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.