Physiology

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.

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.

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

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?

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.

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

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.,
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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.

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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.

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