Let us take the right side of the heart and consider it first. Blood, let us suppose, is pour ing into the right auricle from veins that join it, bringing blood from all parts of the body. The tricuspid valve is open, its daps hanging down into the ventricle. The blood, there fore, having entered the upper chamber, flows through the opening into the lower chamber; but the auricle, being smaller than the ven tricle, fills sooner. As soon as it is full the muscular walls are stimulated by the pressure of blood on them and contract vigorously. The contraction is accomplished at the expense of the cavity, just as, if a hollow elastic ball be squeezed by the hand, its cavity will be abolished, and the blood which filled the cavity is driven out. In what direction will it go? It may pass in two directions, either back wards into the veins, from which it has come, or downwards into the ventricle. But the mouths of the veins contract along with the auricle, and, besides, the veins are already full of blood, waiting the relaxation of the auricle to enter it. Thus all the blood will be driven down into the ventricle. The auricle having emptied itself begins to relax, and blood, entering from the veins, begins to fill it again. The ventricle contained already a considerable quantity of blood before the emptying of the auricle into it, and the additional quantity, re ceived on the auricular contraction, just fills it. Its muscular walls are stimulated by the force of blood, and thus, as soon as the auricle has discharged its contents into it, the ventricle begins to contract. It contracts with much greater vigour than the upper chamber, be cause of the increased thickness and power of its walls, and the blood is driven out of it with considerable force. In what direction will it go? It may be forced upwards, back into the auricle, or it may be forced into the pulmonary artery, which passes, as already noted, from the right ventricle to the lungs. Now the tricuspid valve collies into play to determine which of these two directions the blood shall take. The free edges of the flaps are hanging downwards into the ventricle, but the blood is driven by the contraction against them, and forces them up so that the three flaps meet in the middle line and the opening is effectually closed. The action may be roughly illustrated thus: suppose a room filled with people, and the door of the room, which opens only in wards, to be standing half-open; suppose now a sudden movement of all the people towards the door, at once the door would be pressed on from behind, and would be speedily shut if the people were moving blindly onwards. The blood, therefore, is prevented passing back wards into the auricle. It is urged, then, into the pulmonary artery, which also is provided with valves, but they open away from the ventricle into the artery. The flaps are, conse quently, pressed close against the wall of the artery and the passage is quite free. Suppose the room, already thought of, to have a second door, opening outwards into a passage; the stream of people pressing against it would drive it widely open so that the people might pass out. The ventricle remains contracted for a measurable period of time and then relaxes. The tricuspid valve, no longer pressed close, opens, partly forced to do so by the weight of blood now pouring into the chamber above it, and blood is again permitted to flow into the ventricle from the auricle. Now what is the purpose of the valves at the entrance to the pulmonary artery? While the ventricle remains contracted the blood, driven out of it, is being forced onwards along the artery to the lungs, but because of the ever-increasing small• ness of the branches of the artery the blood encounters some resistance to its flow. As soon as the ventricle begins to relax, the pressure, urging on the blood from behind, is no longer exerted, and so the resistance in front tends to cause a back wave which would drive part of the blood back again into the ventricle. The back wave, however, gets behind the pouches of the semilunar valves, forces the three flaps to meet in the middle line, the passage is barred, and the return of any of the blood to the ventricle prevented. To return to our illustration: suppose the room to be emptied of the crowd, who have all passed into the pas sage, and suppose then the first of the crowd find their way along the passage not easy and try to return. At once the backward pressure
of the crowd, catching the edge of the open door, will force it to close, and communication with the room will be cut off.
A similar series of occurrences is found to take place on the left side of the heart. The left auricle is filled with blood entering by veins. It contracts and forces the blood through the opening of the mitral valve into the left ventricle. A forcible contraction of the ventricle immediately follows; the blood, pressing against the flaps of the mitral valve, brings them together, thus preventing its pass ing up again into the auricle. It is forced to flow out into the artery arising from the ven tricle—the aorta—whose valves open widely. As soon as the ventricle begins to relax, there is a tendency for the blood in the artery to recoil because of the resistance to its onward movement ; and the back wave, getting into the pouches of the aortic semilunar valves, causes them to close, so that the return of blood to the ventricle is prevented.
Thus the valves determine the direction of the flow of blood through the heart. What is likely to arise should any of the valves fail from any cause to act properly, will be con sidered in detail under DISEASES OF THE HEART in the succeeding section. One example will, however, not be out of place here. Sup pose the mitral valve to be incompetent, as the phrase goes, that is, not to shut completely. Then when the ventricle contracts, only part of the blood will pass into the aorta, and some will escape upwards into the auricle. The auricle, which already gets sufficient blood by the veins from the lungs, will always be over-full, and will be unduly stretched to ac commodate the additional quantity. The blood will not pass on quickly enough, and the veins, in turn, will become over-full. The over-ful ness will speedily pass backwards to the lungs. Its veins will become overcrowded, and a state of congestion will arise which may extend back wards to other organs—liver, stomach, &c.
We have not yet completely exposed, how ever, the wonderful character of these valvular arrangements. It would readily occur to one that the pressure of blood on the tricuspid and mitral valves, during the contraction of the would not only be sufficient to raise the flaps to the horizontal position, but would orce them beyond that, and make them open upwards into the auricles, so that the blood would still escape upwards. To prevent this there exists a remarkable arrangement. In the ventricles there are 'numbers of fleshy pillars projecting from the inner surface. They Ire called musculi papillares (papillary mus 31es). They have connected with their free points tendinous cords (chordm tendinem), whose other ends are fastened to the free edges of the valves. Now when the ventricles contract, these papillary muscles contract with them and pull on the cords. The cords are of such a length that when the valve is closed they become tight and a doubling upwards of the flap of the valve is thus prevented. In fig. 135, P points to the tip of one of the muscles, front which cords are seen passing off to a valve. As seen in that figure the inside of the heart is rough and irregular with bands of muscular fibres akin to the musculi papillares. Some of the bands, however, are connected at both ends with the muscular substance of the ven tricle, others are like little cones of flesh pro jecting from the surface, with the apex free but with no tendinous cords. To them the term fleshy columns (columna carnese) is applied. It is only to those from whose free extremity tendinous cords pass to the valves that the name papillary muscles is given. The setuilunar valves of the pulmonary artery and aorta require no such cords, because they are not simply flaps connected by one edge only, but have a semicitcular connection with the artery, just as a pocket may be sown on to a garment. Each of the three portions of the valve forms thus a little pouch, and when the three pouches are bulged out by the recoil of the blood they meet one another. The greater the backward force of the blood the more do the pouches press back to back against one another, and any doubling back is impossible. In fig. 135, u points to the entrance to the pul monary artery opened up, and one pouch and a half are seen, while H indicates the entrance to the aorta cut across at the level of the valve. The shape of the pouches is in both slighly in dicated.