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. [Illustration: FIG. 11.--_Left Side of the Heart of a Sheep (laid open)._ _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; _Ao_^{2}. 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). ] To return to the inside of the left ventricle. Up at the top of the ventricle, close to the opening from the auricle, there is one other opening, and only one. If you put your finger into this, you will find that it leads into a tube which first of all dips under or behind the pulmonary artery and then comes up and to the front again. This tube is what you already know as the =aorta=. If you slit it up from the ventricle (and to do this you must cut through the pulmonary artery), you will find that on the left side, as on the right, the red fleshy wall of the ventricle suddenly changes into the yellow firm wall of the artery, and that just at this line there are three semilunar valves exactly like those in the pulmonary artery. =On the left side of the heart, then, we have also two valves, the mitral between the auricle and the ventricle, and the semilunar between the ventricle and the aorta. These let the blood pass one way and not the other. You can easily drive fluid from the pulmonary veins through auricle and ventricle into the aorta, but you cannot send it back the other way from the aorta.=