The valves in the veins, then, let the blood pass easily from the capillaries to the heart, but won’t let it go the other way. If you bare your arm you may see some of the veins in the skin, in which the blood is running up from the hand towards the shoulder. If with your finger you press one of these veins back towards the hand it will swell up, and if you look carefully you may see little knots here and there caused by the bulging out of the watch-pocket valves. If you press it the other way, towards the elbow, you will empty it easily, and if with another finger you prevent the blood getting into it from behind, that is from the hand, the vein will remain empty a very long time.
The presence of valves in the veins, then, is one reason why the blood moves in one direction, but other reasons, and these the chief ones, are to be found in the heart.
Let us now go back to the sheep’s heart.
[30.] You know from the diagram that the two great veins, the superior and inferior vena cava, open into the right auricle. If you slit up these two veins in the sheep’s heart, you will find that they end by separate openings in a small cavity, the inside of which is for the most part smooth, and the walls of which, made, as you will at once see, of muscle, are not very thick. This small cavity is the right auricle, shown in [Fig. 8], R.A., where the great veins have not been slit up, but the front of the auricle has been cut away. In this auricle, beside the openings into the two great veins and another one which belongs to a vein coming from the heart itself ([Fig. 8], b) there is quite a large one, leading straight downwards, into which you
R.A. cavity of right auricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; (a piece of whalebone has been passed through each of these;) a, a piece of whalebone passed from the auricle to the ventricle through the auriculo-ventricular orifice; b, a piece of whalebone passed into the coronary vein.
R.V. cavity of right ventricle; tv, tv, two flaps of the tricuspid valve: the third is dimly seen behind them, the a, piece of whalebone, passing between the three. Between the two flaps, and attached to them by chordæ tendineæ, is seen a papillary muscle, PP, cut away from its attachment to that portion of the wall of the ventricle which has been removed. Above, the ventricle terminates somewhat like a funnel in the pulmonary artery, P.A. One of the pockets of the semilunar valve, sv, is seen in its entirety, another partially.