Now slit up the pulmonary artery as far as you can, and note when you cut it how stout and firm are its walls. You will find that it soon divides into two branches, one for the right lung, one for the left. Each of these, when it gets to the lung, divides into branches, and these again into others, as far as you can follow them. You know from what you have learnt already that these branches end in capillaries all over the lungs.
[32.] Not far from the two main branches of the pulmonary artery you will find, covered up perhaps with fat and other matters, some tubes which you will at once recognize as veins, and if you open any one of these you will find that you can put a thin rod into it, and that it leads in one direction to the lungs, and in the other into the left side of the heart. These are the pulmonary veins, and if you slit them right up you will find they open (by four openings) into a cavity on the left side of the heart, almost exactly like that cavity on the right side which we called the right auricle ([Fig. 11]). This cavity is, in fact, the left auricle; out of it there is an opening into the left ventricle, very like the opening from the right auricle into the right ventricle. It too is guarded by flap valves, exactly like the tricuspid valve, only there are but two flaps instead of three ([Fig. 9], m.v. 1, m.v. 2). Hence this valve is called the bicuspid, or more frequently the mitral valve. Its flaps have little threads by which they are fastened to the walls of the ventricle, and in fact, except for there being two flaps instead of three, the mitral valve is exactly like the tricuspid valve, and acts exactly the same way.
If you cut with a pair of scissors from the auricle into the ventricle, you will find the left ventricle ([Fig. 11]) very much like the right ventricle, only its walls are very much thicker, so much thicker that the left ventricle takes up the greater part of the heart. You will see this if you now look at the outside of a fresh heart.
The auricles are so small and so covered up by fat that from the outside you can hardly see them at all. What you chiefly see are two little fleshy corners, one of each auricle ([Fig. 5], R.A. L.A.), often called “the auricular appendages.” By far the greater part is taken up by the ventricles—and if you look you will see a band of fat slanting across the heart ([Fig. 5], 3). This marks the line of the fleshy division, or septum as it is called, between the two ventricles. You will notice that the point or apex of the heart belongs altogether to the left ventricle.
P.V. pulmonary veins opening into the left auricle by four openings, as shown by the styles or pieces of whalebone placed in them: a, a style passed from auricle into ventricle through the auriculo-ventricular orifice; b, a style passed into the coronary vein, which, though it has no connection with the left auricle, is, from its position, necessarily cut across in thus laying open the auricle.
M.V. the two flaps of the mitral valve (drawn somewhat diagrammatically): pp, papillary muscles, belonging as before to the part of the ventricle cut away; c, a style passed from ventricle in Ao. aorta; Ao2. branch of aorta (see [Fig. 5], Áó); P.A. pulmonary artery; S.V.C. superior vena cava.
1, wall of ventricle cut across; 2, wall of auricle cut away around auriculo-ventricular orifice; 3, other portions of auricular wall cut across; 4, mass of fat around base of ventricle (see [Fig. 5], 2).