Part 9

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 artery=. We shall have more to say about it directly.