The commonwealth of cells

I.

In the preceding essay we regarded protoplasm as a chemical factor in the universe.

We have seen how it is always changing, always taking in food, always giving off waste materials. We have seen, too, that it grows and that it does work, and that in a large mass the cells which compose it share the labour instead of each component cell performing all the vital functions. We have now to consider the work which protoplasm does—in a word, the mechanical effect of the chemical actions just described.

The simplest movement of protoplasm is to be seen by the aid of the microscope in certain vegetable cells, where granules seem always streaming about in different directions. A step higher, and we find this streaming movement converted into movements of the whole cell. In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. Along the fibrils at intervals are minute globules, and by watching the distance between them it is seen that the fibrils undergo changes in length, pulling in the membrane when they shorten, and letting the cell flow out in any direction when they relax. By adjusting these two movements to balance one another, the cell can move in any direction, surround and engulf particles of food, and assume a strange variety of shapes. (See Diagram 1.)

In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. (See Diagram 15.)

Others manage differently. In them the nucleus simply bursts, and turns its essential elements, a number—always a constant number—of coarse threads, adrift. Meanwhile, two centrosomes have moved to opposite ends of the cell, and there anchored themselves by fibrils; other fibrils springing from them become attached to the nuclear threads, and when all is ready pull them apart, equally divided, to their respective ends, where they re-form into two fresh nuclei. (See Diagram 16.)

Unicellular animals, which are constant in shape and swim instead of flowing when they want to get anywhere, have at first sight nothing in common with those which do the latter. From their surfaces spring fringes of free protoplasmic threads, called cilia, from their fancied resemblance to eyelashes, which serve as motor organs, and beat the water like oars. (See Diagram 2.) Waves of movement, as they lash one after another, all in the same direction, seem to pass over the cell, and it is propelled through the water; while others, which are situated in the neighbourhood of the cell’s mouth, stir the water into eddies, and drive food particles into it.

These cilia are important, as they are adapted for many purposes in large animals. The cells which line the cavity at the back of the nose, the tubes of the lungs, and other parts of the body, have a few cilia on their free surface, and it is in them that the structure of these organs can best be made out. At the foot of each cilium is a minute globule, from which a fine fibril passes into the cell, and the fibrils, collectively forming a leash, are attached to its opposite end. (See Diagram 17.) It seems highly probable that the globule is a centrosome giving rise to two fibrils, one attached as described, the other passing up one side of the cilium, and fast to its apex. The result of this arrangement is that when the fibrils contract the cilium is bent over with a jerk to the side up which the fibril runs, and when they relax it slowly straightens itself. There is, therefore, no fundamental difference between this and the other mode of progression; both are dependent upon the centrosome.

Finally we have muscle cells. These are only found in a fairly complicated animal, since they are a product of the division of labour principle, and their sole business is movement. There are two varieties of muscle, but the principle is the same in both—a long thin cell, with fibrils traversing its length whose contraction causes the cell to shorten and thicken, thus reducing the distance between its two ends. At present the development of muscle and the way in which it ‘contracts,’ to use the word accepted in this case for describing a redistribution of bulk, are little understood, and there are accordingly many opinions; but I think careful study will eventually show that some modification of the centrosome, with its contractile fibrils, is responsible for the movement.

The two varieties are: the smooth, or involuntary, and the striped, or voluntary, muscle. Smooth muscle consists of spindle-shaped cells with one elongated nucleus. (See Diagram 18, Fig. 1.) It only contracts very slowly, and is not under control of the will; but it is very abundant in the body, since it effects practically all the movements of the alimentary canal and bloodvessels. Voluntary or striped muscle, so called from its appearance under the low power of a microscope, consists of long fibres, each containing many nuclei. (See Diagram 18, Fig. 2.) Its protoplasm is rich in hæmoglobin, and in it, under powerful microscopes, can be made out two kinds of fibrils: Rutherford’s fibrils, the complicated structure of which gives muscle its striped appearance; and Marshall’s fibrils, which are much finer and more difficult to see. The muscle of the heart, though not under control of the will, is striped; but it differs from ordinary striped muscle in being made up of small branched cells with only one nucleus.

The way in which the three elements of striped muscle contribute to a contraction is practically unknown, and the subject of much dispute. In fact, one could hardly wish for a better soil for theories, and some which grow in it are very wonderful indeed. We have reason for supposing that there are two contractile substances—one which gives a sharp twitch, the other a slow, hard pull; and on the whole there seems good reason to believe that Rutherford’s fibrils give the sudden movements, while Marshall’s give the more forcible ones; and that the ordinary protoplasm of the cell is restricted to the duty of nourishing the fibrils.

The muscle cells are modified from among those of the bud forming the middle layers of the embryo. (See Diagram 5.) Other cells of this bud form connective tissue, by, so to speak, spinning long fibres of the substance called collagen, which turns to gelatin when boiled. (See Diagram 20.) This connective tissue permeates the whole body, affording a firm foundation for the many layers of cells which form the skin and the single layer of digestive cells; supporting the other organs throughout, and keeping the different parts of the body in their places, in doing which, however, it is assisted by other fibres which are not collagenous, but elastic. It also forms tracts which become lymph and blood vessels.

In parts of the animal which require special support it forms solid rods, the collagen combining with calcium salts to form a clear, hard substance—cartilage. At one period in the development of an animal or animals we find the only solid support is cartilage, but cartilage is not sufficiently rigid for a very large beast, especially on land, so is only used for outlying parts, the main framework being bone.

Bone is formed very much as if Nature were rectifying a mistake. When a rod of cartilage is unequal to its work it is eaten hollow, and fresh connective-tissue cells immigrate and fill up the cavity, eventually laying down a fine network of cells in its place, the meshes of which are filled with inorganic calcium salts, chiefly phosphate of lime. Nature then benefits by experience, and the last bones to be formed are not preceded by any makeshift cartilage, but built up straight away in ordinary connective tissue.

This brings us back again to muscle, for the object of nearly all the voluntary muscle is to cause movement among the bones. For this purpose the muscle cells or fibres are arranged parallel to one another, and bound up together by connective tissue, the whole bundle being known as ‘a muscle.’ The two ends of a muscle are attached to two bones by connective tissue, which sometimes forms a short cord, or tendon. Then, when the muscle contracts, the two places of its attachment are pulled towards one another, and something has to move. But before saying more about the way in which the bones are jointed and muscles attached—in fact, what movements are possible in the human body—it would be as well here to describe the chief properties of muscle and the way in which they are studied.

II.

The way in which voluntary muscle is studied is very simple. A frog is killed by thrusting a probe into the brain and down the spinal cord, and a muscle is then dissected out and attached to a piece of apparatus (see Diagram 21) in such a way that on its contracting it raises a lever, and draws a line on a moving surface. The rate at which the surface is moving is ascertained, so that the nature of the curve, which is a graphic record of the contraction, can be analyzed. (See Diagram 22.) For instance, when an electric shock is used to make the muscle contract, we find that a slight shock causes a small contraction, as shown by a low curve, while a stronger one, up to a certain point, causes an increase.

But having described how muscle is studied, it is only necessary to state a few facts concerning it; to discuss muscle, fully describing the experiments by which its more obscure properties have been elucidated, and the devices by which causes of error have been eliminated, would fill volumes.

Muscle is thrown into a state of contraction by an impulse reaching it from a nerve, but it contracts quite as readily if excited directly by a mechanical or electrical shock. A second shock causes a second contraction, or, if the muscle is still in a state of contraction owing to the first, causes it to contract still more. (See Diagram 23.) If a number of stimuli are applied to a muscle in such rapid succession that the effect of the preceding one has not passed off by the time the next arrives, it will contract as far as possible, and remain contracted—a state known as tetanus. (See Diagram 24.) A muscle is therefore kept in a state of contraction by a continuous nervous effort, not arranged and then left contracted.

Various conditions alter the character of a muscular response. With repeated stimuli at short intervals a muscle fatigues, and each contraction becomes smaller in extent and longer in duration. (See Diagrams 25 and 26.) If the muscle has to lift a load it has a certain check on its contraction, and its relaxation time is shortened. Temperature also affects muscular contraction, moderate increase causing a sharper, and moderate cooling a slower, rise and fall of the lever on stimulation. (See Diagram 27.) Lastly, we have drugs which exert an influence, but the only one of these which it is necessary to mention here is veratria, which makes the slowly contracting fibrils continue their activity after the quick ones have subsided. (See Diagram 28.)

Finally, there are the electrical changes in muscle. These, again, may be passed over briefly, since they are not easily understood or described. To put the facts in a nutshell, the part of a muscle which is in activity is negative to all other parts. Thus, if a muscle be dissected out and cut across, the activity at the seat of the injury, while it lasts, causes a current to pass through a galvanometer from uninjured parts to the wounded. (See Diagram 29.) Again, if a muscle be dissected out without injury, connected at two points with a galvanometer, and then stimulated at one end, as the wave of contraction passes along it, first one, then the other, contact becomes negative. (See Diagram 30.) S, Stimulating electrodes; N, contraction which marks the wave of excitation passing along the muscle; G, galvanometer which shows that the seat of activity (N) is negative to the rest of the muscle.

In passing, it may be mentioned that, as the heart is a muscle slung obliquely across the body, and waves of contraction are continually passing down its long axis, the whole body is affected by continual electrical changes. By very delicate instruments it can be demonstrated that with each beat the two hands alternately become electrically positive and negative to each other.

Whilst dealing with the electrical phenomena of muscle, it may be as well to state that nerve fibres, which are studied with very much the same apparatus, show the same electrical changes, the point of injury or of the greatest activity being negative to all the rest. Single cells are less easily investigated, but in glands it is possible to show that the same rule holds.

Undoubtedly the most curious fact about the generation of electricity by protoplasm is that, by a modification of muscle and nerve, which causes them to lose their ordinary properties, they are converted into a special organ for giving electric shocks. Armed with powerful batteries of this description, an otherwise rather helpless class of fish are enabled to defend themselves from their enemies, and deal unexpected death to their more agile prey.

Having now run over a few of the physical properties of protoplasm, we may pass on to a brief investigation of the movements we find in the body of man.

III.

In describing the movements of the body, we shall have to treat them as several and distinct, as indeed they are; but the fact should not be lost sight of that they cannot really be isolated: one idea embraces the whole. Two kinds of movement may, however, be distinguished in the vital functions: movement of the actual cells, such as muscles; and movement of non-protoplasmic elements acted upon by the cells—e.g., lymph.

There is a parallel to this in the chemical side of life, where we find some phenomena peculiar to the living elements, and others, like digestion, going on in the living body, but outside the cells.

Taking the movements in the natural order—that is, proceeding from the simpler to the more complex—the first to be considered is undoubtedly that of the leucocytes, or general scavengers of the tissues. The body consists, so far as we have defined its anatomy, of three layers of cells, and its shape is that of a tube with hollow walls. (See Diagram 6.) Within the cavity of the body are various organs, such as the muscles, which are formed from the middle layer; and its space is largely reduced by glands, lungs, and other ramifications of the inner layer which forms the alimentary canal.

These organs hang more or less freely in the body cavity, slung to its walls by enveloping sheets of connective tissue, the whole being bathed in lymph. Now, in such an arrangement the products of wear and tear must accumulate. Cells here and there die for various reasons, and pieces of cells become detached even in adult animals. The interior of a bone is always being eaten away to decrease its weight, or in order that it may be replaced by fresh bone of a closer texture, and in young animals and embryos there are many structures which, useful for a time, have eventually to be removed; as an instance, we may quote the tadpole’s tail. In fact, if the tissues were left to themselves, the body would soon be choked with débris, and to avoid this it is supplied with an army of scavengers, the leucocytes.

The leucocytes are detached cells which owe their origin to the middle layer. In size they are, of course, very small, quite invisible to the naked eye. In appearance they resemble unicellular organisms of the amœba type, which we have had occasion to mention several times already (Essay II., Section II.; Essay III., Section I., Diagram 1). They are of several different varieties, some being larger and more active than others; but they all wander about in the lymph and blood like independent animals, creeping in and out between the cells of the organs, and devouring any foreign matter they come across. They sometimes multiply, like independent animals, by division, especially in the presence of inflammation, or when they have much work to do, and a rapid increase in their numbers is needed; and they have been induced to live, and feed, and multiply, outside the body (in which case they must be considered to have become independent organisms), thanks to the careful attentions of the experimenter.

Apart from their duties of devouring the inside layers of bones and clearing away dead tissue, they are supposed by some to assist in the absorption of food by creeping between the cells lining the alimentary canal, and, after throwing out arms to engulf particles of food, returning with their spoils into the body. Perhaps, however, the most interesting, or at any rate most romantic, of their many and important functions are what may be called their emergency duties. Frequently people, especially those who live in smoky towns, draw into their lungs particles of dust and soot, which if left adhering to the walls of the air cavities would cause dangerous irritation. As if by magic a leucocyte will discover the presence of such a nuisance, and, crawling between the cells forming the wall of the lung, in which, by the way, it is outside the body proper, will engulf it and carry it away with him. This exploit, however, pales beside the warfare which goes on in the body between leucocytes and invading bacteria. A bacterium thrives in the blood or lymph, since it finds itself in a warm alkaline fluid containing complex organic substances, by breaking down which it can easily obtain energy. Unfortunately, the products of such a process are frequently virulent poisons, the effect of which upon neighbouring cells produces the distressing symptoms which we associate with disease. No sooner, however, has the bacterium begun to generate poisons, than leucocytes, influenced by chemical attraction (Essay I.), swarm upon it. First come leucocytes of a small kind, full of zymogen granules, which crowd round the bacterium till they have covered it. After a time they creep away, leaving it dead. They are now in an exhausted condition, and no longer contain granules, having doubtless discharged them as a destructive ferment upon their enemy. Then a leucocyte of another kind moves to the attack, or, rather, to clear up the remains, for he is a large, non-granular, active fellow, and eats up the dead bacterium by the simple process of engulfing him whole. (See Diagram 31.)

A natural question arising out of the study of leucocytes is, What becomes of them? Particles of soot and similar refuse can hardly be considered nutritious, or even digestible, food, and one is rather drawn to the conclusion that the leucocyte performs its functions for the good of the body at large, not of itself, and that when its work is done it must die. Many leucocytes, probably, loaded with unconsidered and undesirable trifles, cast themselves into the alimentary canal, and are got rid of with the useless portions of the food; but they do not always have the luck or energy to get to a natural outlet. An unpleasantly familiar phenomenon is the boil. Here we have some irritating substance under the skin setting up inflammation, and leucocytes swarm up to remove the cause of the trouble. Before, however, this is done, many have perished in the fray, and they have collected in numbers to the formation of what is commonly known as pus, or matter. Their dispersal into the body is now neither easy nor desirable, and the surgeon usually lets them escape from the surface by a touch of the lancet.

Such, then, is very briefly the story of the leucocyte, neglecting such problems as the differences between those found in the blood, called white corpuscles to distinguish them from the red corpuscles, with which they have no sort of connection; those found in the lymph, called lymphocytes to distinguish them from those found in the blood; those caught in the act of devouring bone, called osteoclasts; and those found with bacteria inside them, therefore known as phagocytes; and without speculating on how long an individual lives, and whether the different varieties differ in origin or are merely at progressive stages of development. The study of leucocytes is one of the most fascinating in physiology, but we have many other things calling for our attention, and we have said enough about the part they play in the life of the body to justify our passing on to consider another essential movement.

IV.

Next in natural order for consideration come the movements of the alimentary canal.

So far we have considered this structure as a chemical laboratory, a tube consisting of a single layer of cells which secrete ferments into the lumen, where digestion takes place, and then absorb the products, and we have not yet accounted for the food travelling along the tube, without which its functions, as described in the earlier part of the book, could not be performed. That the passage of the food is not due to gravitation is obvious from the many directions of the tube’s coils—not to quote the old instance of a horse drinking, in which case the liquid first travels upwards. One must therefore conclude in favour of some muscular method of propulsion.

We have so far described the alimentary canal as a single layer of cells, but it must be obvious that these soft secreting portions of the tube are not capable of vigorous movement. The canal proper is surrounded by a tough sheath of connective tissue which prevents its being overdistended or ruptured, and, by means of a layer—or, rather, two layers—of non-striped muscle which it contains, produces the movements which result in the passage of its contents along the tube. These two layers lie well to the outside of the connective-tissue sheath. The fibres of the inner layer are arranged circularly, so as to form rings round the tube; those of the outer have a longitudinal direction, running, therefore, parallel with its long axis. When the former contract, the diameter of the tube is reduced, while contraction of the latter has the effect of enlarging it. (See Diagram 32.)

The movements of the intestine are what is known as peristaltic. Contraction of the muscle fibres is not simultaneous in all parts, but passes in waves along it. Just in front of the food the longitudinal fibres contract, and thus offer less resistance, while just behind the circular fibres reduce the size of the tube, and so get up a pressure. The result of a number of successive waves of contraction passing down the alimentary canal is that the food is propelled along it.

The arrangement of the muscle varies in places to suit special needs. Where the tube suddenly enlarges to form the stomach, and where the stomach suddenly narrows to the intestine, there are two strong rings of muscle, whose constricting influence converts the enlargement into a closed chamber during gastric digestion; while the coats which actually clothe it here run obliquely, and their activity causes the contents to be slowly churned about inside.

Thus it will be seen that it is not only the voluntary muscles which give the alimentary system its opportunities; without these unobtrusive non-striped cells we should toil for our bread and swallow it in vain.

V.

Our next step, after having surveyed the principle of movement by which the chemical necessities of the body are exposed to its absorbing surface, must be to see how the fluid which transports them is made to pass along the tubes containing it. We have already had occasion to describe how these blood and lymph vessels ramify through all the organs, when we were dealing with the chemical influence of the blood and lymph.

The tubes through which the lymph is brought back to the blood-stream have thin walls, and no muscle of their own. They are subjected, however, to a constantly varying pressure by the movements of the limbs and trunk, and as, owing to valves inside them, the lymph can only escape in one direction, there is a constant flow towards the junction with the bloodvessels.

The bloodvessels are quite different. A far more certain and expeditious current is necessary—hence the steady circulation through a system of closed tubes.

In order to understand this passage of the blood, it is necessary to keep in mind the great principle with which hydrostatics supplies us, viz., that a liquid always flows from a region of high to a region of lower pressure. The problem of the vascular system is, therefore: How can the pressure within a ring of tube be so arranged as to maintain a regular flow always in the same direction?

Let us begin with the structure of the system. The tube through which the blood first passes on leaving the heart is composed of four distinct and essential elements: A lining of endothelial cells, which we need not discuss at length; a main substance of tough white fibrous connective tissue; elastic fibres and muscle fibres, the two last arranged in the substance of the connective tissue. All these parts are present in the main arteries which leave the heart, but in the fine meshwork of capillaries to which the arteries give rise by repeated branchings there is nothing left of the outer coats, only the lining of endothelial cells separating the blood from the organ traversed. In the veins which these capillaries unite to form, the connective-tissue sheath reappears, and also some muscle; but the elastic coat is quite absent. The heart is really a double coil of the tube (see Diagram 12), in which the muscular coat is predominant, and is divided into four chambers by the valves, which insure the blood flowing in the right direction when it contracts. (See Diagram 33.)

The way in which these structures work is as follows: Two of the chambers of the heart (the auricles) receive blood from the veins, and when full suddenly contract, driving their contents into the other two chambers (the ventricles). The blood does not run back into the veins, although the pressure in them is very low and there are no valves to prevent it, because there is still less pressure in the ventricles, and also because the veins enter the auricles obliquely, and the tendency of the increasing pressure is to close their orifices. Having discharged the blood into the ventricles, the auricles relax, and the pressure within being a minus quantity, they are speedily filled with blood from the veins, blood not being able to return after entering the ventricles, as valves close automatically to prevent it.

Stimulated by the blood distending them, the ventricles then contract simultaneously like the auricles, only with much greater force: for the right ventricle has to drive the blood all through the vessels pervading the lungs back to the left auricle; whilst the left ventricle, which is proportionately stronger than the right, has to send its contents to the furthest extremities of the body. They then relax, in order that conditions of their internal pressure may favour another inflow from the auricles, return of blood from the arteries being, as in the preceding case, prevented by valves.

The pressure in the arteries during life is always fairly high; indeed, the ventricles have to get up a considerable force before the valves leading from them will open. The result of this is not only that the blood is driven along them with a rush, but also that they are slightly distended at each beat; and so, owing to the elasticity of their walls, the blood continues to flow forwards even between the beats of the heart. The rest of the journey is quite simple; the pressure in the capillaries is lower than in the arteries, and the pressure in the veins lower than in the capillaries, and lower in the veins, too, as they approach the heart, till, where they join the auricle, it is actually minus, and the blood has no other course open to it but to return to the auricle. It looks as though accidents might happen in the veins owing to there being so low a pressure there to direct the current, but this is prevented by the presence of valves at intervals, to stop any return.

The rate at which the blood travels is another point which has an important bearing on the nutrition. It does its work—i.e., gives out nutriment and picks up refuse—whilst flowing through the capillaries; so here one finds that it moves slowly. On the other hand, the sooner it reaches them the better, so it races fast through the arteries. Finally, its return to the heart need not be delayed, so it is quickened up again through the veins. The principle by which this variation in the rate of flow is obtained is simple and inevitable. If a tube through which liquid is flowing is not the same size all the way along, the liquid will be found to flow faster in the narrow parts than in the wider ones. Now, in branching, the arteries do not keep becoming smaller in regular proportion, and the result is that the capillaries have collectively a diameter five hundred times larger than the aorta; hence the blood flows through them only one-five-hundredth of the pace at which it leaves the heart. But in uniting again to form the veins their cross-section is reduced once more, so that that of the large veins near the heart is only two and a half times larger than that of the aorta, and hence a flow only two and a half times slower results.

The pace of the blood-stream must depend, obviously, on the pressure of the blood in the arteries. This pressure is altered either by changing the rapidity of the heart-beat or the diameter of the arteries, which are capable of considerable variation owing to their muscular coat. The regulation of the blood-pressure is managed by the nervous system, so does not belong here, and we may leave it after mentioning one or two facts. High pressure is due to a large quantity of blood being in the arteries, and this may be due either to the rapidity with which it is injected by the heart or to the reduced capacity of the bloodvessels themselves. High pressure, due to the latter cause, throws a great strain upon the heart, owing to the hard work it has in pumping blood into the arteries; with a low pressure the heart beats feebly, having less resistance to overcome.

Blood-pressure can be raised by stimulating the muscular coat and reducing the capacity of the bloodvessels, and lowered by causing the heart to beat more slowly or by removing blood from the body. This latter operation was a favourite way with doctors of the old school; but as our knowledge of physiology, and with it our control over the vital functions, increases, such crude and heroic remedies are able to be replaced by others which are less dangerous.

VI.

Comparisons are rightly regarded as objectionable, so it would hardly be safe to say that the group of movements whose primary object is filling the lungs, and which we must study next, is the most important in the body, especially when we have just been speaking of the circulation, which, however, would be of but little use if the blood could not be oxidized; but we can at least say that its importance cannot be overrated, so far-reaching are its effects.

The lungs are, as we have described them above, a pair of delicate membranous sacs connected by a tube, the trachea, with the alimentary canal, from which they originally budded out. They are subdivided, though how we need not describe in detail, into a vast number of small compartments, so as to give the maximum surface in the space accorded them, and the whole somewhat resembles a cluster of grapes, the stalks being the branches of the trachea. The membranous parts are pervaded by an elastic network, enveloping the compartments in such a way that it would reduce them permanently to the resemblance of a bunch of raisins rather than grapes, were it not that they are enclosed in an airtight box—the thorax—from the walls of which they cannot shrink without causing a vacuum. Owing, however, to the latter arrangement and the trachea being open to the external world, they are always more or less distended with air.

The thorax, which they thus must always exactly fill, is a conical-shaped box, its walls being the ribs, and its floor a sheet of muscle known as the diaphragm. It contains, besides the lungs, only the heart and large bloodvessels. The problem, therefore, of drawing air into the lungs and (after the gaseous interchange described in Essay II., Section IV., has taken place) of expelling it again, becomes solely a matter of increasing and decreasing the capacity of the thorax. (See Diagram 34.) This can be done in two ways: the diameter through the ribs can be increased, or the diaphragm can be pulled down, increasing its depth. Actually, both these methods come into play together. Diagram 35 will probably give a better idea of how this is done than could easily be conveyed by a verbal description. An attempt is here made to show the action of the ribs and the diaphragm—first, of each separately, then of the two combined. The elasticity of the lungs themselves is sufficient to drive out the tidal air if the diaphragm and the muscles of the ribs are relaxed, though in hard breathing a muscular movement may depress the ribs and a contraction of the abdominal muscles force up the diaphragm.

But though the primary object of raising the ribs and depressing the diaphragm may be to fill the lungs, its secondary influence upon the trunk as a whole is hardly less important. The effect upon the circulation is profound. The compartments of the lungs are enveloped in innumerable capillary bloodvessels, and, as these lie around and between them in the cavity of the thorax, they must, when breath is drawn in, be subjected to a negative pressure before the lung itself, and be the first to experience a positive pressure when the air is expelled. Here, again, a diagram is the best explanation. (See Diagram 36.)

The pulmonary vessels, moreover, are not the only ones influenced. The reader who attentively examined Diagram 13 must have been struck by the peculiarities of the circulation through the spleen, intestine and liver, and the obstacles which this repeated breaking up into fine vessels must offer to the flow of blood, as described in Section V. of this essay.

The liver forms the crux of the situation. (See Diagram 37.) A vein carrying blood from the intestine and spleen is broken up into fine capillaries to pass through that organ, and the pressure in this vein is extremely low. How is a sufficiently rapid flow of blood to be maintained? The answer to this riddle is best given by Diagram 38, which shows how, by the contraction of the diaphragm at each breath, the large veins entering the heart are subjected to a negative pressure which draws blood out of the liver, while, simultaneously, that organ is squeezed and the blood it contains forced out. Obviously this natural pump influences not only the flow of blood, but also that of the lymph, and what was said about the hepatic vessels also holds good for the thoracic duct, up which the lymph, rich with fat absorbed from the intestine, passes to be emptied into the large veins near the heart. So, though vigour in the action of the diaphragm is more favourable to health than necessary to life, deep breathing is an essential factor in the well-being of the body.