were to leave the blood in the pail for<span class="pagenum"><a name="page_047" id="page_047"></a>{47}</span> a few hours or for a -day, you would find, instead of the large jelly quite filling the pail, -a smaller but firmer jelly covered by or floating in a colourless or -very pale yellow liquid. This smaller, firmer jelly, which in the course -of a day or so would get still firmer and smaller, would in fact go on -shrinking in size, you may still call the <b>clot</b>; the clear fluid in which -it is floating is called <b>serum</b>.</p> - -<p>What has taken place is as follows. Soon after blood is shed there is -formed in it a something which was not present in it before. This -something, which we call <b>fibrin</b>, starts as a multitude of fine tender -threads which run in all directions through the mass of blood, forming a -close network everywhere. So the blood is shut up in an immense number -of little chambers formed by the meshes of the fibrin; and it is this -which makes it seem a jelly. But each thread of fibrin as soon as it is -formed begins to shrink, and the blood in each of these little chambers -is squeezed by the shrinking of its walls of fibrin, and tries to make -its way out. The corpuscles get caught in the meshes, but all the rest -of the blood passes between the threads and comes out on the top and -sides of the pail. And this goes on until you have left in the clot very -little besides corpuscles entangled in a network of fibrin, and all the -rest of the blood has been squeezed outside the clot, and is then called -serum. <b>Serum, then, is blood out of which the corpuscles have been -strained by the process of clotting.</b></p> - -<p>Now I dare say you are ready to ask the question, If blood clots so -readily when it is shed, why does it not clot inside the body? Why is -our blood ever<span class="pagenum"><a name="page_048" id="page_048"></a>{48}</span> fluid? This is rather a difficult question to answer. -When blood is shed from the warm body it soon gets cool. But it does not -clot and become solid because it gets cool, as ordinary jelly does. If -you keep it from getting cool it clots all the same, in fact quicker, -and if kept cold enough will not clot at all. Nor does it clot when -shed, because it has become still, and is no longer rushing round -through the blood-vessels. Nor is it because it is exposed to the air. -Perhaps we don’t know exactly why it is, and you will have much to learn -hereafter about the coagulation of blood. All I will say at present is -that as long as the blood is in the body there is something at work to -keep it from clotting. It does clot sometimes in the body, and -blood-vessels get plugged with the clots; but that constitutes a very -dangerous disease.</p> - -<p><b><a name="sect_24" id="sect_24">24.</a></b> Well, blood is thicker than water because it contains solid -corpuscles and fibrin. But even the serum, <i>i.e.</i> blood out of which -both fibrin and corpuscles have been taken, is thicker than water.</p> - -<p>You know that if you were to take a basinful of pure water and boil it, -it would boil away to nothing. It would all go off in steam. But if you -were to try to boil a basinful of serum, you would find several curious -things happen.</p> - -<p>In the first place you would not be able to boil it at all. Before you -got it as hot as boiling water, your serum, which before seemed quite as -liquid as water, only feeling a little sticky if you put your finger in -it, would all become quite solid. You know the difference between a raw -and a boiled egg. The white of the raw egg, though very sticky and ropy, -or viscid as<span class="pagenum"><a name="page_049" id="page_049"></a>{49}</span> it is called, is still liquid; you will find it hard work -if you try to cut it with a knife. The white of the hard boiled egg, on -the other hand, is quite solid, and you can cut it into ever so thin -slices. It has been “set” by boiling. Well, the serum of blood is in -this respect very like white of egg. In fact they both contain the same -substance, called <b>albumin</b>, which has this property of “setting” or -becoming solid when heated nearly to boiling-point. Both the serum of -blood and white of egg even when “set” are wet, <i>i.e.</i> contain a great -deal of water. You may dry them in the proper manner into a transparent -horny substance. When quite dry they will readily burn. They are -therefore things which can be oxidized. When burnt they give off -carbonic acid, water, and ammonia; the latter you might easily recognize -by its effect on your nose if you were to burn a piece of dried blood in -a flame. Now, when I say that <b>albumin</b> in burning gives off carbonic -acid, water, and ammonia, you know from your Chemistry that it must -contain carbon to form the carbonic acid, hydrogen to form water, and -nitrogen to form ammonia. It need not contain oxygen, for as you know it -could get all the oxygen it wanted from the air; still it does contain -some oxygen. <b>Albumin, then, is an oxidizable or combustible body made up -of nitrogen, carbon, hydrogen, and oxygen.</b> It is important you should -remember this; but I will not bother you with how much of each—it is a -very complex substance, built up in a wonderful way, far more complex -than any of the things you had to learn about in your Chemistry Primer. -And this albumin, dissolved in a great deal of water, forms the serum of -blood.<span class="pagenum"><a name="page_050" id="page_050"></a>{50}</span></p> - -<p>I did not say anything about what fibrin was made of; but it, like -albumin, is made up of nitrogen, carbon, hydrogen, and oxygen. It is not -quite the same thing as albumin, but first cousin to it. There is -another first cousin to both of them, also containing nitrogen, carbon, -hydrogen, and oxygen, which together with a great deal of water forms -muscle; another forms a great part of the red corpuscles; and scattered -all over the body in various places, there are first cousins to albumin, -all containing nitrogen, carbon, hydrogen, and oxygen, all combustible, -and all when burnt giving off carbonic acid, water, and ammonia. All -these first cousins go under one name; they are all called <b>proteids</b>.</p> - -<p><b><a name="sect_25" id="sect_25">25.</a></b> Well, then, blood is thicker than water by reason of the proteids in -the corpuscles, in the fibrin, and in the serum, but there is something -else besides. I will not trouble you with the crowd of things of which -there are perhaps just a few grains in a gallon of blood, like the -little pinches of things a cook puts into a savoury dish; though, as you -go on in your studies, you will find that these, like many other little -things in the world, are of great importance.</p> - -<p>But I will ask you to remember this. If you take some dried blood and -burn it, though you may burn all the proteids (and some other of the -trifles I spoke of just now) away, you will not be able to burn the -whole blood away. Burn as long as you like, you will always have left a -quantity of what you have learnt from your Chemistry to call <b>ash, and if -you were to examine this ash you would find it contained ever so many -elements; sulphur, phosphorus, chlorine, potassium, sodium, calcium, and -iron, being the most abundant and most important</b>.<span class="pagenum"><a name="page_051" id="page_051"></a>{51}</span></p> - -<p>Blood, then, is a very wonderful fluid: wonderful for being made up of -coloured corpuscles and colourless fluid, wonderful for its fibrin and -power of clotting, wonderful for the many substances, for the proteids, -for the ashes or minerals, for the rest of the things which are locked -up in the corpuscles and in the serum.</p> - -<p>But you will not wonder at it when you come to see that the blood is the -great circulating market of the body, in which all the things that are -wanted by all parts, by the muscles, by the brain, by the skin, by the -lungs, liver, and kidney, are bought and sold. What the muscle wants, -it, as we have seen, buys from the blood; what it has done with it sells -back to the blood; and so with every other organ and part. As long as -life lasts this buying and selling is for ever going on, and this is why -the blood is for ever on the move, sweeping restlessly from place to -place, bringing to each part the things it wants, and carrying away -those with which it has done. When the blood ceases to move, the market -is blocked, the buying and selling cease, and all the organs die, -starved for the lack of the things which they want, choked by the -abundance of things for which they have no longer any need.</p> - -<p>We have now to learn how the blood is thus kept continually on the move.</p> - -<h2><a name="HOW_THE_BLOOD_MOVES_V" id="HOW_THE_BLOOD_MOVES_V"></a>HOW THE BLOOD MOVES. § V.</h2> - -<p><b><a name="sect_26" id="sect_26">26.</a></b> You have already learnt to recognize the blood-vessels of the -rabbit, and to distinguish two kinds of blood-vessels—the arteries, -which in a dead animal generally contain little or no blood, and have<span class="pagenum"><a name="page_052" id="page_052"></a>{52}</span> -rather firm stout walls; and the veins, which are generally full of -blood, and have thinner and flabby walls. The arteries when you cut them -generally gape and remain open; the veins fall together and collapse. -The larger the arteries, the stouter and firmer they are, and the -greater the difference between them and the veins.</p> - -<p>You have also studied the capillaries in the frog’s foot; you have seen -that they are minute channels, with the thinnest and tenderest walls, -forming a close network in which the smallest arteries end, and from -which the smallest veins begin.</p> - -<p>You have moreover been told that all over your own body, in every part, -there are, though you cannot see them, networks of capillaries like -those in the frog’s foot which you can see; that all the arteries of -your body end in capillaries, and all the veins begin in capillaries. -Let me repeat that, one or two structures excepted, there is no part of -your body in which, could you put it under a microscope, you would not -see a small artery branching out and losing itself in a network of -capillaries, out of which, as out of so many roots, a small vein gathers -itself together again.</p> - -<p>In some places the network is very close, the capillaries lying closer -together than even in the frog’s foot; in others the network is more -open, and the capillaries wider apart; but everywhere, with a few -exceptions which you will learn by and by, there are capillaries, -arteries, and veins.</p> - -<p>Suppose you were a little lone red corpuscle, all by yourself in the -quite empty blood-vessels of a dead body, squeezed in the narrow pathway -of a capillary, say<span class="pagenum"><a name="page_053" id="page_053"></a>{53}</span> of the biceps muscle of the arm, able to walk -about, and anxious to explore the country in which you found yourself. -There would be two ways in which you might go. Let us first imagine that -you set out in the way which we will call backwards. Squeezing your way -along the narrow passage of the capillary in which you had hardly room -to move, you would at every few steps pass, on your right hand and on -your left, the openings into other capillary channels as small as the -one in which you were. Passing by these you would presently find the -passage widening, you would have more room to move, and the more -openings you passed, the wider and higher would grow the tunnel in which -you were groping your way. The walls of the tunnel would grow thicker at -every step, and their thickness and stoutness would tell you that you -were already in an artery, but the inside would be delightfully smooth. -As you went on you would keep passing the openings into similar tunnels, -but the further you went on, the fewer they would be. Sometimes the -tunnels into which these openings led would be smaller, sometimes -bigger, sometimes of the same size as the one in which you were. -Sometimes one would be so much bigger, that it would seem absurd to say -that it opened into your tunnel. On the contrary, it would appear to you -that you were passing out of a narrow side passage into a great wide -thoroughfare. I dare say you would notice that every time one passage -opened into another the way suddenly grew wider, and then kept about the -same size until it joined the next. Travelling onwards in this way, you -would after a while find yourself in a great wide tunnel, so big that<span class="pagenum"><a name="page_054" id="page_054"></a>{54}</span> -you, poor little corpuscle, would seem quite lost in it. Had you anyone -to ask, they would tell you it was the main artery of the arm. Toiling -onwards through this, and passing a few but for the most part large -openings, you would suddenly tumble into a space so vast that at first -you would hardly be able to realize that it was the tunnel of an artery -like those in which you had been journeying. This you would learn to be -the <b>aorta</b>, the great artery of all; and a little further on you would be -in the heart.</p> - -<p>Suppose now you retraced your steps, suppose you returned from the aorta -to the main artery of the arm, and thus back through narrower and -narrower tunnels till you came again to the spot from which you started, -and then tried the other end of the capillary. You would find that that -led you also, in a very similar way, into wider and wider passages. Only -you could not help noticing that though the inside of all the passages -was as smooth as before, the walls were not nearly so thick and stout. -You would learn from this that you were in the veins, and not in the -arteries. You would meet too with something, the like of which you did -not see in the arteries (except perhaps just close to the heart). Every -now and then you would come upon what for all the world looked like one -of those watch-pockets that sometimes are hung at the head of a -bedstead, a watch-pocket with its opening turned the way you were going. -This you would find was called a <b>valve</b>, and was made of thin but strong -membrane or skin. Sometimes in the smaller veins you would meet with one -watch-pocket by itself, sometimes with two or even three abreast, and I -dare say you would notice<span class="pagenum"><a name="page_055" id="page_055"></a>{55}</span> that very frequently, directly you had passed -one of these valves, you came to a spot where one vein joined another.</p> - -<p>Well, but for these differences, your journey along the veins would be -very like your journey along the arteries, and at last you would find -yourself in a great vein, whose name you would learn to be the <b>vena -cava</b>, or hollow vein (and because, though there is but one aorta, there -are two great “hollow veins,” <b>the superior vena cava</b> or <b>upper hollow -vein</b>), and from thence your next step would be into the heart again. So -you see, starting from the capillary (you started from a capillary in -the arm, but you might have started from any capillary anywhere), -whether you go along the arteries or whether you go along the veins, you -at last come to the heart.</p> - -<p>Before we go on any further we must learn something about the heart.</p> - -<p><b><a name="sect_27" id="sect_27">27.</a></b> Go and ask the butcher for a sheep’s pluck. There will most probably -be one hanging up in his shop. Look at it before he takes it down. The -hook on which it is hanging has been thrust through the windpipe. You -will see that the sheep’s windpipe is, like the rabbit’s, all banded -with rings of cartilage, only very much larger and coarser. Below the -windpipe come the spongy lungs, and between them lies the heart, which -perhaps is covered up with a skin and so not easily seen. Hanging to the -heart and lungs is the great mass of the liver. When you have got the -pluck home, cut away the liver, cut away the skin (pericardium, it is -called) which is covering the heart, if it has not been cut away -already, and lay the lungs out on a table with the heart between them.<span class="pagenum"><a name="page_056" id="page_056"></a>{56}</span> -You will then have something very much like what</p> - -<p><a name="fig_5" id="fig_5"></a><span class="pagenum"><a name="page_057" id="page_057"></a>{57}</span></p> - -<div class="figcenter"> -<a href="images/i_p056_lg.jpg"> -<br /> -<img src="images/i_p056_sml.jpg" width="430" height="641" alt="Image unavailable: Fig. 5.—Heart of Sheep, as seen after Removal from the Body, -lying upon the Two Lungs. The Pericardium has been cut away, but no -other Dissection made. - -R.A. Auricular appendage of right auricle; L.A. auricular appendage -of left auricle; R.V. right ventricle; L.V. left ventricle; S.V.C. -superior vena cava; I.V.C. inferior vena cava; P.A. pulmonary -artery; Ao, aorta; Áó, innominate branch from aorta dividing into -subclavian and carotid arteries; L. lung; Tr. trachea. 1, solid cord -often present, the remnant of a once open communication between the -pulmonary artery and aorta. 2, masses of fat at the bases of the -ventricle hiding from view the greater part of the auricles. 3, line of -fat marking the division between the two ventricles. 4, mass of fat -covering the trachea." /></a> -<br /> -<span class="caption">Fig. 5.—Heart of Sheep, as seen after Removal from the Body, -lying upon the Two Lungs. The Pericardium has been cut away, but no -other Dissection made.</span> -<p class="caption"> -R.A. Auricular appendage of right auricle; L.A. auricular appendage -of left auricle; R.V. right ventricle; L.V. left ventricle; S.V.C. -superior vena cava; I.V.C. inferior vena cava; P.A. pulmonary -artery; Ao, aorta; Áó, innominate branch from aorta dividing into -subclavian and carotid arteries; L. lung; Tr. trachea. 1, solid cord -often present, the remnant of a once open communication between the -pulmonary artery and aorta. 2, masses of fat at the bases of the -ventricle hiding from view the greater part of the auricles. 3, line of -fat marking the division between the two ventricles. 4, mass of fat -covering the trachea.</p> - -</div> - -<p class="nind">is represented in <a href="#fig_5">Fig. 5</a>. If you could look through the front of your -own chest, and see your own heart and lungs in place, you would see -something not so very very different.</p> - -<p>If now you handle the heart—and if you want to learn physiology you -must handle things—you will have no great difficulty in finding the -great yellowish tubes marked <i>Ao</i> and <i>Áó</i> in the figure. Your butcher -perhaps may not have cut them across exactly where mine has done, but -that will not prevent your recognizing them. You will notice what thick -stout walls they have, and how they gape where they are cut. <i>Ao</i> is the -<b>aorta</b>, and <i>Áó</i> is a great branch of the aorta, going to the head and -neck of one side, perhaps the branch along which we imagined just now -that you, a poor little red blood-corpuscle, were travelling. If you -were to put a wire through <i>Áó</i> you would be able to bring it out -through <i>Ao</i>, or <i>vice versâ</i>. But what is <i>P.A.</i> which looks so much -like the aorta, though you will find that it has no connection with it? -You cannot pass a wire from the aorta into it. It also is an artery, the -<b>pulmonary<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> artery</b>. We shall have more to say about it directly.</p> - -<p>Now try and find what are marked in the figure as <i>S.V.C.</i> and <i>I.V.C.</i> -You will perhaps have a little difficulty in this; and when you have -found them you will understand why. They are the great veins<span class="pagenum"><a name="page_058" id="page_058"></a>{58}</span> of the -body. <i>S.V.C.</i> is the <b>superior vena cava</b>, to form which all the veins -from the head and neck and arms join, the vein in which you were -journeying a little while ago. <i>I.V.C.</i> is the <b>inferior vena cava</b>, made -out of all the veins from the trunk and the legs. Being veins, they have -thin flabby walls; and their sides fall flat together, so that they seem -nothing more than little folds of skin, and it becomes very hard to find -the passage inside them. But when you have found the opening into them, -you will see that you can stretch them out into quite wide tubes, and -that their walls, though very much thinner than those of the aorta, so -thin indeed that they are almost transparent, are still after a fashion -strong. If you put a penholder or thin rod through either you will find -that they both seem to lead right into the middle of the heart. With a -little care you can pass a rod up <i>I.V.C.</i> and bring the end of it out -at the top of <i>S.V.C.</i> Of course you will understand that both of these -veins have been cut off short.</p> - -<p><b><a name="sect_28" id="sect_28">28.</a></b> Before we go on any further with the sheep’s heart, let me tell you -something about it, by help of the diagram in <a href="#fig_6">Fig. 6</a>, which is meant to -represent the whole circulation. You must remember that this figure is a -<b>diagram</b>, and not a picture; it does not represent the way the -blood-vessels are really arranged in your own body. If you had no arms -and no legs, and if you only had a few capillaries at the top of your -head and at the bottom of your body, it might be more like than it is.</p> - -<p>In the centre of the figure is the heart. This you will see is -completely divided by an upright partition into two halves, a right half -and a left half. Each half<span class="pagenum"><a name="page_059" id="page_059"></a>{59}</span> is further marked off, but not completely -divided, into</p> - -<p><a name="fig_6" id="fig_6"></a><span class="pagenum"><a name="page_060" id="page_060"></a>{60}</span></p> - -<div class="figcenter"> -<a href="images/i_p059_lg.jpg"> -<br /> -<br /> -<img src="images/i_p059_sml.jpg" width="294" height="643" alt="Image unavailable: Fig. 6.—Diagram of the Heart and Vessels, with the Course of the -Circulation, viewed from behind so that the proper left of the -Observer corresponds with the left side of the Heart in the -Diagram. - -L.A. left auricle; L.V. left ventricle; Ao. aorta; A1. -arteries to the upper part of the body; A2. arteries to the lower -part of the body; H.A. hepatic artery, which supplies the liver with -part of its blood; V2. veins of the upper part of the body; -V2. veins of the lower part of the body; V.P. vena portæ; H.V. -hepatic vein; V.C.I. inferior vena cava; V.C.S. superior vena cava; -R.A. right auricle; R.V. right ventricle; P.A. pulmonary artery; -Lg. lung; P.V. pulmonary vein; Lct. lacteals; Ly. lymphatics; -Th.D. thoracic duct; Al. alimentary canal; Lr. liver. The arrows -indicate the course of the blood, lymph, and chyle. The vessels which -contain arterial blood have dark contours, while those which carry -venous blood have light contours." /></a> -<br /> -<span class="caption">Fig. 6.—Diagram of the Heart and Vessels, with the Course of the -Circulation, viewed from behind so that the proper left of the -Observer corresponds with the left side of the Heart in the -Diagram.</span> -<p class="caption"> -L.A. left auricle; L.V. left ventricle; Ao. aorta; A1. -arteries to the upper part of the body; A2. arteries to the lower -part of the body; H.A. hepatic artery, which supplies the liver with -part of its blood; V2. veins of the upper part of the body; -V2. veins of the lower part of the body; V.P. vena portæ; H.V. -hepatic vein; V.C.I. inferior vena cava; V.C.S. superior vena cava; -R.A. right auricle; R.V. right ventricle; P.A. pulmonary artery; -Lg. lung; P.V. pulmonary vein; Lct. lacteals; Ly. lymphatics; -Th.D. thoracic duct; Al. alimentary canal; Lr. liver. The arrows -indicate the course of the blood, lymph, and chyle. The vessels which -contain arterial blood have dark contours, while those which carry -venous blood have light contours.</p> -</div> - -<p class="nind">two chambers, an upper chamber and a lower chamber; so that altogether -we have four chambers,—two upper chambers, one on each side, marked -<i>R.A.</i> and <i>L.A.</i>, these are called the <b>right and left auricles</b>; and two -lower chambers, one on each side, marked <i>R.V.</i> and <i>L.V.</i>, these are -called the <b>right and left ventricles</b>. The right auricle, <i>R.A.</i>, opens -in the direction of the arrow into the right ventricle, <i>R.V.</i>, the -opening being guarded, as we shall see, by a valve. The left auricle, -<i>L.A.</i>, opens into the left ventricle, <i>L.V.</i>, the opening being -likewise guarded by a valve; but you have to go quite a roundabout way -to get from either the right auricle or ventricle to the left auricle or -ventricle. Let us see how we can get round the figure. Suppose we begin -with the two tubes marked <i>V.C.S.</i> and <i>V.C.I.</i>, the walls of which are -drawn with thin lines. These both open into the right auricle. They are -the vena cava superior and inferior, which you have just made out in the -sheep’s heart. From the right auricle you pass easily into the right -ventricle; thence, following the arrow, the way is straight into the -tube marked <i>P.A.</i> This is the pulmonary artery, the outside of which -you saw in the sheep’s heart (<a href="#fig_5">Fig. 5</a>, <i>P.A.</i>) Travelling along this -pulmonary artery, you come to the lungs, and after passing through -branches not represented in the figure, picking your way through -arteries which continually get smaller and smaller,<span class="pagenum"><a name="page_061" id="page_061"></a>{61}</span> you find yourself -at last in the capillaries of the lungs. Squeezing your way through -these, you come out into veins, and gradually advancing through larger -and larger veins, you, still following the arrow, find yourself in one -of four large veins (only one of them is represented in the diagram) -which land you in the left auricle. From the left auricle it is but a -jump into the left ventricle. From the left ventricle the way is open, -as indicated by the arrow, into the tube marked <i>Ao</i>. This is intended -to represent the aorta, which you have already seen in the sheep’s heart -(<a href="#fig_5">Fig. 5</a>, <i>Ao</i>). It is here drawn for simplicity’s sake as dividing into -two branches, but you have already been told, and must bear in mind, -that it does not in reality divide in this way, but gives off a good -many branches of various sizes. However, taking the figure as it stands, -suppose we travel along <i>A</i><sup>2</sup>. Following the arrow, and shooting -through arteries which continually get smaller and smaller, we come at -last to capillaries somewhere, in the skin or in some muscle, or in a -bone, or in the brain, or almost anywhere, in fact, in the upper part of -the body. Out of the capillaries we pass into veins, which, joining -together and so forming larger and larger trunks, bring us at last to -the point from which we started, the superior vena cava, <i>V.C.S.</i> If we -had taken the other road, <i>A</i><sup>2</sup>, we should have passed through -capillaries somewhere in the lower part of the body instead of the -upper, and come back by the vena cava inferior, <i>V.C.I.</i>, instead of the -vena cava superior. <b>Starting from the right auricle, whichever way we -took we should always come back to the right auricle again, and in our<span class="pagenum"><a name="page_062" id="page_062"></a>{62}</span> -journey should always pass through the following things in the following -order: right auricle, right ventricle, pulmonary artery, arteries, -capillaries, and veins of the lungs, pulmonary vein, left auricle, left -ventricle, aorta, arteries, capillaries, and veins somewhere in the -body, and either superior or inferior vena cava.</b> That is the course of -the circulation. But there is something still to be added. Among the -many large branches, not drawn in the diagram, given off by the aorta to -the lower part of the body, there are two branches which are drawn and -which deserve special notice.</p> - -<p>One is a large branch carrying blood to the tube <i>A.L.</i>, which is meant -in the diagram to stand for the stomach, intestines, and some other -organs. This branch, like all other branches of the aorta, divides into -small arteries, and these into capillaries, which again are gathered up -into veins, forming at last a large vein marked in the diagram <i>V.P.</i> -and called the <b>vena portæ</b> or <b>portal vein</b>. Now the remarkable thing is -that this vein does not, like all the other veins, go straight to join -the vena cava, but makes for the liver, where it divides into smaller -and smaller veins, until at last it breaks completely up in the liver -into a set of capillaries again. These capillaries gather once more into -veins, forming at last the large trunk, called the <b>hepatic<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> vein</b>, -<i>H.V.</i>, which does what the portal vein ought to have done but did not; -it opens straight into the vena cava.</p> - -<p>The other branch of the aorta of which we are speaking goes straight to -the liver, and is called the<span class="pagenum"><a name="page_063" id="page_063"></a>{63}</span> <b>hepatic artery</b>, <i>H.A.</i>: there it breaks up -in the liver into small arteries, and then into capillaries, which -mingle with the capillaries of the portal vein, and form one system, out -of which the hepatic veins spring. So you see it makes a great -difference to a red corpuscle which is travelling along the lower part -of the aorta <i>A</i><sup>2</sup>, whether it takes a turn into the branch going to -the alimentary canal, or whether it goes straight on into, for instance, -a branch going to some part of the leg. In the latter case, having got -through a set of capillaries, it is soon back into the vena cava and on -its road to the heart. But if it takes the turn to the alimentary canal, -it finds after it has passed through the capillaries and got into the -portal vein, that it has still to go through another set of capillaries -in the liver before it can pass through the hepatic vein into the vena -cava.</p> - -<p>This then is the course of the circulation. Right side of the heart, -pulmonary artery, capillaries of the lungs, pulmonary vein, left side of -the heart, aorta, capillaries somewhere, sometimes two sets, sometimes -one, vena cava, right side of the heart again. A little corpuscle cannot -get from the right to the left side of the heart without going through -the capillaries of the lungs. It cannot get from the left side of the -heart to the right without going through some capillaries somewhere in -the body, and if it should happen to take the turn to the stomach, it -has to go through two sets of capillaries instead of one.</p> - -<p>You see, you really have two circulations, and you have two hearts -joined together into one. If you were very skilful you might split the -heart in half and pull the two sides asunder, and then you would have<span class="pagenum"><a name="page_064" id="page_064"></a>{64}</span> -one heart receiving all the veins from the body and sending its arteries -(branches of the pulmonary artery) all to the lungs, and another heart -receiving all the veins from the lungs and sending its arteries -(branches of the aorta) all over the body. And you would have two -circulations, one through the lungs, and another through the rest of the -body, both joining each other. Very often two circulations are spoken -of, and because the lungs are so much smaller than the rest of the body, -the circulation through the lungs is called the lesser circulation, that -through the rest of the body the greater circulation.</p> - -<p><b><a name="sect_29" id="sect_29">29.</a></b> I have described the circulation as if the blood always went in one -direction from the right side of the heart to the left, from arteries to -veins, the way the arrows point in the diagram. And so it does. It -cannot go the other way round. <b>Why does it go that way? Why cannot it go -the other way round?</b></p> - -<p>The reasons are to be found partly in the heart, partly in the veins.</p> - -<p><b>In the veins the blood will only pass from the capillaries to the heart.</b> -Why not from the heart to the capillaries? You remember the little -watch-pocket-like valves, here and there, sometimes singly, sometimes -two or three abreast. <b>You remember that the mouths of the watch-pockets -were always turned towards the heart.</b> Now suppose a crowd of little -corpuscles hurrying along a vein towards the heart. When they came to -one of these watch-pocket valves they would simply trample it down flat, -and so pass over it without hardly knowing it was there,<span class="pagenum"><a name="page_065" id="page_065"></a>{65}</span> and go on -their way as if nothing had happened. But suppose they were journeying -the other way, from the heart to the capillaries. When they came to the -open mouth of a watch-pocket valve, some of them would be sure to run -into the pocket, and then the pocket would bulge out, and the more it -bulged out the more blood would run into it, until at last it would be -so full of blood that it would press close against the top of the vein, -as is shown in <a href="#fig_7">Fig. 7</a> (or, if there were two or three, they would all -meet together), and so quite block the vein up. If you doubt this, make -a watch-pocket out of a piece of silk or cotton, fasten it on to a piece -of brown paper, and roll the paper up into a tube, so that the valve is -nicely inside the tube. If you pour some peas down the tube with the -mouth of the valve looking away from you, they will run through at once; -but if you try to pour them the other way, your tube will soon be -choked, and if you carefully unroll the tube you will find the -watch-pocket crammed full of peas.</p> - -<p><a name="fig_7" id="fig_7"></a></p> - -<div class="figcenter"> -<a href="images/i_p065_lg.jpg"> -<br /> -<br /> -<img src="images/i_p065_sml.jpg" width="203" height="138" alt="Image unavailable: Fig. 7.—Diagrammatic Sections of Veins with Valves. - -In the upper, the blood is supposed to be flowing in the direction -of the arrow, towards the heart; in the lower, the reverse way. C, -capillary side; H, heart side. -" /></a> -<br /> -<span class="caption">Fig. 7.—Diagrammatic Sections of Veins with Valves. -</span> -<p class="caption"> -In the upper, the blood is supposed to be flowing in the direction -of the arrow, towards the heart; in the lower, the reverse way. C, -capillary side; H, heart side.</p> - -</div> - -<p><span class="pagenum"><a name="page_066" id="page_066"></a>{66}</span></p> - -<p><b>The valves in the veins, then, let the blood pass easily from the -capillaries to the heart, but won’t let it go the other way.</b> If you bare -your arm you may see some of the veins in the skin, in which the blood -is running up from the hand towards the shoulder. If with your finger -you press one of these veins back towards the hand it will swell up, and -if you look carefully you may see little knots here and there caused by -the bulging out of the watch-pocket valves. If you press it the other -way, towards the elbow, you will empty it easily, and if with another -finger you prevent the blood getting into it from behind, that is from -the hand, the vein will remain empty a very long time.</p> - -<p>The presence of valves in the veins, then, is one reason why the blood -moves in one direction, but other reasons, and these the chief ones, are -to be found in the heart.</p> - -<p>Let us now go back to the sheep’s heart.</p> - -<p><b><a name="sect_30" id="sect_30">30.</a></b> You know from the diagram that the two great veins, the superior and -inferior vena cava, open into the right auricle. If you slit up these -two veins in the sheep’s heart, you will find that they end by separate -openings in a small cavity, the inside of which is for the most part -smooth, and the walls of which, made, as you will at once see, of -muscle, are not very thick. This small cavity is the right auricle, -shown in <a href="#fig_8">Fig. 8</a>, <i>R.A.</i>, where the great veins have not been slit up, -but the front of the auricle has been cut away. In this auricle, beside -the openings into the two great veins and another one which belongs to a -vein coming from the heart itself (<a href="#fig_8">Fig. 8</a>, <i>b</i>) there is quite a large -one, leading straight downwards, into which you<span class="pagenum"><a name="page_067" id="page_067"></a>{67}</span></p> - -<p><a name="fig_8" id="fig_8"></a></p> - -<div class="figcenter"> -<a href="images/i_p067_lg.jpg"> -<br /> -<br /> -<img src="images/i_p067_sml.jpg" width="404" height="496" alt="Image unavailable: Fig. 8.—Right Side of the Heart of a Sheep. - -R.A. cavity of right auricle; S.V.C. superior vena cava; -I.V.C. inferior vena cava; (a piece of whalebone has been passed -through each of these;) a, a piece of whalebone passed from the -auricle to the ventricle through the auriculo-ventricular orifice; -b, a piece of whalebone passed into the coronary vein. - -R.V. cavity of right ventricle; tv, tv, two flaps of the -tricuspid valve: the third is dimly seen behind them, the a, -piece of whalebone, passing between the three. Between the two -flaps, and attached to them by chordæ tendineæ, is seen a -papillary muscle, PP, cut away from its attachment to that -portion of the wall of the ventricle which has been removed. Above, -the ventricle terminates somewhat like a funnel in the pulmonary -artery, P.A. One of the pockets of the semilunar valve, sv, is -seen in its entirety, another partially. - -1, the wall of the ventricle cut across; 2, the position of the -auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of -fat lodged between the auricle and pulmonary artery." /></a> -<br /> -<span class="caption">Fig. 8.—Right Side of the Heart of a Sheep. -</span> -<p class="caption"> -R.A. cavity of right auricle; S.V.C. superior vena cava; -I.V.C. inferior vena cava; (a piece of whalebone has been passed -through each of these;) a, a piece of whalebone passed from the -auricle to the ventricle through the auriculo-ventricular orifice; -b, a piece of whalebone passed into the coronary vein.</p> -<p class="caption"> -R.V. cavity of right ventricle; tv, tv, two flaps of the -tricuspid valve: the third is dimly seen behind them, the a, -piece of whalebone, passing between the three. Between the two -flaps, and attached to them by chordæ tendineæ, is seen a -papillary muscle, PP, cut away from its attachment to that -portion of the wall of the ventricle which has been removed. Above, -the ventricle terminates somewhat like a funnel in the pulmonary -artery, P.A. One of the pockets of the semilunar valve, sv, is -seen in its entirety, another partially. -</p> -<p class="caption"> -1, the wall of the ventricle cut across; 2, the position of the -auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of -fat lodged between the auricle and pulmonary artery. -</p> -</div> - -<p class="nind">can put your three fingers. This is the opening into<span class="pagenum"><a name="page_068" id="page_068"></a>{68}</span> the right -ventricle; and you will have no difficulty in putting your fingers from -the auricle into the ventricle and bringing them out again.</p> - -<p><a name="fig_9" id="fig_9"></a></p> - -<div class="figcenter"> -<a href="images/i_p068_lg.jpg"> -<br /> -<img src="images/i_p068_sml.jpg" width="411" height="381" alt="Image unavailable: Fig. 9.—The Orifices of the Heart seen from above, the -Auricles and Great Vessels being cut away. - -P.A. pulmonary artery, with its semilunar valves; Ao. aorta, -do. - -R.A.V. right auriculo-ventricular orifice with the three flaps -(lv. 1, 2, 3) of tricuspid valve. - -L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, -flaps of mitral valve; b, piece of whalebone passed into coronary -vein. On the left part of L.A.V. the section of the auricle is -carried through the auricular appendage; hence the toothed -appearance due to the portions in relief cut across." /></a> -<br /> -<span class="caption">Fig. 9.—The Orifices of the Heart seen from above, the -Auricles and Great Vessels being cut away. -</span> -<p class="caption"> -P.A. pulmonary artery, with its semilunar valves; Ao. aorta, -do.</p><p class="caption"> - -R.A.V. right auriculo-ventricular orifice with the three flaps -(lv. 1, 2, 3) of tricuspid valve. -</p><p class="caption"> -L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, -flaps of mitral valve; b, piece of whalebone passed into coronary -vein. On the left part of L.A.V. the section of the auricle is -carried through the auricular appendage; hence the toothed -appearance due to the portions in relief cut across. -</p> -</div> - -<p>But hold the heart in one hand with the auricle upwards, and try to pour -some water into the ventricle. The first few spoonfuls will go in all -right, and then you will see some thin white skin or membrane come -floating up into the opening and quite block up the entrance from the -auricle into the ventricle; the<span class="pagenum"><a name="page_069" id="page_069"></a>{69}</span></p> - -<p><a name="fig_10" id="fig_10"></a></p> - -<div class="figcenter"> -<a href="images/i_p069_lg.jpg"> -<br /> -<img src="images/i_p069_sml.jpg" width="530" height="420" -alt="Image unavailable: Fig. 10.—View of the Orifices of the Heart from below, -the whole of the Ventricles having been cut away. - -R.A.V. right auriculo-ventricular orifice surrounded by the three -flaps, t.v. 1, t.v. 2, t.v. 3, of the tricuspid valve; these -are stretched by weights attached to the chordæ tendineæ. - -L.A.V. left auriculo-ventricular orifice surrounded in same way -by the two flaps, m.v. 1, m.v. 2, of mitral valve; P.A. the -orifice of pulmonary artery, the semilunar valves having met and -closed together; Ao. the orifice of the aorta with its semilunar -valves. The shaded portion, leading from R.A.V. to P.A., -represents the funnel seen in Fig. 8." /></a> -<br /> -<span class="caption">Fig. 10.—View of the Orifices of the Heart from below, -the whole of the Ventricles having been cut away. -</span> -<p class="caption"> -R.A.V. right auriculo-ventricular orifice surrounded by the three -flaps, t.v. 1, t.v. 2, t.v. 3, of the tricuspid valve; these -are stretched by weights attached to the chordæ tendineæ.</p> -<p class="caption"> -L.A.V. left auriculo-ventricular orifice surrounded in same way -by the two flaps, m.v. 1, m.v. 2, of mitral valve; P.A. the -orifice of pulmonary artery, the semilunar valves having met and -closed together; Ao. the orifice of the aorta with its semilunar -valves. The shaded portion, leading from R.A.V. to P.A., -represents the funnel seen in <a href="#fig_8">Fig. 8</a>.</p> -</div> - -<p class="nind">water will immediately fill the auricle and run over. If you look at the -membrane carefully as it comes bulging up, you will notice that it is -made up of three pieces joined together as is shown in <a href="#fig_9">Fig. 9</a> (<i>lv.</i> 1, -<i>lv.</i> 2, <i>lv.</i> 3). These three pieces form the valve between the right -auricle and ventricle, called the <b>tricuspid</b>, or three-peaked valve. Why -it is so<span class="pagenum"><a name="page_070" id="page_070"></a>{70}</span> called you will understand if you lay open the right ventricle -by cutting with a pair of scissors from the auricle into the ventricle -along the side of the heart, or by cutting away the front of the -ventricle as has been done in <a href="#fig_8">Fig. 8</a>. You will then see that the valve -is made up of three little triangular flaps, which grow together round -the opening with their points hanging down into the cavity of the -ventricle (<a href="#fig_10">Fig. 10</a>, <i>t.v.</i>) They do not, however, hang quite loosely. -You will notice fastened to the sides of the flaps, thin delicate -threads, the other ends of which are fastened to the sides of the -ventricle, and often to little fleshy projections called papillary -muscles (<a href="#fig_8">Fig. 8</a>, <i>P.P.</i>)</p> - -<p>How do these valves act? In this way. When the ventricle is empty, and -blood or water or any other fluid is poured into it from the auricle, -the valves are pushed on one side against the walls of the ventricle, -and thus there is a great wide opening from the auricle into the -ventricle. But as the ventricle fills, the blood or water gets behind -the flaps and floats them up towards the auricle. The more fluid in the -ventricle the higher they float, until when the ventricle is quite full -they all meet together in the middle of the opening between the auricle -and ventricle and completely block it up. But why do they not turn right -over into the auricle, and so open up again the wrong way? Because of -those little threads (the <i>chordæ tendineæ</i>, as they are called) which -fasten them to the walls of the ventricle. The flaps float back until -these threads are stretched quite tight, and the threads are just long -enough to let the flaps reach to the middle of the opening, but no -further. The tighter the threads are<span class="pagenum"><a name="page_071" id="page_071"></a>{71}</span> stretched the closer the flaps fit -together, and the more completely do they block the way from the -ventricle back into the auricle.</p> - -<p><b>The tricuspid valve, then, lets blood flow easily from the right auricle -into the right ventricle, but prevents it flowing from the ventricle -into the auricle.</b></p> - -<p><b><a name="sect_31" id="sect_31">31.</a></b> Now look at the cavity of the ventricle. Its walls are fleshy, that -is muscular, and you will notice that they are much stouter and thicker -than those of the auricle. Besides the opening from the auricle there is -but one other, which is at the top of the ventricle, side by side with -the former. If you put a penholder or your finger through this second -opening, you will find that it leads into the large vessel which you -have already learnt to recognize as the pulmonary artery (<a href="#fig_5">Fig. 5</a>, -<i>P.A.</i>)</p> - -<p>Slit up the pulmonary artery from the ventricle with a pair of scissors, -as has been done in <a href="#fig_8">Fig. 8</a>, <i>P.A.</i> You will notice at once the line -where the red soft flesh of the muscular ventricle leaves off, and the -yellow firmer material of which the artery is made begins. Just at that -line you will see a row of three (perhaps you may have cut one of the -three with your scissors) most beautiful, watch-pocket valves, made on -just the same principle as those in the veins, only larger, and more -exquisitely finished. These are called <b>semilunar valves</b>, because each -pocket is of the shape of a half-moon. Lift them up carefully and see -how tender and yet how strong they are. There is no need to tell you the -use of these. You know it at once. <b>They are to let the blood flow from -the ventricle into the pulmonary artery, and<span class="pagenum"><a name="page_072" id="page_072"></a>{72}</span> to prevent the blood going -back from the artery into the ventricle.</b></p> - -<p>On the right side of the heart we have, then, two great valves, the -tricuspid valve between the auricle and the ventricle, and the semilunar -valve between the ventricle and the pulmonary artery. These let the -blood flow easily one way, but not the other. If you doubt this, try it. -Put a tube into either the superior or inferior vena cava of a fresh -heart, tying the other vena cava and another tube into the pulmonary -artery. If with a funnel you pour water into the tube in the vein, it -will run through auricle and ventricle and out through the tube of the -pulmonary artery as easily as possible; but if you try to pour water the -other way down the pulmonary artery, you will find you cannot do it; the -tube gets blocked directly, and only a few drops come back through the -heart into the vein.</p> - -<p>Now slit up the pulmonary artery as far as you can, and note when you -cut it how stout and firm are its walls. You will find that it soon -divides into two branches, one for the right lung, one for the left. -Each of these, when it gets to the lung, divides into branches, and -these again into others, as far as you can follow them. You know from -what you have learnt already that these branches end in capillaries all -over the lungs.</p> - -<p><b><a name="sect_32" id="sect_32">32.</a></b> Not far from the two main branches of the pulmonary artery you will -find, covered up perhaps with fat and other matters, some tubes which -you will at once recognize as veins, and if you open any one of these -you will find that you can put a thin rod into it, and that it leads in -one direction to the lungs, and in the other into the left side of the -heart. These are the<span class="pagenum"><a name="page_073" id="page_073"></a>{73}</span> <i>pulmonary veins</i>, and if you slit them right up -you will find they open (by four openings) into a cavity on the left -side of the heart, almost exactly like that cavity on the right side -which we called the right auricle (<a href="#fig_11">Fig. 11</a>). This cavity is, in fact, -the <b>left auricle</b>; out of it there is an opening into the left ventricle, -very like the opening from the right auricle into the right ventricle. -It too is guarded by flap valves, exactly like the tricuspid valve, only -there are but two flaps instead of three (<a href="#fig_9">Fig. 9</a>, <i>m.v.</i> 1, <i>m.v.</i> 2). -Hence this valve is called the <b>bicuspid</b>, or more frequently the <b>mitral</b> -valve. Its flaps have little threads by which they are fastened to the -walls of the ventricle, and in fact, except for there being two flaps -instead of three, the mitral valve is exactly like the tricuspid valve, -and acts exactly the same way.</p> - -<p>If you cut with a pair of scissors from the auricle into the ventricle, -you will find the left ventricle (<a href="#fig_11">Fig. 11</a>) very much like the right -ventricle, only its walls are very much thicker, so much thicker that -the left ventricle takes up the greater part of the heart. You will see -this if you now look at the outside of a fresh heart.</p> - -<p>The auricles are so small and so covered up by fat that from the outside -you can hardly see them at all. What you chiefly see are two little -fleshy corners, one of each auricle (<a href="#fig_5">Fig. 5</a>, <i>R.A.</i> <i>L.A.</i>), often -called “the auricular appendages.” By far the greater part is taken up -by the ventricles—and if you look you will see a band of fat slanting -across the heart (<a href="#fig_5">Fig. 5</a>, 3). This marks the line of the fleshy -division, or <b>septum</b> as it is called, between the two ventricles. You -will notice that the point or apex of the heart belongs altogether to -the left ventricle.<span class="pagenum"><a name="page_074" id="page_074"></a>{74}</span></p> - -<p><a name="fig_11" id="fig_11"></a></p> - -<div class="figcenter"> -<a href="images/i_p074_lg.jpg"> -<br /> - -<img src="images/i_p074_sml.jpg" width="399" height="555" alt="Image unavailable: Fig. 11.—Left Side of the Heart of a Sheep (laid -open). - -P.V. pulmonary veins opening into the left auricle by four -openings, as shown by the styles or pieces of whalebone placed in -them: a, a style passed from auricle into ventricle through the -auriculo-ventricular orifice; b, a style passed into the coronary -vein, which, though it has no connection with the left auricle, is, -from its position, necessarily cut across in thus laying open the -auricle. - -M.V. the two flaps of the mitral valve (drawn somewhat -diagrammatically): pp, papillary muscles, belonging as before to -the part of the ventricle cut away; c, a style passed from -ventricle in Ao. aorta; Ao2. branch of aorta (see Fig. 5, -Áó); P.A. pulmonary artery; S.V.C. superior vena cava. - -1, wall of ventricle cut across; 2, wall of auricle cut away around -auriculo-ventricular orifice; 3, other portions of auricular wall -cut across; 4, mass of fat around base of ventricle (see Fig. 5, -2)." /></a> -<br /> -<span class="caption">Fig. 11.—Left Side of the Heart of a Sheep (laid -open). -</span> -<p class="caption"> -P.V. pulmonary veins opening into the left auricle by four -openings, as shown by the styles or pieces of whalebone placed in -them: a, a style passed from auricle into ventricle through the -auriculo-ventricular orifice; b, a style passed into the coronary -vein, which, though it has no connection with the left auricle, is, -from its position, necessarily cut across in thus laying open the -auricle.</p> -<p class="caption"> -M.V. the two flaps of the mitral valve (drawn somewhat -diagrammatically): pp, papillary muscles, belonging as before to -the part of the ventricle cut away; c, a style passed from -ventricle in Ao. aorta; Ao2. branch of aorta (see <a href="#fig_5">Fig. 5</a>, -Áó); P.A. pulmonary artery; S.V.C. superior vena cava. -</p> -<p class="caption"> -1, wall of ventricle cut across; 2, wall of auricle cut away around -auriculo-ventricular orifice; 3, other portions of auricular wall -cut across; 4, mass of fat around base of ventricle (see <a href="#fig_5">Fig. 5</a>, -2). -</p> -</div> - -<p><span class="pagenum"><a name="page_075" id="page_075"></a>{75}</span></p> - -<p>To return to the inside of the left ventricle. Up at the top of the -ventricle, close to the opening from the auricle, there is one other -opening, and only one. If you put your finger into this, you will find -that it leads into a tube which first of all dips under or behind the -pulmonary artery and then comes up and to the front again. This tube is -what you already know as the <b>aorta</b>. If you slit it up from the ventricle -(and to do this you must cut through the pulmonary artery), you will -find that on the left side, as on the right, the red fleshy wall of the -ventricle suddenly changes into the yellow firm wall of the artery, and -that just at this line there are three semilunar valves exactly like -those in the pulmonary artery.</p> - -<p><b>On the left side of the heart, then, we have also two valves, the mitral -between the auricle and the ventricle, and the semilunar between the -ventricle and the aorta. These let the blood pass one way and not the -other. You can easily drive fluid from the pulmonary veins through -auricle and ventricle into the aorta, but you cannot send it back the -other way from the aorta.</b></p> - -<p>These then are the reasons why the blood will only pass one way, the way -I said it did. There are sets of valves opening one way and shutting the -other. These valves are the tricuspid between the right auricle and -right ventricle, the pulmonary semilunar valves between the right -ventricle and the pulmonary artery, the mitral valve between the left -auricle and the left ventricle, the aortic semilunar valves between the -left ventricle and the aorta, and the valves which are scattered among -the veins of the body. Of these by<span class="pagenum"><a name="page_076" id="page_076"></a>{76}</span> far the most important are the -valves in the heart: they do the chief work; those in the veins do -little more than help.</p> - -<p><b><a name="sect_33" id="sect_33">33.</a></b> Well, then, we understand now, do we not? why the blood, if it moves -at all, moves in the one way only. There still remains the question, <b>Why -does the blood move at all?</b></p> - -<p>You know that during life it does keep moving. You have seen it moving -in the web of a frog’s foot—and whenever any part of the body can be -brought under the microscope, the same rush of red corpuscles through -narrow channels may be seen. You know it moves because when you cut a -blood-vessel the blood runs out. If you cut an artery across, the blood -gushes out from the end which is nearest the heart; if you cut a vein -across, the blood comes most from the end nearest the capillaries. If -you want to stop an artery bleeding, you tie it between the cut and the -heart; if you want to stop a vein bleeding, you tie it between the cut -and the capillaries. You understand now why there is this difference -between a cut artery and a cut vein. And you see that this is by itself -a proof that the blood moves in the arteries from the heart to the -capillaries, and in the veins from the capillaries to the heart.</p> - -<p>The blood is not only always moving, but moves very fast. It flies along -the great arteries at perhaps ten inches in a second. Through the little -bit of capillaries along which it has to pass it creeps slowly, but -manages sometimes to go all the way round from vein to vein again in -about half a minute.</p> - -<p>It is always moving at this rapid rate, and when it ceases to move, you -die.<span class="pagenum"><a name="page_077" id="page_077"></a>{77}</span></p> - -<p><b>What makes it move?</b></p> - -<p>Suppose you had a long thin muscle, fastened at one end to something -firm, and with a weight hanging at the other end. You know that every -time the muscle contracted it would pull on the weight and draw it up. -But suppose, instead of hanging a weight on to the muscle, you wrapped -the muscle round a bladder full of water. What would happen then each -time the muscle contracted? Why, evidently it would squeeze the bladder, -and if there were a hole in the bladder some of the water would be -squeezed out. That is just what takes place in the heart. You have -already learnt that the heart is muscular. Each cavity of the heart, -each auricle, and each ventricle is, so to speak, a thin bag with a -number of muscles wrapped round it. In an ordinary muscle of the body, -the bundles of fibres of which the muscle is made up are placed -carefully and regularly side by side. You can see this very well in a -round of boiled beef, which is little more than a mass of great muscles -running in different directions. You know that if you try to cut a thin -slice right across the round, at one part your carving-knife will go -“with the grain” of the meat, <i>i.e.</i> you will cut the fibres lengthways; -at another part it will go “against the grain,” <i>i.e.</i> you will cut the -fibres crossways. In both parts, the bundles of fibres will run very -regularly. But in the heart the bundles are interlaced with each other -in a very wonderful fashion, so that it is very difficult to make out -the grain. They are so arranged in order that the muscular fibres may -squeeze all parts of each bag at the same time.</p> - -<p>Each cavity of the heart, then, auricle or ventricle,<span class="pagenum"><a name="page_078" id="page_078"></a>{78}</span> is a thin bag -with a network of muscles wrapped round it, and each time the muscles -contract they squeeze the bag and try to drive out whatever is in it. -There are more muscles in the ventricles than in the auricles, and more -in the left ventricle than in the right, for we have already seen how -much thicker the ventricles are than the auricles, and the left -ventricle than the right; and the thickness is all muscle.</p> - -<p>And now comes the wonderful fact. These muscles of the auricles and -ventricles are always at work contracting and relaxing, shortening and -lengthening, of their own accord, as long as the heart is alive. The -biceps in your arm contracts only when you make it contract. If you keep -quiet, your arm keeps quiet and your biceps keeps quiet. But your heart -never keeps quiet. Whether you are awake or whether you are asleep, -whether you are running about or lying down quite still, whatever you -are doing or not doing, as long as you are alive your heart keeps on -steadily at work. Every second, or rather oftener, there comes a short -sharp squeeze from the auricles, from both exactly at the same time, and -just as the auricles have finished their squeeze, there comes a great -hug from the ventricles, from both at the same time, but a much stronger -hug from the left than from the right; and then for a brief space there -is perfect quiet. But before the second has quite passed away, the -auricles have begun again, and after them the ventricles once more, and -thus the contracting and relaxing of the walls of the heart’s cavities, -this beat of the heart as it is called, this short snap of the two -auricles, this longer, steadier pull of the two ventricles, have gone on -in your own body since before you were born, and will<span class="pagenum"><a name="page_079" id="page_079"></a>{79}</span> go on until the -moment comes when friends gathering round your bedside will say that you -are “gone.”</p> - -<p><b><a name="sect_34" id="sect_34">34.</a> But how does this beat of the heart make the blood move?</b> Let us see.</p> - -<p>Remember that you have, or when you are grown up will have, bottled up -in the closed blood-vessels of your body about 12 lbs. of blood. You -have seen that the heart and the blood-vessels form a system of closed -tubes; the walls are in some places, in the capillaries for instance, -very thin, but they are sound and whole—and though the road is quite -open from the capillaries through the veins, heart, and arteries to the -capillaries again, there is no way out of the tubes except by making a -breach somewhere in the walls.</p> - -<p>This closed system of heart and tubes is pretty well filled by the 12 -lbs. of blood.</p> - -<p>What then must happen each time the heart contracts?</p> - -<p>Let us begin with the right ventricle. Suppose it is full of blood. It -contracts. The blood in it, squeezed on all sides, tries to go back into -the right auricle, but the tricuspid flaps have been driven back and -block the way. The more the blood presses on them, the tighter they -become, and the more completely they shut out all possibility of getting -into the auricle.</p> - -<p>The way into the pulmonary artery is open, the blood can go there. But -stay, the artery is already full of blood, and so are the capillaries -and veins in the lung. Yes, but the artery will stretch ever so much. -Take a piece of pulmonary artery, and having tied one end, pump or pour -water into the other; you will see how much it will stretch. Into the -pulmonary artery, then, goes the blood, stretching<span class="pagenum"><a name="page_080" id="page_080"></a>{80}</span> it in order to find -room. As the ventricle squeezes and squeezes, until its walls meet in -the middle, all the blood that was in it finds its way out into the -artery. But the beat of the ventricle soon ceases, the squeeze is over -and gone, and back tumbles the blood into the ventricle, or would -tumble, only the first few drops that shoot backwards are caught by the -watch-pocket semilunar valves. Back fly these valves with a sharp click -(for the things of which we are speaking happen in a fraction of a -second), and all further return is cut off. The blood has been squeezed -out of the ventricle, and is safely lodged in the pulmonary artery.</p> - -<p>But the pulmonary artery is ever so much on the stretch. It was fairly -full before it received this fresh lot of blood; now it is over-full—at -least that part of it which is nearest to the heart is over-full. What -happens next? What happens when you stretch a piece of india-rubber and -then let it go? It returns to its former size. The ventricle has -stretched the piece of pulmonary artery near it, beyond the natural -size, and then (when it ceased to contract) has let it go. Accordingly -the piece of pulmonary artery tries to return to its former size, and -since it cannot send the blood back to the ventricle, squeezes it on to -the next piece of the artery nearer the capillaries, stretching that in -turn.</p> - -<p>This again in turn sends it on the next piece—and so on right to the -capillaries. The over-full pulmonary artery, stretched to hold more than -it fairly can, empties itself through the capillaries into the pulmonary -veins until it is not more than comfortably full. But the pulmonary -veins also are already full,—what<span class="pagenum"><a name="page_081" id="page_081"></a>{81}</span> are they to do? To empty the surplus -into the left auricle. Oftener than every second there will come a time -when they can do so.</p> - -<p>For at the same time that the right ventricle pumped a quantity of blood -into the pulmonary artery and safely lodged it there, the left ventricle -pumped a like quantity into the aorta, safely lodged it there, and was -left empty itself. But just at that moment the left auricle began to -contract and to squeeze the blood that was in it.</p> - -<p>Where could that blood go? It could not go back into the pulmonary -veins, for they were already full, and the blood in them was being -pressed behind by the over-full pulmonary arteries. But it could pass -easily into the empty ventricle—and in it tumbled, the mitral flaps -readily flying back and opening up a wide way. And so the auricle -emptied itself into the ventricle. But now the auricle ceases to -contract—its walls no longer squeeze—it is empty and wants filling, -and so comes the moment when the pulmonary veins can pour into it the -blood which has been driven into them by the over-full pulmonary artery.</p> - -<p>Thus the right ventricle drives the blood into the over-full pulmonary -artery, the pulmonary artery overflows into the pulmonary veins, the -pulmonary veins carry the surplus to the empty left auricle, the left -auricle presses it into the empty left ventricle, the left ventricle -pumps it into the aorta—(the stretching of the aorta and of its -branches is what we call the pulse)—the over-full aorta overflows just -as did the pulmonary artery, through the capillaries of the body into -the great venæ cavæ—through these the blood falls into the empty right -auricle, the right auricle drives it into<span class="pagenum"><a name="page_082" id="page_082"></a>{82}</span> the empty right ventricle, -and the full right ventricle is the point at which we began.</p> - -<p><b>Thus the alternate contractions of auricles and ventricles, thanks to -the valves in the heart and in the veins, pump the blood, stroke by -stroke, through the wide system of tubes; and thus in every capillary -all over the body we find blood pressed upon behind by over-full -arteries, with a way open to it in front, thanks to the auricles, which -are, once a second or oftener, empty and ready to take up a fresh supply -from the veins.</b> Thus it comes to pass that every little fragment of your -body is bathed by blood, which a few moments ago was in your heart, and -a few moments before that was in some other part of your frame. Thus it -is that no part of your body can keep itself to itself; the blood makes -all things common as it flies from spot to spot. The red corpuscle that -a minute ago was in your brain, is now perhaps in your liver, and in -another minute may be in a muscle of your arm or in a bone of your leg: -wherever it goes it has something to bring, and something to fetch. A -restless heart is for ever driving a busy blood, which wherever it goes -buys and sells, making perhaps an occasional bargain as it shoots along -the great arteries and great veins, but busiest of all as it lingers in -the narrow pathways of the capillaries.</p> - -<p><b><a name="sect_35" id="sect_35">35.</a></b> When you look down upon a great city from a high place, as upon -London from St. Paul’s, you see stretching below you a network of -streets, the meshes of which are filled with blocks of houses. You can -watch the crowds of men and carts jostling through<span class="pagenum"><a name="page_083" id="page_083"></a>{83}</span> the streets, but the -work within the houses is hidden from your view. Yet you know that, busy -as seems the street, the turmoil and press which you see there are but -tokens of the real business which is being carried on in the house.</p> - -<p>So is it with any piece of the body upon which you look through the -microscope. You can watch the red blood jostling through the network of -capillary streets. But each mesh bounded by red lines is filled with -living flesh, is a block of tiny houses, built of muscle, or of skin, or -of brain, as the case may be. You cannot <i>see</i> much going on there, -however strong your microscope; yet that is where the chief work goes -on. In the city the raw material is carried through the street to the -factory, and the manufactured article may be brought out again into the -street, but the din of the labour is within the factory gates. In the -body the blood within the capillary is a stream of raw material about to -be made muscle, or bone, or brain, and of stuff which, having been -muscle, or bone, or brain, is no longer of any use, and is on its way to -be cast out. The actual making of muscle, or of bone, or of brain, is -carried on, and the work of each is done, outside the blood, in the -little plots of tissue into which no red corpuscle comes.</p> - -<p>The capillaries are closed tubes; they keep the red corpuscles in their -place. But their walls are so thin and delicate that they let the watery -plasma of the blood, the colourless fluid in which the corpuscles float, -soak through them into the parts inside the mesh. You probably know that -many things will pass through thin skins and membranes in which no holes -can be found even after the most careful search.<span class="pagenum"><a name="page_084" id="page_084"></a>{84}</span> If you put peas into a -bladder and tie the neck, the peas will not get out until the bladder is -untied or torn. But if you were to put a solution of sugar or of salt -into the bladder, and place the bladder with its neck tied ever so -tightly in a basin of pure water, you would find that very soon the -water in the basin would begin to taste of sugar or salt—and that -without your being able to discover any hole, however small, in the -bladder. By putting various substances in the bladder, you will find -that solid particles and things which will not dissolve in water keep -inside the bladder, whereas sugar and salt, and many other things which -dissolve in water, will make their way through the bladder into the -water outside, and will keep on passing until the water in the basin is -as strong of sugar or salt as the water in the bladder. This property -which membranes such as a bladder have of letting certain substances -pass through them is called <b>osmosis</b>. You will at once see how important -a part it plays in your own body. It is by osmosis chiefly that the raw -nourishing material in the blood gets into the little islets of flesh -lying, as we have seen, in the meshwork of the capillaries. It is by -osmosis chiefly that the worn-out stuff from the same islets gets back -into the blood. It is by osmosis chiefly that food gets out of the -stomach into the blood. It is by osmosis chiefly that the waste, -worn-out matters are drained away from the blood, and so cast out of the -body altogether. By osmosis the blood nourishes and purifies the flesh. -By osmosis the blood is itself nourished and kept pure.</p> - -<p>There are two chief things by which the blood, and<span class="pagenum"><a name="page_085" id="page_085"></a>{85}</span> through the blood -the body, is nourished. These are food and air. The air we have always -with us, we have no need to buy it or toil for it; hence we take it as -we want it, a little at a time, and often. We gather up no store of it; -and cannot bear the lack of it for more than a few moments.</p> - -<p>For our food we have to labour; we store it up in our bodies from time -to time, at intervals of hours, in what we call meals, and can go hours -or even days without a fresh supply.</p> - -<p>Let us first of all see how the blood, and, through the blood, the body, -is nourished by air.</p> - -<h2><a name="HOW_THE_BLOOD_IS_CHANGED_BY_AIR_BREATHING_VI" id="HOW_THE_BLOOD_IS_CHANGED_BY_AIR_BREATHING_VI"></a>HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI.</h2> - -<p><b><a name="sect_36" id="sect_36">36.</a></b> I have already said, perhaps more than once, that our muscles burn, -burn in a wet way without giving light. And when I say our muscles, I -might say our whole body, some parts burning more fiercely than others.</p> - -<p>You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens -when a candle is placed in a closed jar of pure air. The oxygen gets -less, carbonic acid comes in its place, and after a while the candle -goes out for want of oxygen to carry on that oxidation which is the -essence of burning. You also know that exactly the same thing would -happen if you were (only you need not do it) to put a bird or a mouse in -the jar instead of a candle. The oxygen would go, carbonic acid would -come, and the little flame of life in the mouse would flicker and go -out, and after a while its body would be cold.</p> - -<p>But suppose you were to put a fish or a snail in a<span class="pagenum"><a name="page_086" id="page_086"></a>{86}</span> jar of pure fresh -water, and cork the jar tight. There seems at first sight to be no air -in the jar. But there is. If you were to take that fresh water, and put -it under an air-pump, you could pump bubbles of air out of it; and if -you were to examine these bubbles you would find them to contain oxygen -and nitrogen, with very little carbonic acid. <b>The water contains -dissolved air.</b> After the fish or the snail had been some time in the -jar, you would see its flame of life flicker and die out, just like that -of the bird in air; and if you then pumped the air out of the water you -would find that the oxygen was nearly gone and that carbonic acid had -come in its place.</p> - -<p>You see, then, that air can be breathed, as we call it, even when it is -dissolved in water.</p> - -<p>Now to return to our muscle. When you were watching the circulation in -the frog’s foot, you could tell the artery from the vein, because in the -artery the blood was flowing <i>to</i> the capillaries, and in the vein -<i>from</i> them. Both artery and vein were rather red, and of about the same -tint of colour. But if you could see in your own body a large artery -going to your biceps muscle, and a large vein coming away from it, you -would be struck at once with the difference of colour between them. The -artery would look bright scarlet, the vein a dark purple; and if you -were to prick both, the blood would gush from the artery in a bright -scarlet jet, and bubble from the vein in a dark purple stream. And -wherever you found an artery and a vein (with a great exception of which -I shall have to speak directly), the blood in the artery would be bright -scarlet, and that in the vein dark purple. Hence we call the bright -scarlet blood which is found<span class="pagenum"><a name="page_087" id="page_087"></a>{87}</span> in the arteries <b>arterial blood</b>, and the -dark purple blood which is found in the veins <b>venous blood</b>.</p> - -<p>What is the difference between the two? If you were to pump away at some -arterial blood, as you did at the water in which you put your fish, you -would be able to obtain from it some air, or, more correctly, some gas; -a great deal more gas, in fact, than you did from the water. A pint of -blood would yield you half a pint of gas. This gas you would find on -examination not to be air, <i>i.e.</i> not made up of a great deal of -nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be -very little nitrogen, but a good deal of oxygen, and still more carbonic -acid.</p> - -<p>If you were to pump away at some venous blood you would get about as -much gas, but it would be very different in composition. The little -nitrogen would remain about the same, but the oxygen would be about half -gone, while the carbonic acid would be much increased.</p> - -<p>This, then, is one great difference (for there are others) between -venous and arterial blood, that while <b>both contain, dissolved in them, -oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen -and more carbonic acid than arterial blood</b>.</p> - -<p><b><a name="sect_37" id="sect_37">37.</a> In passing through the capillaries on its way to the vein, the blood -in the artery has lost oxygen and gained carbonic acid.</b> Where has the -oxygen gone to? Whence comes the carbonic acid? To and from the islets -of flesh between the capillaries, to the bloodless muscular fibre or bit -of nerve or skin which the blood-holding capillaries wrap round. The -oxygen has passed from the<span class="pagenum"><a name="page_088" id="page_088"></a>{88}</span> blood within the capillaries to the flesh -outside; from the flesh outside the carbonic acid has passed to the -blood within the capillaries. And this goes on all over the body. -Everywhere the flesh is breathing blood, is breathing gas dissolved in -the blood, just as a fish breathes water, <i>i.e.</i> breathes the air -dissolved in the water.</p> - -<p>Goes on everywhere with one great exception. There is one great artery, -with its branches, in which blood is not bright, scarlet, arterial, but -dark, purple, venous. There are certain great veins in which the blood -is not dark, purple, venous, but bright, scarlet, arterial. You know -which they are. The pulmonary artery and the pulmonary veins. The blood -in the pulmonary veins contains more oxygen and less carbonic acid than -the blood in the pulmonary artery. <b>It has lost carbonic acid and gained -oxygen, as it passed through the capillaries of the lungs.</b></p> - -<p><b><a name="sect_38" id="sect_38">38.</a></b> What are the lungs? As you saw them in the rabbit, or as you may see -them in the sheep, they are shrunk and collapsed. We shall presently -learn why. But if you blow into them through the windpipe, which divides -into branches, one for either lung, you can blow them out ever so much -bigger. They are in reality bladders which can be filled with air, but -which, left to themselves, at once empty themselves again.</p> - -<p>They are bladders of a peculiar construction. Imagine a thick short bush -or tree crowded with leaves; imagine the trunk and the branches, small -and great, down to the veriest twigs, all hollow; imagine further that -the leaves themselves were little<span class="pagenum"><a name="page_089" id="page_089"></a>{89}</span> hollow bladders, stuck on to the -smallest hollow twigs, and made of some delicate, but strong and -exceedingly elastic, substance. If you blew down the trunk you might -stretch and swell out all the hollow leaves; when you left off blowing -they would all fall together, and shrink up again.</p> - -<p>Around such a framework of hollow branches called <b>bronchial-tubes</b>, and -hollow elastic bladders called <b>air-cells</b>, is wrapped the intricate -network of pulmonary arteries, veins, and capillaries, in such a way -that each air-cell, each little bladder, is covered by the finest and -most close-set network of capillaries, very much as a child’s -india-rubber ball is covered round with a network of string. Very thin -are the walls of the air-cell, so thin that the blood in the capillary -is separated from the air in the air-cell by the thinnest possible sheet -of finest membrane. As the dark purple blood rushes through the crowded -network, its carbonic acid escapes through this thin membrane, from the -blood into the air, and oxygen slips from the air into the blood.</p> - -<p>Thus the dark purple venous blood coming along the pulmonary artery, as -it glides in the pulmonary capillaries along the outside of the inflated -air-cells, by loss of carbonic acid and gain of oxygen is changed into -the bright scarlet blood of the pulmonary veins.</p> - -<p>This then is the mystery of our constant need of air. <b>The flesh of the -body of whatever kind, everywhere all over the body, breathes blood, -making pure arterial blood venous and impure, all over the body except -in the lungs, where the blood itself breathes air,<span class="pagenum"><a name="page_090" id="page_090"></a>{90}</span> and changes from -impure and venous to pure and arterial.</b></p> - -<p><b><a name="sect_39" id="sect_39">39.</a></b> Through the capillaries of the muscle a stream of blood is ever -flowing so long as life lasts and the heart has power to beat; every -instant a fresh supply of bright, pure, arterial blood comes to take the -place of that which has become dark, venous, and impure. Without this -constant renewal of its blood the muscle would be choked, and its vital -flame would flicker and die out.</p> - -<p>In the lungs, the air filling the air-cells would if left to itself soon -lose all its oxygen and become loaded with carbonic acid; and the blood -in the capillaries of the lungs would no longer be changed from venous -to arterial, but would travel on to the pulmonary vein as dark and -impure as in the pulmonary artery. <b>Just as the blood in the muscle must -be constantly renewed, so must the air in the lungs be continually -changed.</b></p> - -<p>How is this renewal of the air in the lungs brought about?</p> - -<p>In the dead rabbit you saw the lungs, shrunk, collapsed, emptied of much -of their air, and lying almost hidden at the back of the chest (<a href="#fig_1">Fig. 1</a>, -<i>G.G.</i>) The cavity of the chest seemed to be a great empty space, hardly -half filled by the lungs and heart. But this is quite an unnatural -condition of the lungs. Take another rabbit, and before you touch the -chest at all, open the abdomen and remove all its contents—stomach, -liver, intestines, &c. You will then get a capital view of the -<b>diaphragm</b>, which as you already know forms a complete partition between -the chest and the belly. You will notice that it is arched<span class="pagenum"><a name="page_091" id="page_091"></a>{91}</span> up towards -the chest, so that the under surface at which you are looking is quite -hollow. If you hold the rabbit up by its hind legs with its head hanging -down, and pour some water into the abdomen, quite a little pool will -gather in the shallow cup of the diaphragm.</p> - -<p>In the rabbit the diaphragm is very transparent; you can see right -through it into the chest, and you will have no difficulty in -recognizing the pink lungs shining through it. <b>You will notice that they -cover almost all the diaphragm—in fact they fill up the whole of the -cavity of the chest that is not occupied by the heart.</b></p> - -<p>If you seize the diaphragm carefully in the middle with a pair of -forceps, and pull it down towards the abdomen, you will find that you -cannot create a space between the lungs and the diaphragm, but that the -lungs follow the diaphragm, and are quite as close to it when it is -pulled down as when it is drawn up.</p> - -<p><b>In other words, when the diaphragm is arched up as you find it on -opening the abdomen, the lungs quite fill the chest; and when the -diaphragm is drawn down and the cavity of the chest made bigger, the -lungs swell out so that they still fill up the chest.</b></p> - -<p><b><a name="sect_40" id="sect_40">40.</a></b> How do they swell out? By drawing air in through the windpipe. If -you listen, you will perhaps hear the air rush in as you pull the -diaphragm down—and if you tie the windpipe, or quite close up the nose -and mouth, you will find it much harder to pull down the diaphragm, -because no fresh air can get into the lungs.<span class="pagenum"><a name="page_092" id="page_092"></a>{92}</span></p> - -<p>Now prick a hole through the diaphragm into the cavity of the chest, -without wounding the lungs. You will hear a sudden rush of air, and the -lungs will shrink up almost out of sight. They are no longer close -against the diaphragm as they were before; and if you open the chest you -will find that they have shrunk to the back of the thorax as you saw -them in the first rabbit. The rush of air is partly a rush of air out of -the lungs, and partly a rush of air into the chest between the chest -walls and the outside of the lungs.</p> - -<p>But before you lay open the chest, pull the diaphragm up and down as you -did before you made the hole in the diaphragm. You will find that you -have no effect whatever on the lungs. They remain perfectly quiet, and -do not swell up at all. By working the diaphragm up and down, you only -drive air through the hole you have made, <b>in and out of the cavity of -the chest</b>, not <b>in and out of the lungs</b> as you did before.</p> - -<p>We see then that the <b>chest is an air-tight chamber</b>, and that the lungs, -when the chest walls are whole, <b>are always on the stretch</b>, are on the -stretch even when the diaphragm is arched up as high as it can go.</p> - -<p>Why is it that the lungs are thus always on the stretch? Because the -chest is air-tight, so that no air can get in between the outside of the -lungs and the inside of the chest wall. You know from your Physics -Primer (Art. 29, p. 34) that the atmosphere is always pressing on -everything. It is pressing on all parts of the rabbit; it presses on the -inside of the windpipe and on the inside of the lungs.<span class="pagenum"><a name="page_093" id="page_093"></a>{93}</span> It presses on -the outside of the abdomen, and so presses on the under surface of the -diaphragm, and drives it up into the chest as far as it will go. But it -will not go very far, because its edges are fastened to the firm walls -of the chest. The air also presses on the outside of the chest, but -cannot squeeze that much, because its walls are stout.</p> - -<p>If the walls of the chest were soft and flabby, the atmosphere would -squeeze them right up, and so through them press on the outside of the -lungs; since they are firm it cannot. The chest walls keep the pressure -of the atmosphere off the outside of the lungs.</p> - -<p>The lungs then are pressed by the atmosphere on their insides and not on -their outsides; and it is this inside pressure which keeps them on the -stretch or expanded. When you blow into a bladder, you put it on the -stretch and expand it because the pressure of your breath inside the -bladder is greater than the pressure of the atmosphere outside the -bladder. If, instead of making the pressure inside <i>greater</i> than that -outside, you were to make the pressure outside <i>less</i> than that inside, -as by putting the bladder under an air-pump, you would get just the same -effect; you would expand the bladder. That is just what the chest walls -do; they keep the pressure outside the lungs less than that inside the -lungs, and that is why the lungs, as long as the chest walls are sound, -are always expanded and on the stretch.</p> - -<p>When you make a hole into the chest, and let the air in between the -outside of the lungs and the chest wall, the pressure of the atmosphere -gets at the outside of the lungs; there is then the same atmospheric -pressure outside as inside the lungs; there is nothing<span class="pagenum"><a name="page_094" id="page_094"></a>{94}</span> to keep them on -the stretch, and so they shrink up to their natural size, just as does -the bladder when you leave off blowing into it, or when you take it out -of the air-pump.</p> - -<p>When before you made the hole in the diaphragm you pulled the diaphragm -down, you <b>still further lessened the pressure on the outside of the -lungs</b>; hence the pressure inside the lungs caused them to swell up and -follow the diaphragm. But this put the lungs still more on the stretch, -so that when you let go the diaphragm and ceased to pull on it, the -lungs went back again to their former size, emptying themselves of part -of their air and pulling the diaphragm up with them. When there is a -hole in the chest wall, pulling the diaphragm down does not make any -difference to the pressure outside the lungs. They are then always -pressed upon by the same atmospheric pressure inside and outside, and so -remain perfectly quiet.</p> - -<p><b>When in an air-tight chest the diaphragm is pulled down, the pressure of -the atmosphere drives air into the lungs through the windpipe and swells -them up. When the diaphragm is let go, the stretched lungs return to -their former size, emptying themselves of the extra quantity of air -which they had received.</b></p> - -<p>Suppose now the diaphragm were pulled down and let go again regularly -every few seconds: what would happen? Why, every time the diaphragm went -down a certain quantity of air would enter into the lungs, and every -time it was let go that quantity of air would come out of the lungs -again.<span class="pagenum"><a name="page_095" id="page_095"></a>{95}</span></p> - -<p><a name="fig_12" id="fig_12"></a></p> - -<div class="figcenter"> -<a href="images/i_p095_lg.jpg"> -<br /> -<img src="images/i_p095_sml.jpg" width="408" height="469" alt="Image unavailable: Fig. 12.—The Diaphragm of a Dog viewed from the Lower -or Abdominal Side. - -V.C.I. the vena cava inferior; O. the œsophagus; Ao. the -aorta; the broad white tendinous middle (B) is easily -distinguished from the radiating muscular fibres (A) which pass -down to the ribs and into the pillars (C D) in front of the -vertebræ. -" /></a> -<br /> -<span class="caption">Fig. 12.—The Diaphragm of a Dog viewed from the Lower -or Abdominal Side. -</span><p class="caption"> -V.C.I. the vena cava inferior; O. the œsophagus; Ao. the -aorta; the broad white tendinous middle (B) is easily -distinguished from the radiating muscular fibres (A) which pass -down to the ribs and into the pillars (C D) in front of the -vertebræ.</p> - -</div> - -<p>This is what does take place in breathing or respiration. Every few -seconds, about seventeen times a minute, the diaphragm does descend, and -a quantity of air rushes into the lungs through the windpipe. This is -called <b>inspiration</b>. As soon as that has taken place, the diaphragm -ceases to pull downwards, the<span class="pagenum"><a name="page_096" id="page_096"></a>{96}</span> stretched lungs return to their former -size, carrying the diaphragm up with them, and squeeze out the extra -quantity of air. This is called <b>expiration</b>.</p> - -<p>As the diaphragm descends it presses down on the abdomen; when it ceases -to descend, the contents of the abdomen help to press it up. If you -place your hand on your stomach, you can feel the abdomen bulging out -each time the diaphragm descends in inspiration, and going in again each -time the diaphragm returns to its place in expiration.</p> - -<p><b><a name="sect_41" id="sect_41">41.</a></b> But what causes the diaphragm to descend?</p> - -<p>If you look at the diaphragm of the rabbit (or of any other animal) a -little carefully, you will see that it is in reality a flat thin muscle, -rather curiously arranged; for the red fleshy muscular fibres are on the -outside all round the edge (<a href="#fig_12">Fig. 12</a>, <i>A</i> and <i>C</i>), while the centre <i>B</i> -is composed of a whitish transparent tendon. These muscular fibres, like -all other muscular fibres, have the power of contracting. What must -happen when they contract and become shortened?</p> - -<p>When these muscular fibres are at rest, as in the dead rabbit, the whole -diaphragm is arched up, as we have seen, towards the thorax, somewhat as -is shown in <a href="#fig_13">Fig. 13</a>, <i>B</i>. It is partly pushed up by all the contents of -the abdomen (for the cavity of the abdomen, you will remember, is quite -filled by the liver, stomach, intestines, and other organs), partly -pulled up by the lungs, which, as we know, are always on the stretch. -When the muscular fibres contract, they pull at the central tendon (just -as the biceps pulls at its lower tendon), <b>and pull the diaphragm flat</b>; -and some of the fibres, such as those at <i>C</i>, <a href="#fig_12">Fig. 12</a>, also pull it -<b>down</b>. <b>The diaphragm during its contraction<span class="pagenum"><a name="page_097" id="page_097"></a>{97}</span> is flattened and descends</b>, -somewhat as is shown in <a href="#fig_13">Fig. 13</a>, <i>A</i>.</p> - -<p><a name="fig_13" id="fig_13"></a></p> - -<div class="figcenter"> -<a href="images/i_p097_lg.jpg"> -<br /> -<img src="images/i_p097_sml.jpg" width="386" height="448" alt="Image unavailable: Fig. 13.—Diagrammatic Sections of the Body in - -A. inspiration; B. expiration. Tr. trachea; St. sternum; -D. diaphragm; Ab. abdominal walls. The shading roughly -indicates the stationary air. The unshaded portion at the top of -A is the tidal air." /></a> -<br /> -<span class="caption">Fig. 13.—Diagrammatic Sections of the Body in -</span><p class="caption"> -A. inspiration; B. expiration. Tr. trachea; St. sternum; -D. diaphragm; Ab. abdominal walls. The shading roughly -indicates the stationary air. The unshaded portion at the top of -A is the tidal air.</p> - -</div> - -<p><b>The descent of the diaphragm in inspiration is caused by a contraction -of its muscular fibres. During expiration the diaphragm is at rest; its -muscular fibres relax; and it goes up because it is partly drawn up by -the lungs, partly pushed up by the contents of the abdomen.</b><span class="pagenum"><a name="page_098" id="page_098"></a>{98}</span></p> - -<p><b><a name="sect_42" id="sect_42">42.</a></b> Other structures besides the diaphragm assist in pumping air in and -out of the lungs. By the action of the diaphragm the chest is -alternately lengthened and shortened. But if you watch anyone, and -especially a woman, breathing, you will notice that with every breath -the chest rises and falls; the front of the chest, the sternum, as you -have learnt to call it, comes forward and goes back; and a little -attention will convince you that it comes forward during inspiration, -<i>i.e.</i> while the diaphragm is descending, and falls back during -expiration. But this coming forward of the sternum means a widening of -the chest from back to front, and the falling back of the sternum means -a corresponding narrowing. So that while the chest is being lengthened -by the descent of the diaphragm, it is also being widened by the coming -forward of the sternum. In inspiration the lungs are expanded not only -downwards, by the movement of the diaphragm, but also outwards, by the -movement of the walls of the chest.</p> - -<p>What thrusts forward the sternum? If you were to watch closely the sides -of the chest of a very thin person, you would be able to notice that at -every breathing in, at every inspiration, the ribs are pulled up a -little way. Now, each rib is connected with the backbone behind by a -joint, and is firmly fastened to the sternum in front by cartilage (see -Frontispiece). If you were to fasten a piece of string to the middle of -one of the ribs and to pull it, you would find you were working on a -lever, with the fulcrum at the backbone, with the weight acting at the -sternum, and the power at the point where your string was tied. Every -time you pulled the string the<span class="pagenum"><a name="page_099" id="page_099"></a>{99}</span> rib would move on its fulcrum at the -backbone, in such a way that the front end of the rib would rise up, and -the sternum would be thrust out a little. When you left off pulling, the -sternum, which in being thrust forward had been put on the stretch, -would sink back, and the rib would fall down to its previous position.</p> - -<p><a name="fig_14" id="fig_14"></a></p> - -<div class="figcenter"> -<a href="images/i_p099_lg.jpg"> -<br /> -<img src="images/i_p099_sml.jpg" width="401" height="410" alt="Image unavailable: Fig. 14.—View of Four Ribs of the Dog with the -Intercostal Muscles. - -a. The bony rib; b, the cartilage; c, the junction of bone -and cartilage; d, unossified; e, ossified, portions of the -sternum. A. External intercostal muscle. B. Internal -intercostal muscle. In the middle interspace, the external -intercostal has been removed to show the internal intercostal -beneath it." /></a> -<br /> -<span class="caption">Fig. 14.—View of Four Ribs of the Dog with the -Intercostal Muscles. -</span><p class="caption"> -a. The bony rib; b, the cartilage; c, the junction of bone -and cartilage; d, unossified; e, ossified, portions of the -sternum. A. External intercostal muscle. B. Internal -intercostal muscle. In the middle interspace, the external -intercostal has been removed to show the internal intercostal -beneath it.</p> -</div> - -<p>Between the ribs are certain muscles called <b>intercostal muscles</b> (<a href="#fig_14">Fig. -14</a>). The exact action of these you will learn at some future time. -Meanwhile it<span class="pagenum"><a name="page_100" id="page_100"></a>{100}</span> will be enough to say that they act like the piece of -string we are speaking of. <b>When they contract, they pull up the ribs and -thrust out the sternum; when they leave off contracting, the ribs and -sternum fall back to their previous position.</b></p> - -<p>There are many other muscles which help in breathing, especially in hard -or deep breathing, but it will be sufficient for you to remember that in -ordinary breathing there are two chief movements taking place exactly at -the same time, by means of which air is drawn into the chest, both -movements being caused by the contraction of muscles. First, the -diaphragm contracts and flattens itself, making the chest deeper or -longer; secondly, at the same time the ribs are raised and the sternum -thrust out by the contraction of the intercostal muscles, making the -chest wider. But as the chest becomes wider and longer, the lungs become -wider and longer too. In order to fill up the extra room thus made in -the lungs, air enters into them through the windpipe. This is -<b>inspiration</b>. But soon the diaphragm and the intercostal muscles cease to -contract; the diaphragm returns to its arched condition, the ribs sink -down, the sternum falls back, and the extra air rushes back again out of -the lungs through the windpipe. This is <b>expiration</b>. An inspiration and -an expiration make up a whole breath; and thus we breathe some seventeen -times in every minute of our lives.</p> - -<p><b><a name="sect_43" id="sect_43">43.</a></b> But what makes the diaphragm and intercostal muscles contract and -rest in so beautifully regular a fashion? The biceps of the arm, we saw, -was made to contract by our will. It is not our will, however,<span class="pagenum"><a name="page_101" id="page_101"></a>{101}</span> which -makes us breathe. We breathe often without knowing it; we breathe in our -sleep when our will is dead; we breathe whether we will or no, because -we cannot help it. We can quicken our breathing, we can take a short or -deep breath as we please, we can change our breathing by the force of -our will; but the breathing itself goes on without, and in spite of, our -will. It is <b>an involuntary act.</b></p> - -<p>Though breathing is not an effort of the will, it is an effort of the -brain; an effort, too, of one particular part of the brain, that part -where the brain joins on to the spinal cord. Nerves run from the -diaphragm and the intercostal and other muscles through the spinal cord, -to this part of the brain. And seventeen times a minute a message comes -down along these nerves, from the brain, bidding them contract; they -obey, and you breathe. Why and how that message comes, you will learn at -some future time. When your head is cut off, or when that part of the -brain which joins on to the spinal cord is injured by accident or made -powerless by disease, the message ceases to be sent, and you cease to -breathe.</p> - -<p><b><a name="sect_44" id="sect_44">44.</a></b> At every breath, then, a certain quantity of air goes in and out of -the chest; but only a small quantity. <b>You must not think the lungs are -quite emptied and quite filled at each breath.</b> On the contrary, you only -take in each time a mere handful of air, which reaches about as far as -the large branches of the windpipe, and does not itself go into the -air-cells at all. This is often called <b>tidal</b> air; and the rest of the -air in the lungs, which does not move, is often called the <b>stationary</b> -air (see <a href="#fig_13">Fig. 13</a>).<span class="pagenum"><a name="page_102" id="page_102"></a>{102}</span></p> - -<p>How then does the carbonic acid at the bottom of the lungs get out? How -do the capillaries in the air-cells get their fresh oxygen?</p> - -<p>The stationary air mingles with the tidal air at every breath. If you -want to ventilate a room, you are not obliged to take a pair of bellows -and drive out every bit of the old air in the room, and supply its place -with new air: it will be enough if you open a window or a door and let -in a draught of pure air across one corner, say, of the room. That -current of pure air flowing across the corner will mingle with all the -rest of the air until the whole air in the room becomes pure; and the -mingling will take place very quickly. So it is in the lungs. The tidal -air comes in with each inspiration as pure air from without; but before -it comes out at the next expiration it gives up some of its oxygen to -the stationary air, and robs the stationary air of some of its carbonic -acid. For each breath of tidal air the stationary air is so much the -better, having lost some of its carbonic acid and gained some fresh -oxygen. The tidal air rapidly purifies the stationary air, and the -stationary air purifies the blood.</p> - -<p>Thus it comes to pass that the tidal air, which at each pull of the -diaphragm and push of the sternum goes into the chest as pure air with -twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred -parts, comes out, when the diaphragm goes up and the sternum falls back, -as impure air with only sixteen parts oxygen, but with five parts -carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried -through the stationary air to the blood in the capillaries, and the -gained carbonic acid came through the stationary air from the blood in -the capillaries. So<span class="pagenum"><a name="page_103" id="page_103"></a>{103}</span> each breath helps to purify the blood, and the -pumping of air in and out of the chest changes the impure, hurtful, -venous, to pure, refreshing, arterial blood; the blood breathes air in -the lungs, that all the body may in turn breathe blood.</p> - -<h2><a name="HOW_THE_BLOOD_IS_CHANGED_BY_FOOD_DIGESTION_VII" id="HOW_THE_BLOOD_IS_CHANGED_BY_FOOD_DIGESTION_VII"></a>HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII.</h2> - -<p><b><a name="sect_45" id="sect_45">45.</a></b> The blood is not only purified by air, it is also renewed and made -good by food. The food we eat becomes blood. But our food, though -frequently moist, is for the most part solid. We cut it into small -pieces on the plate, and with our teeth we crush and tear it into still -smaller morsels in our mouth. Still, however well chewed, a great deal -of it, most of it in fact, is swallowed solid. In order to become blood -it must first be dissolved. It is dissolved in the alimentary canal, and -we call the dissolving <b>digestion</b>. Let us see how digestion is carried -on.</p> - -<p>Your skin, though sometimes quite moist with perspiration, is as -frequently quite dry. The inside of your mouth is always moist—very -frequently quite filled with fluid; and even when you speak of it as -being dry, it is still very moist. Why is this? The inside of your mouth -is also very much redder than your skin. The redness and the moisture go -together.</p> - -<p>In speaking of the capillaries, I said that almost all parts of the body -were completely riddled with them, but <b>not quite all</b>. A certain part of -the skin, for instance, has no capillaries or blood-vessels at all. You -know that where your skin is thick, you can shave off pieces of skin -without “fetching blood;” if your<span class="pagenum"><a name="page_104" id="page_104"></a>{104}</span></p> - -<p><a name="fig_15" id="fig_15"></a></p> - -<div class="figcenter"> -<a href="images/i_p104_lg.jpg"> -<br /> -<img src="images/i_p104_sml.jpg" width="433" height="644" alt="Image unavailable: Fig. 15.—Section of Skin, highly magnified. - -a, horny epidermis; b, softer layer; c, dermis; d, -lowermost vertical layer of epidermic cells; e, cells lining the -sweat duct continuous with epidermic cells; h, corkscrew canal of -sweat duct. To the right of the sweat duct the dermis is raised -into a papilla, in which the small artery, f, breaks up into -capillaries, ultimately forming the veins, g." /></a> -<br /> -<span class="caption">Fig. 15.—Section of Skin, highly magnified. -</span><p class="nind"> -a, horny epidermis; b, softer layer; c, dermis; d, -lowermost vertical layer of epidermic cells; e, cells lining the -sweat duct continuous with epidermic cells; h, corkscrew canal of -sweat duct. To the right of the sweat duct the dermis is raised -into a papilla, in which the small artery, f, breaks up into -capillaries, ultimately forming the veins, g.</p> -</div> - -<p><span class="pagenum"><a name="page_105" id="page_105"></a>{105}</span></p> - -<p class="nind">knife were very sharp and you very skilful, you might do the same in -every part of your skin. If you were to put some of the skin you had -thus cut off under the microscope, you would find that it was made up of -little scales. And if you were to take a very thin upright slice running -through the whole thickness of the skin, and examine that under a high -power of the microscope, you would find that the skin was made up of two -quite different parts or layers, as shown in <a href="#fig_15">Fig. 15</a>. The upper layer, -<i>a</i>, <i>b</i>, is nothing but a mass of little bodies packed closely -together. At the top they are pressed flat into scales, but lower down -they are round or oval, and at the same time soft. They are called -<b>cells</b>. As you advance in your study of Physiology you will hear more and -more about cells. This layer of cells, either soft and round, or -flattened and dried into scales, is called the <b>epidermis</b>. No -blood-vessel is ever found in the epidermis, and hence, when you cut it, -it never bleeds. As long as you live it is always growing. The top -scales are always being rubbed off. Whenever you wash your hands, -especially with soap, you wash off some of the top scales; and you would -soon wash your skin away, were it not that new round cells are always -being formed at the bottom of the epidermis, along the line at <i>d</i> (<a href="#fig_15">Fig. -15</a>), and always moving up to the top, where they become dried into -scales. Thus the skin, or more strictly the epidermis, is always being -renewed. Sometimes, as after scarlet fever, the new skin grows quickly, -and the old skin comes away in great flakes or patches.</p> - -<p>The lower layer below the epidermis is what is called the <b>dermis</b>, or -<b>true skin</b>. This is full of capillaries and blood-vessels, and when the -knife or<span class="pagenum"><a name="page_106" id="page_106"></a>{106}</span> razor gets down to this, you bleed. It is not made up of cells -like the epidermis, but of that fibrous substance which you early learnt -to call connective tissue (see p. 9). Its top is rarely level, but -generally raised into little hillocks, called <b>papillæ</b>, as in the figure; -the epidermis forming a thick cap over each papillæ, and filling up the -hollows between them. Most of the papillæ are full of blood-vessels.</p> - -<p>Now, then, I think you will understand why your skin is not red, but -flesh-coloured, and why it is generally dry. The true skin under the -epidermis is always moist, because of the blood-vessels there; the waste -and fluid parts of the blood pass readily through the walls of the -capillaries, as you have learnt, by osmosis, and so keep everything -round them moist. But this moisture is not enough to soak through the -thick coating of epidermis, and so the top part of the epidermis remains -dry and scaly.</p> - -<p>The true skin underneath the epidermis is always red; you know that if -you shave off the surface of your skin anywhere, it gets redder and -redder the deeper you go down, even though you do not fetch blood. It is -red because of the immense number of capillaries, all full of red blood, -which are crowded into it. When you look at these capillaries through a -great thickness of epidermis, the redness is partly hidden from you, as -when you put a sheet of thin white paper over a red cloth, and the skin -seems pink or flesh-coloured; and where the epidermis is very thick, as -at the heel, the skin is not even pink, but white or yellow, more or -less dirty according to circumstances.</p> - -<p><b><a name="sect_46" id="sect_46">46.</a></b> But if the moist true skin is thus everywhere covered by a thick -coat of epidermis, which keeps the<span class="pagenum"><a name="page_107" id="page_107"></a>{107}</span> moisture in, how is it that the skin -is nevertheless sometimes quite moist, as when we perspire?</p> - -<p><a name="fig_16" id="fig_16"></a></p> - -<div class="figcenter"> -<a href="images/i_p107_lg.jpg"> -<br /> -<img src="images/i_p107_sml.jpg" width="348" height="360" alt="Image unavailable: Fig. 16.—Coiled end of a Sweat Gland, Epithelium not -shown. - -a, the coil; b, the duct; c, network of capillaries, inside which -the duct gland lies." /></a> -<br /> -<span class="caption">Fig. 16.—Coiled end of a Sweat Gland, Epithelium not -shown. -<br /> -a, the coil; b, the duct; c, network of capillaries, inside which -the duct gland lies.</span> -</div> - -<p>If you look at <a href="#fig_15">Fig. 15</a>, you will see that the epidermis is at one point -pierced by a canal (<i>h</i>) running right through it. You will notice that -this canal is not closed at the bottom of the epidermis, but runs right -into the dermis or true skin, where the canal becomes a tube, with just -one layer (<i>e</i>) of cells, like the cells of the epidermis, for its -walls. There is no room in <a href="#fig_15">Fig. 15</a> to show what becomes of this tube, -but it runs some way down under the skin all among the blood-vessels, -and then twisting itself up into a knot, ends blindly, as is shown in -<a href="#fig_16">Fig. 16</a>, where <i>b</i> is<span class="pagenum"><a name="page_108" id="page_108"></a>{108}</span> a continuation on a smaller scale of the same -tube which is seen in <a href="#fig_15">Fig. 15</a>. This knot is covered by a close network -of capillaries, which at <i>c</i> are supposed to be unravelled and taken -away from the knotted tube in order to show them. The capillaries, you -will understand, though inside the knot, are always outside the tube. If -you were to drop a very diminutive marble in at <i>h</i> (<a href="#fig_15">Fig. 15</a>), it would -rattle down the corkscrew passage through the thick epidermis, shoot -down the straight tube <i>b</i> (<a href="#fig_16">Fig. 16</a>), and roll through the knot <i>a</i>, -until it came to rest at the blind end of the tube. Along its whole -course it would touch nothing but cells, like the cells of the -epidermis, a single layer of which forms the walls of the tube where it -runs below the epidermis. If it got lodged at <i>h</i> (<a href="#fig_15">Fig. 15</a>), or got -lodged in the knot at <i>a</i> (<a href="#fig_16">Fig. 16</a>), it would in both cases be touching -epidermic cells. But there would be this great difference. At <i>h</i> it -would be ever so far removed from any blood capillary; at <i>a</i> it would -only have to make its way through a thin layer of single cells, and it -would be touching a capillary directly. At <i>h</i> it might remain dry for -some time; at <i>a</i> it would get wet directly, for there is nothing to -prevent the fluid parts of the blood oozing out through the thin wall of -the capillaries, and so through the thin wall of the tube into the canal -of the tube, on to the marble.</p> - -<p>In fact, the inside of the knot is always moist and filled with fluid. -When the capillaries round the knot get over-full of blood, as they -often do, a great deal of colourless watery fluid passes from them into -the tube. The tube gets full, the fluid wells up right into the -corkscrew portion in the thickness of the<span class="pagenum"><a name="page_109" id="page_109"></a>{109}</span> epidermis, and at last -overflows at the mouth of the tube over the skin. We call this fluid -sweat or <b>perspiration</b>. We call the tube with its knotted end <b>a gland</b>; -and we call the act by which the colourless fluid passes out of the -blood capillaries into the canal of the tube, <b>secretion</b>. We speak of the -<b>sweat gland secreting sweat out of the blood brought by the capillaries -which are wrapped round the gland</b>.</p> - -<p><b><a name="sect_47" id="sect_47">47.</a></b> Now we can understand why the inside of the mouth is red and moist. -The mouth has a skin just like the skin of the hand. There is an outside -epidermis, made up of cells and free from capillaries, and beneath that -a dermis or true skin crowded with capillaries. Only the epidermis of -the mouth is ever so much thinner than that of the hand. The red -capillaries easily shine through it, and their moisture can make its way -through. Hence the mouth is red and moist. Besides there are many glands -in it, something like the sweat gland, but differing in shape; these -especially help to keep it moist.</p> - -<p>Because it is always red and moist and soft, the skin of the inside of -the mouth is generally not called a skin at all, but <b>mucous membrane</b>, -and the upper layer is not called epidermis, but <b>epithelium</b>. You will -remember, however, that a mucous membrane is in reality a skin in which -the epidermis is thin and soft, and is called epithelium.</p> - -<p>The mouth is the beginning of the alimentary canal. Throughout its whole -length the alimentary canal is lined by a skin or mucous membrane like -that of the mouth, only over the greater part of it the epithelium is -still thinner than in the mouth, and indeed is made up<span class="pagenum"><a name="page_110" id="page_110"></a>{110}</span> of a single -layer only of cells. The whole of the inside of the canal is therefore -red and moist, and whatever lies in the canal is separated by a very -thin partition only from the blood in the capillaries, which are found -in immense numbers in the walls of the canal. The alimentary canal is, -as you know, a long tube, wide at the stomach but narrow elsewhere. In -all parts of its length the tube is made up of mucous membrane on the -inside, and on the outside of muscles, differing somewhat from the -muscles of the body and of the heart, but having the same power of -contracting, and by contracting of squeezing the contents of the tube, -just as the muscles of the heart squeeze the blood in its cavities. The -muscles, and especially the mucous membrane, are crowded with -blood-vessels.</p> - -<p>Though the epithelium of the mucous membrane is very thin, the mucous -membrane itself is thick, in some places quite as thick as the skin of -the body. This thickness is caused by its being <b>crowded with glands</b>. In -the skin the sweat glands are generally some little distance apart, but -in the mucous membrane of the stomach and of the intestines they are -packed so close together, that the membrane seems to be wholly made up -of glands.</p> - -<p>These glands vary in shape in different parts. Nowhere are they exactly -like the sweat glands, because none of them are long thin tubes coiled -up at the end in a knot, and none of them have a great thickness of -epidermis to pass through. Most of them are short, rather wide tubes; -some of them are branched at the deep end. They all, however, resemble -the sweat glands in being tubes or pouches closed at the bottom but open -at top, lined by a single layer of<span class="pagenum"><a name="page_111" id="page_111"></a>{111}</span> cells, and wrapped round with blood -capillaries. From these capillaries, a watery fluid passes into the -tubes, and from the tubes into the alimentary canal. This watery fluid -is, however, of a different nature from sweat, and is not the same in -all parts of the canal. The fluid which is, as we say, secreted by the -glands in the walls of the stomach is an <b>acid fluid</b>, and is called -<b>gastric juice</b>; that by the glands in the walls of the intestines is <b>an -alkaline fluid</b>, and is called <b>intestinal juice</b>.</p> - -<p><b><a name="sect_48" id="sect_48">48.</a></b> But besides these glands in the mucous membrane of the mouth, the -stomach, and the intestines, there are other glands, which seem at first -sight to have nothing to do with the mucous membrane.</p> - -<p>Beneath the skin, underneath each ear, just behind the jaw, is a soft -body, which ordinarily you cannot feel, but which, when inflamed by what -is called “the mumps,” swells up into a great lump. In a sheep’s head -you would find just the same body, and if you were to examine it you -would notice fastened to it a fleshy cord running underneath the skin -across the cheek towards the mouth. By cutting the cord across you would -discover that what seemed a cord was in reality a narrow tube coming -from the soft body we are speaking of and opening into the mouth. Just -close to the soft body this tube divides into two smaller tubes, these -divide again into still smaller ones, or give off small branches; all -these once more divide and branch like the boughs of a tiny tree; and so -they go on branching and dividing, getting smaller and smaller, until -they end in fine tubes with blind swollen ends. All the tubes, great and -small, are lined with epithelium and wrapped round with<span class="pagenum"><a name="page_112" id="page_112"></a>{112}</span> blood-vessels, -and being packed close together with connective tissue, make up the soft -body we are speaking of. This body is in fact a <b>gland</b>, and is called a -<b>salivary gland</b>; as you see it is not a simple gland like a sweat gland, -but is made up of a host of tube-like glands all joined together, and -hence is called a <b>compound gland</b>. Being placed far away from the mouth, -it has to be connected with the cavity of the mouth by a long tube, -which is called its <b>duct</b>. You cannot fail to notice how like such a -gland is, in its structure, to a lung. The lung is in fact a gland -secreting carbonic acid: and the duct of the two lungs is called the -trachea. The salivary gland beneath the ear is called the <b>parotid gland</b>; -there is another very similar one underneath the corner of the jaw on -either side, called the <b>submaxillary gland</b>. By each of them a watery -fluid is secreted, which, flowing along their ducts into the mouth and -being there mixed with the moisture secreted by the other glands in the -mouth, is called <b>saliva</b>.</p> - -<p>In the cavity of the abdomen lying just below the stomach is a much -larger but altogether similar compound gland called <b>the pancreas</b>, which -pours its secretion called <b>pancreatic juice</b> into the alimentary canal -just where the small intestine begins (<a href="#fig_17">Fig. 17</a>, <i>g.</i>)</p> - -<p>That large organ the liver, though the plan of its construction is not -quite the same as that of the pancreas or salivary glands, as you will -by and by learn, is nevertheless a huge gland, secreting from the blood -capillaries into which the portal vein (see p. 62) breaks up, a fluid -called <b>bile</b> or <b>gall</b>, which by a duct, the <b>gall duct</b>, is poured into the -top of the intestine (Fig.<span class="pagenum"><a name="page_113" id="page_113"></a>{113}</span> 17, <i>e</i>). When bile is not wanted, as when -we are fasting, it turns off by a side passage from the duct into the -gall-bladder (<a href="#fig_17">Fig. 17</a>, <i>f</i>), to be stored up there till needed.</p> - -<p><b><a name="sect_49" id="sect_49">49.</a></b> What are the uses of all these juices and secretions? To dissolve -the food we eat.</p> - -<p><a name="fig_17" id="fig_17"></a></p> - -<div class="figcenter"> -<a href="images/i_p113_lg.jpg"> -<br /> -<img src="images/i_p113_sml.jpg" width="363" height="333" alt="Image unavailable: Fig. 17.—The Stomach laid open behind. - -a, the œsophagus or gullet; b, one end of the stomach; d, -the other end joining the intestine; e, gall duct; f, the -gall-bladder; g, the pancreatic duct; h, i, the small -intestine. -" /></a> -<br /> -<span class="caption">Fig. 17.—The Stomach laid open behind. -</span> -<p class="nind"> -a, the œsophagus or gullet; b, one end of the stomach; d, -the other end joining the intestine; e, gall duct; f, the -gall-bladder; g, the pancreatic duct; h, i, the small -intestine.</p> - -</div> - -<p>We eat all manner of dishes, but in all of them that are worth eating we -find the same kind of things, which we call <b>food-stuffs</b>.</p> - -<p>We eat various kinds of meat; but all meats are made up chiefly of two -things: the substance of the muscular fibre, which you have already -learnt is a <b>proteid</b> matter containing nitrogen, and the <b>fat</b> which<span class="pagenum"><a name="page_114" id="page_114"></a>{114}</span> wraps -round the lean muscular flesh. Now, proteids are, when cooked, insoluble -in water (see p. 49); and fat, you know, will not mingle with water. -Both these parts of meat, both these food-stuffs, must be acted upon -before they can pass from the inside of the alimentary canal, through -the epithelium of the mucous membrane, into the blood capillaries.</p> - -<p>Besides meat we eat bread. Bread is chiefly composed of <b>starch</b>; but -besides starch we find in it a substance containing nitrogen, -exceedingly like the proteid matter of muscle or of blood.</p> - -<p>Potatoes contain a very great deal of starch with a very small quantity -of proteid matter; and nearly all the vegetables we eat contain starch, -with more or less proteid matter.</p> - -<p>Then we generally eat more or less sugar, either as such or in the form -of sweet fruits. We also take salt with our meals, and in almost -everything we eat, animal or vegetable, meat, bread, potatoes or fruit, -we swallow a quantity of mineral substances, that is, various kinds of -salts, such as potash, lime, magnesia, iron, with sulphuric, -hydrochloric, phosphoric, and other acids.</p> - -<p><b>In everything on which we live we find one or more of the following -food-stuffs:—Proteid matter, starch or sugar, and fat, together with -certain minerals and water. It is on these we live: any article which -contains either proteid matter, or starch, or fat, is useful for food. -Any article which contains none of them is useless for food, unless it -be for the sake of the minerals or water it holds.</b><span class="pagenum"><a name="page_115" id="page_115"></a>{115}</span></p> - -<p>We are not obliged to eat all these food-stuffs. Proteid matter we must -have always. It is the only food-stuff which contains nitrogen. It is -the only substance which can renew the nitrogenous proteid matter of the -blood and so the nitrogenous proteid matter of the body.</p> - -<p>We might indeed manage to live on proteid matter alone, for it contains -not only nitrogen but also carbon and hydrogen, and out of it, with the -help of a few minerals, we might renew the whole blood and build up any -and every part of the body. But, as you will learn hereafter, it would -be uneconomical and unwise to do so. Starch, sugar, and fats, contain -carbon and hydrogen without nitrogen; and hence, if we are to live on -these we must add some proteid matter to them.</p> - -<p><b><a name="sect_50" id="sect_50">50.</a></b> Of these food-stuffs, putting on one side the minerals, sugar (of -which, as you know, there are several kinds, cane sugar, grape sugar, -and the like) is the only one which is really soluble, and will pass -readily by osmosis through thin membranes (see p. 84). If you take a -quantity of white of egg, or blood serum, or meat, or fibrin, or a -quantity of starch boiled or unboiled, or a quantity of oil or fat, -place it in a bladder, and immerse the bladder in pure water, you will -find that none of it passes through the bladder into the water outside, -as sugar or salt would do. In the same way a quantity of meat, or of -starch, or of fat, placed in your alimentary canal, would never get -through the membrane which separates the inside of the canal from the -inside of the capillaries, and so would remain perfectly useless as food -unless something were done to it. While the food is simply inside the -alimentary canal, it is really<span class="pagenum"><a name="page_116" id="page_116"></a>{116}</span> outside your body. It can only be said -to be inside your body when it gets into your blood.</p> - -<p>In the things we eat, moreover, these food-stuffs are mixed up with a -great many things that are not food-stuffs at all; they are packed away -in all manner of little cases, which are for the most part no more good -for eating than the boxes or paper in which the sweetmeats you buy are -wrapped up. The food-stuffs have to be dissolved out of these boxes and -packing.</p> - -<p><b>The juices secreted by the glands</b> of which we have been speaking, -dissolve the food-stuffs out of their wrappings, act upon them so as to -make them fit to pass into the blood, and leave all the wrappings as -useless stuff which passes out of the alimentary canal without entering -into the blood, and therefore without really forming part of the body at -all.</p> - -<p>This preparation and dissolving of food-stuffs is called digestion.</p> - -<p>Different food-stuffs are acted upon in different parts of the -alimentary canal.</p> - -<p>The saliva of the mouth has a wonderful power of <b>changing starch into -sugar</b>. If you take a mouthful of boiled starch, which is thick, sticky, -pasty, and tasteless, and hold it in your mouth for a few moments, it -will become thin and watery, and will taste quite sweet, because the -starch has been changed into sugar. Now sugar, as you know, will readily -pass through membranes, though starch will not.</p> - -<p>The gastric juice in the stomach does not act much on starch, but it -<b>rapidly dissolves all proteid matters</b>.</p> - -<p>If you take a piece of boiled meat, put it in some gastric juice and -keep the mixture warm, in a very<span class="pagenum"><a name="page_117" id="page_117"></a>{117}</span> short time the meat will gradually -disappear. All the proteid matter will be dissolved, and only the -wrappings of the muscular fibre and the fat be left. You will have a -solution of meat—a solution, moreover, which, strange to say, will -easily pass through membranes, and is therefore ready to get into the -blood.</p> - -<p>The pancreatic juice and the juice secreted by the intestine act both on -starch as saliva does, and on proteids very much as gastric juice does.</p> - -<p><b><a name="sect_51" id="sect_51">51.</a></b> The bile and the pancreatic juice together act upon all fats in a -very curious way.</p> - -<p>You know that if you shake up oil and water together, though by violent -shaking you may mix them a good deal, directly you leave off they -separate again, and all the oil is seen floating on the top of the -water. If, however, you shake up oil with pancreatic juice and bile, the -oil does not separate. You get a sort of creamy mixture, and will have -to wait a very long time before the oil floats to the top. Milk, you -know, contains fat, the fat which is generally called butter. If you -examine milk under the microscope, you will find that the fat is all -separated into the tiniest possible drops. So also, when you shake up -oil or butter, or any other fat, with bile and pancreatic juice, you -will find on examination that the fat or oil is all separated into the -tiniest possible drops. What is the purpose of this?</p> - -<p>If you look at the inside of the small intestine of any animal, you will -find that it is not smooth and shiny like the outside of the intestine, -but shaggy, or, rather, velvety. This is because the mucous membrane is -crowded all over with little tags, like very little tongues, hanging -down into the inside of<span class="pagenum"><a name="page_118" id="page_118"></a>{118}</span></p> - -<p><a name="fig_18" id="fig_18"></a></p> - -<div class="figcenter"> -<a href="images/i_p118_lg.jpg"> -<br /> -<img src="images/i_p118_sml.jpg" width="306" height="263" alt="Image unavailable: Fig. 18.—Semi-diagrammatic View of Two Villi of the -Small Intestines. (Magnified about 50 diameters.) - -a, substance of the villus; b, its epithelium, of which some -cells are seen detached at b2; c d, the artery and vein, -with their connecting capillary network, which envelopes and hides -e, the lacteal which occupies the centre of the villus and opens -into a network of lacteal vessels at its base." /></a> -<br /> -<span class="caption">Fig. 18.—Semi-diagrammatic View of Two Villi of the -Small Intestines. (Magnified about 50 diameters.) -</span> -<p class="caption"> -a, substance of the villus; b, its epithelium, of which some -cells are seen detached at b2; c d, the artery and vein, -with their connecting capillary network, which envelopes and hides -e, the lacteal which occupies the centre of the villus and opens -into a network of lacteal vessels at its base.</p> -</div> - -<p class="nind">the intestine. These are called <b>villi</b>; they are not unlike the papillæ -of the skin (<a href="#fig_15">Fig. 15</a>), if you suppose all the epidermis stripped except -the bottom row of cells (<i>d</i>), and the papilla itself pulled out a good -deal. <a href="#fig_18">Fig. 18</a> is a sketch to illustrate the structure of a <b>villus</b>. The -epithelium (<i>b</i>), you see, is made up of a single row of cells. Beneath -the epithelium, just as in the papilla of the skin, is a network of -blood capillaries, shown, for convenience, in the right-hand villus -only. But besides the blood capillaries, there is in each villus, what -there is not in a papilla of the skin, another capillary (shown, for -convenience, in the left-hand villus only) which does not contain blood, -which is not connected with any artery or with any vein, but which -begins in the villus. This is a <b>lacteal</b>. I have said nothing of these at -present.<span class="pagenum"><a name="page_119" id="page_119"></a>{119}</span> In most parts of the body we find, besides blood capillaries, -fine passages very much like capillaries, except that they contain a -colourless fluid instead of blood, and do not branch off from any larger -vessels like arteries. They seem to start out of the part in which they -are found, like the roots of a plant in the soil. But though unlike -blood capillaries in not branching off from larger trunks, they resemble -capillaries in joining together to form larger trunks corresponding to -veins, and the colourless fluid flows from the fine capillary channels -towards these larger trunks. This colourless fluid is called <b>lymph</b>; it -is very much like blood without the red corpuscles, and the channels in -which it flows are called <b>lymphatics</b>.</p> - -<p>The lymphatics from nearly all parts of the body join at last into a -great trunk called the <b>thoracic duct</b>, which empties itself into the -great veins of the neck, as is shown in the diagram, <a href="#fig_6">Fig. 6</a>, <i>Lct.</i>, -<i>Ly.</i>, <i>Th. D.</i></p> - -<p>Now, many of the lymphatics start from the innumerable villi of the -intestine, and are there called <b>lacteals</b> (<a href="#fig_6">Fig. 6</a>, <i>Lct.</i>); so that -lacteals may be said to be those lymphatics which have their roots in -the villi of the intestine.</p> - -<p>But what has all this to do with the digestion of fat? Lacteal means -<b>milky</b>, and the lymphatics coming from the villi are called lacteals -because, when digestion is going on, the fluid in them, instead of being -transparent as in the rest of the lymphatics, <b>is white and milky</b>. Why is -it thus white and milky? Because it is crowded with minute particles of -fat, and those minute particles of fat come from the inside of the -intestine. They are the same<span class="pagenum"><a name="page_120" id="page_120"></a>{120}</span> minute particles into which the bile and -pancreatic juice have divided the fat taken as food. We know this -because when no fat is eaten the lacteals do not get milky; and when for -any reason bile and pancreatic juice are prevented from getting into the -intestine, though ever so much fat be eaten, it does not get into the -lacteals at all, it remains in the intestine in great pieces, and is -finally cast out as useless.</p> - -<p><b><a name="sect_52" id="sect_52">52.</a></b> This, then, is what becomes of the food-stuffs:—</p> - -<p>The fats are broken up by the bile and pancreatic juice into minute -particles. These minute particles, we do not exactly know how, pass -through the epithelium of the villus into the lacteal vessels, from the -lacteals into the thoracic duct, and from the thoracic duct into the -vena cava. Thus the fats we eat get into the blood.</p> - -<p>The starch is changed into sugar in the mouth by saliva, and in the -intestine by the pancreatic juice; but sugar passes readily through -membranes, and so slips into the blood capillaries of the walls of the -alimentary canal. Thus all the sugar we eat, and all the goodness of the -starch we eat, pass into the blood.</p> - -<p>The proteids are dissolved in the stomach by the gastric juice, and what -passes the stomach is dissolved in the intestine, dissolved in such a -way that it can pass through membranes; and thus proteids pass into the -blood.</p> - -<p>Probably some of the sugar and proteids pass into the lacteals as well.</p> - -<p>The minerals are dissolved either in the mouth, or in the stomach, or in -the intestine, and pass into the blood.<span class="pagenum"><a name="page_121" id="page_121"></a>{121}</span></p> - -<p>And water passes into the blood everywhere along the whole length of the -canal.</p> - -<p>When we eat a piece of bread, while we are chewing it in our mouth it is -getting moistened and mixed with saliva. Part of its starch is thereby -changed into sugar, and all of it is softened and loosened. Passing into -the stomach, some of the proteids are dissolved out by the gastric -juice, and pass into the blood, and all the rest of the bread breaks up -into a pulpy mass. Passing then into the intestine, what is left of the -starch is changed by the pancreatic juice into sugar, and is at once -drained off either into the lacteals or straight into the blood. In the -intestine what remains of the proteids is dissolved, till nothing is -left but the shells of the tiny chambers in which the starch and -proteids were stored up by the wheat-plant as it grew.</p> - -<p>When we eat a piece of meat, it is torn into morsels by the teeth and -well moistened by saliva, but suffers else little change in the mouth. -In the stomach, however, the proteids rapidly vanish under the action of -the gastric juice. The morsels soften, the fibres of the muscle break -short off and come asunder; the fat is set free from the chambers in -which it was stored up by the living ox or sheep, and, melted by the -warmth of the stomach, floats in great drops on the top of the softened -pulpy mass of the half-digested food. Rolled about in the stomach for -some time by the contraction of the muscles which help to form the -stomach walls, losing much of its proteids all the while to the hungry -blood, the much-changed meat is squeezed into the intestine. Here the -bile and the pancreatic juice, breaking up the fat into tiny<span class="pagenum"><a name="page_122" id="page_122"></a>{122}</span> particles, -mix fat, and broken meat, and empty wrappings, and salts, and water, all -together into a thick, dirty, yellowish cream. Squeezed along the -intestine by the contraction of the muscular walls, the goodness of this -cream is little by little sucked up. The fat goes drop by drop, particle -by particle, into the lacteals, and so away into the blood. The -proteids, more and more dissolved the further they travel along the -canal, soak away into blood-vessel or into lacteal. The salts and the -water go the same way, until at last the digested meat, with all its -goodness gone, with nothing left but indigestible wrappings, or perhaps -as well some broken bits of fibre or of fat, is cast aside as no longer -of any use.</p> - -<p><b>Thus all food-stuffs, not much altered, with all their goodness -unchanged, pass either at once into the blood, or first into the -lacteals and then into the blood, and the useless wrappings of the -food-stuffs are cast away.</b></p> - -<p>While we are digesting, the blood is for ever rushing along the branches -of the aorta, through the small arteries and capillaries of the stomach -and intestine, along the branches of the portal vein, and so through the -liver back to the heart; and during the few seconds it tarries in the -intestine, it loads itself with food-stuffs from the alimentary canal, -becoming richer and richer at every round. While we are digesting, the -thoracic duct is pouring, drop by drop, into the great veins of the neck -the rich milky fluid brought to it by the lacteals from the intestine, -and as the blood sweeps by the opening of the thoracic duct on its way -down from the neck to the heart, it carries that rich milky fluid with -it, and the heart scatters it again all over the body.<span class="pagenum"><a name="page_123" id="page_123"></a>{123}</span></p> - -<p><b>Thus the blood feeds on the food we eat, and the body feeds on the -blood.</b></p> - -<h2><a name="HOW_THE_BLOOD_GETS_RID_OF_WASTE_MATTERS_VIII" id="HOW_THE_BLOOD_GETS_RID_OF_WASTE_MATTERS_VIII"></a>HOW THE BLOOD GETS RID OF WASTE MATTERS. § VIII.</h2> - -<p><b><a name="sect_53" id="sect_53">53.</a></b> But if the blood is thus continually being made rich by things, it -must also as continually be getting rid of things. The things with which -it parts are not, however, the same as those which it takes. The blood, -as we have said, is fuel for the muscles, for the brain, and for other -parts of the body. These burn the blood, burn it with heat but without -light. But, as you have learnt from your Chemistry Primer, Art. 4, -burning is only change, not destruction; in burning nothing is lost. If -the muscle burns blood, it burns it into something; that something, -being already burnt, cannot be burnt again, and must be got rid of.</p> - -<p>Into what things does the body burn itself while it is alive?</p> - -<p>I have already said that if you were to take a piece of meat or some -blood, and dry it and burn it, you would find that it was turned into -four things—water, carbonic acid, ammonia, and ashes. The body is made -up of nitrogen, carbon, hydrogen, and oxygen, with sulphur, phosphorus, -and some other elements. The nitrogen and hydrogen go to form ammonia; -the hydrogen, with the oxygen of combustion, forms water; the carbon, -carbonic acid; the phosphorus, sulphur, and other elements go to form -phosphates, sulphates, and other salts.</p> - -<p><b>In whatever way the body be oxidized, whether it be rapidly burnt in a -furnace, whether it be slowly oxidized after death, as<span class="pagenum"><a name="page_124" id="page_124"></a>{124}</span> when it moulders -away either above ground or in the soil, whether it be quickly oxidized -by living arterial blood while still alive—in all these several ways -the things into which it is burnt, into which it is oxidized, are the -same. Whatever be the steps, the end is always water, carbonic acid, -ammonia, and salts.</b></p> - -<p>These are the things which are always being formed in the blood through -the oxidation of the body, these are the things of which the body has -always to be getting rid.</p> - -<p>In addition to the water which comes from the oxidation of the solids of -the body, we are always taking in an immense quantity of water; partly -because it is absolutely necessary that our bodies within should be kept -continually moist, partly because food cannot pass into the blood except -when dissolved in water, and partly because we need washing inside quite -as much as outside; if we had not, so to speak, a stream of water -continually passing through our bodies to wash away all impurities, we -should soon be choked, just as an engine is choked with soot and ashes -if it be not properly cleaned. We have, then, to get rid daily of a -large quantity of washing water over and above that which comes from the -burning of the hydrogen of our food.</p> - -<p>We have already seen that a great deal of the carbonic acid goes out by -the lungs at the same time that the oxygen comes in. A large quantity of -water escapes by the same channel. You very well know that however dry -the air you breathe, it comes out of your body quite wet with water.<span class="pagenum"><a name="page_125" id="page_125"></a>{125}</span></p> - -<p>We have also already seen how the blood secretes sweat into the -sweat-glands, and so on to the skin. Perspiration is little more than -water with a little salt in it. The skin, therefore, helps to purify the -blood through the sweat-glands, by getting rid of water with a little -salt. You must remember that a great deal of water passes away from your -skin without your knowing it. Instead of settling on the skin in drops -of sweat, it passes off at once as vapour or steam. Some carbonic acid -also makes its way from the blood through the skin.</p> - -<p><b><a name="sect_54" id="sect_54">54.</a></b> It only remains for us to inquire, In what way does the blood get -rid of the ammonia and the rest of the saline matters that do not pass -through the skin?</p> - -<p>These are secreted from the blood by the kidney, dissolved in a large -quantity of water in the form of <b>urine</b>.</p> - -<p>What is the kidney? You will learn more about this organ by and by. -Meanwhile it will for our present purpose be sufficient to say that a -kidney is a bundle of long tubular glands, not so very unlike -sweat-glands, all bound together into the rounded mass whose appearance -is familiar to you. <b>Into these glands the blood secretes urine just as -it secretes sweat into the sweat-glands</b>. The glands themselves unite -into a common tube or duct which carries the urine into the receptacle -called the urinary bladder, from whence it is cast out when required.</p> - -<p>What is urine? Urine is in reality water holding in solution several -salts, and in particular containing a quantity of ammonia. The ammonia -in urine is<span class="pagenum"><a name="page_126" id="page_126"></a>{126}</span> generally in a particular condition, being combined with a -little carbonic acid, in the form of what is called <b>urea</b>. If urea is not -actually ammonia, it is at least next door to it.</p> - -<p>The three great channels, then, by which the blood purifies itself, by -which it gets rid of its waste, are the lungs, the kidneys, and the -skin. Through the lungs, carbonic acid and water escape; through the -kidneys, water, ammonia in the shape of urea, and various salts; through -the skin, water and a few salts. As the blood passes through lung, -kidney, and skin, it throws off little by little the impurities which -clog it, one at one place, another at another, and returns from each -purer and fresher. The need to get rid of carbonic acid and to gain a -fresh supply of oxygen is more pressing than the need to get rid of -either ammonia or salts. Hence, while all the blood which leaves the -left ventricle has to pass through the lungs before it returns to the -left ventricle again, only a small part of it passes through the -kidneys, just enough to fill at each stroke the small arteries leading -to those organs. The blood craves for great draughts of oxygen, and -breathes out great mouthfuls of carbonic acid, but is quite content to -part with its ammonia and salts in little driblets, bit by bit.</p> - -<p>The three channels manage between them to keep the blood pure and fresh, -working hard and clearing off much when much food or water is taken or -much work is done, and taking their ease and working slow when little -food is eaten or when the body is at rest.<span class="pagenum"><a name="page_127" id="page_127"></a>{127}</span></p> - -<h2><a name="THE_WHOLE_STORY_SHORTLY_TOLD_IX" id="THE_WHOLE_STORY_SHORTLY_TOLD_IX"></a>THE WHOLE STORY SHORTLY TOLD. § IX.</h2> - -<p><b><a name="sect_55" id="sect_55">55.</a></b> And now you ought to be able to understand how it is that we live on -the food we eat.</p> - -<p>Food, inasmuch as it can be burnt, is a source of power. In burning it -gives forth heat, and heat is power. If we so pleased, we might burn in -a furnace the things which we eat as food, and with them drive a -locomotive or work a mill; if we so pleased, we might convert them into -gunpowder, and with them fire cannon or blast rocks. Instead of doing -so, we burn them in our own bodies, and use their power in ourselves.</p> - -<p>Food passing into the alimentary canal is there digested; the nourishing -food-stuffs are with very little change dissolved out from the -innutritious refuse; they pass into and become part and parcel of the -blood.</p> - -<p>The blood, driven by the unresting stroke of the heart’s pump, courses -throughout the whole body, and in the narrow capillaries bathes every -smallest bit of almost every part. Kept continually rich in combustible -material by frequent supplies of food, the blood as well at every round -sucks up oxygen from the air of the lungs; and thus arterial blood is -ever carrying to all parts of the body, to muscle, brain, bone, nerve, -skin, and gland, stuff to burn and oxygen to burn it with.</p> - -<p>Everywhere oxidation, burning, is going on, in some spots or at some -times fiercely, in other spots or at other times faintly, changing the -arterial blood rich in oxygen to venous blood poor in oxygen. From most -places where oxidation is going on, the venous blood<span class="pagenum"><a name="page_128" id="page_128"></a>{128}</span> goes away hotter -than the arterial which came; and all the hot blood mingling together -and rushing over the whole body keeps the whole body warm. Sweeping as -it continually does through innumerable little furnaces, the blood must -needs be warm. This is why <b>we</b> are warm. But from some places, as from -the skin, the venous blood goes away cooler than the arterial which -came, because while journeying through the capillaries of the skin it -has given up much of its heat to whatever is touching the skin, and has -also lost much heat in turning liquid perspiration into vapour. This is -why so long as we are in health we never get hotter than a certain -degree of temperature, the so-called blood-heat, 98° Fahr., and why we -make warm the clothes which we wear and the bed in which we sleep.</p> - -<p>Everywhere oxidation is going on, oxidation either of the blood itself -or of the structures which it bathes, and whose losses it has to make -good. Everywhere change is going on. Little by little, bit by bit, every -part of the body, here quickly, there slowly, is continually mouldering -away and as continually being made anew by the blood. Made anew -according to its own nature. Though it is the same blood which is -rushing through all the capillaries, it makes different things in -different parts. In the muscle it makes muscle; in the nerve, nerve; in -the bone, bone; in the glands, juice. Though it is the same blood, it -gives different qualities to different parts: out of it one gland makes -saliva, another gastric juice: out of it the bone gets strength, the -brain power to feel, the muscle power to contract.</p> - -<p>When the biceps muscle contracts and raises the<span class="pagenum"><a name="page_129" id="page_129"></a>{129}</span> arm, it does work. The -power to do that work, the muscle got from the blood, and the blood from -the food. All the work of which we are capable comes, then, from our -food, from the oxidation of our food, just as the power of the -steam-engine comes from the oxidation of its fuel. But you know that in -the steam-engine only a very small part of the power, or energy, as it -is called, of the fuel goes to move the wheel. By far the greater part -is lost in heat. So it is with our bodies: all the force we can exert -with our bodies is but a small part of the power of our food; all the -rest goes to keep us warm.</p> - -<p>Visiting all parts of the body, rebuilding and refreshing every spot it -touches, the blood current also carries away from each organ the waste -matters of which that organ has no longer any use. Just as each part or -organ has different properties and different work, so also is the waste -of each not exactly the same, though all are alike inasmuch as they are -all the results of oxidation. The waste of the muscle is not exactly the -same as the waste of the brain or of the liver. Possibly the waste -things which the blood bears from one organ may be useful to another, -and so be made to do double work, just as the tar which the gasworks -throw away makes the fortune of the colour manufacturer.</p> - -<p>Be this as it may, the waste products of all parts, travelling hither -and thither in the body, come at last to be brought down to very simple -things, with all their virtue gone out of them, with all, or all but -all, their power of burning lost, fit for nothing but to be cast away, -come at last to be urea or ammonia, carbonic acid, and salts. In this -shape, the food, after a<span class="pagenum"><a name="page_130" id="page_130"></a>{130}</span> longer or shorter sojourn in the body, having -done its work, having built up this or that part, having helped the -muscle to contract or the liver to secrete, having by its burning given -rise to work or to heat, goes back powerless to the earth and air from -which it came. And so the tale is told.</p> - -<h2><a name="HOW_WE_FEEL_AND_WILL_X" id="HOW_WE_FEEL_AND_WILL_X"></a>HOW WE FEEL AND WILL. § X.</h2> - -<p><b><a name="sect_56" id="sect_56">56.</a></b> One other matter we have to note before we have given the full -answer to the question why we move.</p> - -<p>We have seen that we move by reason of our muscles contracting, and that -in a general way a muscle contracts because a something started in the -brain by our will passes down from the brain through more or less of the -spinal cord, along certain nerves till it reaches the muscle. It is this -something, which we may call a <b>nervous impulse</b>, which causes the muscle -to contract.</p> - -<p>But what leads us to exercise our wills? What starts the nervous -impulse?</p> - -<p>All the nerves in the body do not end in muscles. Many of them end, for -instance, in the skin, in those papillæ of which I spoke a little while -ago. These nerves cannot be used for carrying nervous impulses from the -brain to the skin. By an effort of the will you can make your muscles -contract; but try as much as you can, you cannot produce any change in -your skin.</p> - -<p>What purpose do these nerves serve, then? If you prick or touch your -finger, you feel the prick or touch; you say you have <b>sensation</b> in your -finger. Suppose<span class="pagenum"><a name="page_131" id="page_131"></a>{131}</span> you were to cut across the nerves which lead from the -skin of your finger along your arm up to your brain. What would happen? -If you pricked or touched your finger, you would not feel either prick -or touch. You would say you had lost all sensation in your finger. These -nerves ending in the finger then, have a different use from those ending -in the muscle. <b>The latter carry impulses from the brain to the muscle, -and so, being instruments for causing movements, are called motor -nerves. The former, carrying impulses from the skin to the brain, and -being instruments for bringing about sensations, are called sensory -nerves.</b> All parts of the skin are provided with these sensory nerves, -but not to the same extent. The parts where they abound, as the fingers, -are said to be very sensitive; the parts where they are scanty, as the -back of the trunk, are said to be less sensitive. Other parts besides -the skin have also sensory nerves.</p> - -<p>Motor nerves are of one kind only; they all have one kind of work to -do—to make a muscle contract. But there are several kinds of sensory -nerves, each kind having a special work to do. The several works which -these different kinds of sensory nerves have to do are called <b>the -senses</b>.</p> - -<p>The work of the nerves of the skin, all over the body, is called the -<b>sense of touch</b>. By touch you can learn whether a body is rough or -smooth, wet or dry, hot or cold, and so on.</p> - -<p>You cannot, however, by touch distinguish between salt and sugar. Yet -directly you place either salt or sugar on your tongue you can recognize -it, because<span class="pagenum"><a name="page_132" id="page_132"></a>{132}</span> you then employ sensory nerves of another kind, the nerves -which give us the <b>sense of taste</b>. So also we have nerves of <b>smell</b>, -nerves of <b>hearing</b>, and nerves of <b>sight</b>.</p> - -<p>The nerves of touch, where they end, or rather where they begin in the -skin, sometimes have and sometimes have not, little peculiar structures -attached to them, little <b>organs of touch</b>. So also the nerves of taste, -and smell, end or rather begin in a peculiar way. When we come to the -nerves of hearing and of seeing, we find these beginning in most -elaborate and complicated organs, the ear and the eye.</p> - -<p>Of all these <b>organs of the senses</b> you will learn more hereafter; -meanwhile, I want you to understand that by means of these various -sensory nerves, we are, so long as we are alive and awake, receiving -impressions from the external world, sensations of touch, sensations of -roughness and smoothness, of heat and cold, sensations of good and bad -odours, sensations of tastes of various kinds, sensations of all manner -of sounds, sensations of the colours and forms of things.</p> - -<p>By our skin, by our nose, by our tongue and palate, by our ears, and -above all by our eyes, impressions caused by the external world are for -ever travelling up sensory nerves to the brain; thither come also -impressions from within ourselves, telling us where our limbs are and -what our muscles are doing. Within the brain these impressions become -sensations. They stir the brain to action; and the brain, working on -them and by them, through ways we know not of, governs the body as a -conscious intelligent will.</p> - -<p><span class="pagenum"><a name="page_133" id="page_133"></a>{133}</span> </p> - -<p><span class="pagenum"><a name="page_134" id="page_134"></a>{134}</span> </p> - -<hr /> - -<p class="c"> -<span class="spc">NICHOLSON’S GEOLOGY.</span><br /> -<br /> -<i>Text-Book of Geology, for Schools and Colleges.</i><br /> -<br /> -By <span class="smcap">H. Alleyne Nicholson, M. D., D. Sc., M. A., Ph. D.,<br /> -F. R. S. E., F. G. S.</span>, etc., Professor of Natural History<br /> -and Botany in University College, Toronto.<br /> -<br /> -<i>12mo. 266 pages. Price, $1.50.</i><br /> -</p> - -<p>This work is thoroughly adapted for the use of beginners. At the same -time the subject is treated with such fulness as to render the work -suitable for advanced classes, while it is intended to serve as an -introduction to a larger work which is in course of preparation by the -author.</p> - -<hr /> - -<p class="c"> -<span class="spc">NICHOLSON’S ZOOLOGY.</span><br /> -<br /> -<i>Text-Book of Zoology, for Schools and Colleges.</i><br /> -<br /> -BY SAME AUTHOR AS ABOVE.<br /> -<br /> -<i>12mo. 353 pages. Price, $1.75.</i><br /> -</p> - -<p>In this volume much more space has been devoted, comparatively speaking, -to the Invertebrate Animals, than has usually been the case in works of -this nature: upon the belief that all teachings of Zoology should, where -possible, be accompanied by practical work, while the young student is -much more likely to busy himself practically with shells, insects, -corals, and the like, than with the larger and less attainable -Vertebrate Animals.</p> - -<p>Considerable space has been devoted to the discussion of the principles -of Zoological classification, and the body of the work is prefaced by a -synoptical view of the chief divisions of the animal kingdom.</p> - -<p>⁂ A copy of any of the above works, for examination, will be sent by -mail, post-paid, to any Teacher or School-Officer remitting one-half its -price.</p> - -<p class="c"> -D. APPLETON & CO., <span class="smcap">Publishers,<br /> -549 & 551 Broadway, New York</span>.<br /> -</p> - -<div class="footnotes"><p class="cb">FOOTNOTES:</p> - -<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> It is unusual for muscles to have two tendons at the same -end. Hence the name <b>biceps</b>, or “two-headed.”</p></div> - -<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> From <i>pulmo</i>, <b>lung</b>; the artery of the lung.</p></div> - -<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> From <i>hepar</i>, <b>liver</b>; the vein of the liver.</p></div> - -</div> - -<p><a name="transcrib" id="transcrib"></a></p> - -<table border="0" cellpadding="0" cellspacing="0" summary="" -style="padding:2%;border:3px dotted gray;"> -<tr><th align="center">Typographical errors corrected by the etext transcriber:</th></tr> -<tr><td align="left">would be unvailing=> would be unavailing {pg 34}</td></tr> -<tr><td align="left">cordæ tendineæ=> chordæ tendineæ {pg 70}</td></tr> -<tr><td align="left">the triscuspid valve between=> the tricuspid valve between {pg 72}</td></tr> -<tr><td align="left">the body may may=> the body may {pg 103}</td></tr> -</table> - -<hr class="full" /> - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of Physiology, by M. 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