the veins is further checked by a contraction of the great veins which immediately precedes the systole of the auricles, and is practically continuous with it.

Therefore, when the auricles contract, little or none of the fluid which they contain will flow back into the veins; all the contents or nearly so will pass into and distend the ventricles. As the ventricles fill and begin to resist further distension, the blood, getting behind the auriculo-ventricular valves, will push them towards one another, and indeed almost shut them. The auricles now cease to contract, and immediately that their walls relax, fresh blood flows from the great veins and slowly distends them again.

But the moment the auricular systole is over, the ventricular systole begins. The walls of each ventricle contract vigorously, and the first effect of that contraction is to complete the closure of the auriculo-ventricular valves and so to stop all egress towards the auricle. The pressure upon the valves becomes very considerable, and they might even be driven upwards, if it were not for the chorda tendinea which hold down their edges.

As the contraction continues and the capacities of the ventricles become diminished, the points of the wall of the heart to which the chorda tendinea are attached approach the edges of the valves; and thus there is a tendency to allow of a slackening of these cords, which, if it really took place, might permit the edges of the valves to flap back and so destroy their utility. This tendency, however, is counteracted by the chorde tendineæ being connected, not directly to the walls of the heart, but to those muscular pillars, the papillary muscles, which stand out from its substance. These muscular pillars shorten at the same time as the substance of the heart contracts; and thus, just so far as the contraction of the walls of the ventricles brings the papillary muscles nearer the valves, do they, by their own contraction, pull the chorda tendinea as tight as before.

By the means which have now been described, the fluid in the ventricle is debarred from passing back into the auricle; the whole force of the contraction of the ventricular walls is therefore expended in overcoming the resistance presented by the semilunar valves. This resistance

is partly the result of the mere weight of the vertical column of blood which the valves support; but is chiefly due to the reaction of the distended elastic walls of the great arteries, for as we shall see, these arteries are already so full that the blood within them is pressing on their walls with great force.

It now becomes obvious why the ventricles have so much more to do than the auricles, and why valves are needed between the auricles and ventricles, while none are wanted between the auricles and the veins.

All that the auricles have to do is to fill the ventricles, which offer no active resistance to that process. Hence the thinness of the walls of the auricles, and hence the needlessness of any auriculo-venous valve, the resistance on the side of the ventricle being so insignificant that it gives way, at once, before the pressure of the blood in the veins.

On the other hand, the ventricles have to overcome a great resistance in order to force fluid into elastic tubes which are already full; and if there were no auriculoventricular valves, the fluid in the ventricles would meet with less obstacle in pushing its way backward into the auricles and thence into the veins, than in separating the semilunar valves. Hence the necessity, firstly, of the auriculo-ventricular valves; and, secondly, of the thickness and strength of the walls of the ventricles. And since the aorta, systemic arteries, capillaries, and veins form a system of tubes, which, from a variety of causes, offer more resistance than do the pulmonary arteries, capillaries, and veins, it follows that the left ventricle needs a thicker muscular wall than the right.

Thus, at every systole of the auricles, the ventricles are filled and the auricles emptied, the latter being slowly refilled by the pressure of the fluid in the great veins, which is amply sufficient to overcome the passive resistance of the relaxed auricular walls. And, at every systole of the ventricles, the arterial systems of the body and lungs receive the contents of these ventricles, and the emptied ventricles remain ready to be filled by the auricles.

13. We must now consider what happens in the arteries when the contents of the ventricles are suddenly forced into these tubes (which, it must be recollected, are already full).

If the vessels were tubes of a rigid material, like gaspipes, the forcible discharge of the contents of the left ventricle into the beginning of the aorta would send a shock, travelling with great rapidity, right along the whole system of tubes, through the arteries into the capillaries, through the capillaries into the veins, and through these into the right auricle; and just as much blood would be driven from the end of the veins into the right auricle as had escaped from the left ventricle into the beginning of the aorta; and that, at almost the same instant of time. And the same would take place in the pulmonary vessels between the right ventricle and left auricle.

However, the vessels are not rigid, but, on the contrary, very yielding tubes; and the great arteries, as we have seen, have especially elastic walls. On the other hand, the friction in the capillaries and small arteries is so great that the blood cannot pass through them into the veins as quickly as it escapes from the ventricle into the aorta. Hence the contents of the ventricle, driven by the force of the systole past the semilunar valves, are at first lodged in the first part of the aorta, the walls of which are stretched and distended by the extra quantity of blood thus driven into it. But as soon as the ventricle has emptied itself and no more blood is driven out of it to stretch the aorta, the elastic walls of this vessel come into play; they strive to go back again and make the tube as narrow as it was before; thus they return back to the blood the pressure which they received from the ventricle. The effect of this elastic recoil of the arterial walls is on the one hand to close the semilunar valves, and so prevent the return of blood to the heart, and, on the other hand, to distend the next portion of the aorta, driving an extra quantity of blood into it. And this second portion, in a similar way, distends the next, and this again the next, and so on, right through the whole arterial system. Thus the impulse given by the ventricle travels like a wave along the arteries, distending them as it goes, and ultimately forcing the blood through the capillaries into the veins, and so on to the heart again.

14. Several of the practical results of the working of

the heart and arteries just described now become intelligible. For example, between the fifth and sixth ribs, on the left side, a certain movement is perceptible by the finger and by the eye, which is known as the beating of the heart. It is the result of the striking of the apex of the heart against the pericardium, and through it, on the inner wall of the chest, at this point, at the moment of the systole of the ventricles. Even when the heart is at rest, the apex, in a standing position, lies close under this part of the chest wall; and when the systole takes place, not only does the apex, like the rest of the ventricle, become firm and hard, but by the peculiar movements of the heart and great blood-vessels, is brought sharply in contact with the chest wall at this point. It is this sudden shove of the hardened apex which we feel and see, and which we call the beating, or more correctly the impulse, of the heart.

15. Secondly, if the ear be applied over the heart, certain sounds are heard, which recur with great regularity, at intervals corresponding with those between every two beats. First comes a longish dull sound; then a short sharp sound; then a pause; then the long, then the sharp sound, then another pause; and so on. There are many different opinions as to the cause of the first sound; some physiologists regard it as a muscular sound caused by the contraction of the muscular fibres of the ventricle, while others believe it to be due to the tension of the auriculoventricular valves; whatever be its exact cause it is given out at the same time that the ventricles contract. The second sound is, without doubt, caused by the sudden closure of the semilunar valves when the ventricular systole ends. That such is the case has been proved experimentally, by hooking back the semilunar valves in a living animal, when the second sound ceases at once.

16. Thirdly, if the finger be placed upon an artery, such as that at the wrist, what is termed the pulse will be felt; that is to say, the elastic artery dilates somewhat, at regular intervals, which answer to the beatings of the heart. The pulse which is felt by the finger, however, does not correspond in time precisely with the beat of the heart, but takes place a little after it, and the interval is longer the greater the distance of the artery from the heart.

The beat in the artery on the inner side of the ankle, for example, is a little later than the beat of the artery in the temple. The pulse is in fact nothing but that distension of the arterial walls of which we spoke just now, and which, travelling in the form of a wave from the larger to the smaller arteries, takes longer to reach and distend the more distant branch.

17. Fourthly, when an artery is cut, the outflow of the fluid which it contains is increased by jerks, the intervals of which correspond with the intervals of the beats of the heart. The cause of this is plainly the same as that of the pulse; the force which would be employed in distending the walls of the artery, were the latter entire, is spent in jerking the fluid out when the artery

is cut.

18. Fifthly, under ordinary circumstances, the pulse is no longer to be detected in the capillaries, or in the veins. This arises from several circumstances. One of them is that the capacity of the branches of an artery is greater than the capacity of its trunk, and the capacity of the capillaries, as a whole, is greater than that of all the small arteries put together. Hence, supposing the capacity of the trunk to be 10, that of its branches 50, and that of the capillaries into which these open 100,1 it is clear that a quantity of fluid thrown into the trunk, sufficient to dilate it by one-tenth, and to produce a very considerable and obvious effect, could not distend each branch by more thanth, and each capillary byth of its volume, an effect which might be quite imperceptible.

19. But this is not all. Did the pulse merely become indistinguishable on account of its division and dispersion among so many capillaries, it might be felt again when the blood is once more gathered up into a few large venous trunks. But it is not. The pulse is definitely lost at the capillaries. There is, under ordinary circumstances, no pulse whatever in the veins, except sometimes a backward pulse from the heart along the great venous trunks; but this is quite another matter.

This actual loss, or rather transformation of the pulse,

Ten and one hundred are here taken for simplicity's sake. As a matter of fact, the capacity of the capillaries is not only ten times, but several hundred times greater than that of the aorta.