Chapter XII: Introduction: § I (2)

=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=

_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

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

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.