=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 [Illustration: FIG. 8.--_Right Side of the Heart of a Sheep._ _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. 1, the wall of the ventricle cut across; 2, the position of the auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of fat lodged between the auricle and pulmonary artery. ] can put your three fingers. This is the opening into the right ventricle; and you will have no difficulty in putting your fingers from the auricle into the ventricle and bringing them out again. [Illustration: FIG. 9.--_The Orifices of the Heart seen from above, the Auricles and Great Vessels being cut away._ _P.A._ pulmonary artery, with its semilunar valves; _Ao._ aorta, do. _R.A.V._ right auriculo-ventricular orifice with the three flaps (_lv._ 1, 2, 3) of tricuspid valve. _L.A.V._ left auriculo-ventricular orifice, with _m.v._ 1 and 2, flaps of mitral valve; _b_, piece of whalebone passed into coronary vein. On the left part of _L.A.V._ the section of the auricle is carried through the auricular appendage; hence the toothed appearance due to the portions in relief cut across. ]