But hold the heart in one hand with the auricle upwards, and try to pour some water into the ventricle. The first few spoonfuls will go in all right, and then you will see some thin white skin or membrane come floating up into the opening and quite block up the entrance from the auricle into the ventricle; the [Illustration: FIG. 10.--_View of the Orifices of the Heart from below, the whole of the Ventricles having been cut away._ _R.A.V._ right auriculo-ventricular orifice surrounded by the three flaps, _t.v._ 1, _t.v._ 2, _t.v._ 3, of the tricuspid valve; these are stretched by weights attached to the _chordæ tendineæ_. _L.A.V._ left auriculo-ventricular orifice surrounded in same way by the two flaps, _m.v._ 1, _m.v._ 2, of mitral valve; _P.A._ the orifice of pulmonary artery, the semilunar valves having met and closed together; _Ao._ the orifice of the aorta with its semilunar valves. The shaded portion, leading from _R.A.V._ to _P.A._, represents the funnel seen in Fig. 8. ] water will immediately fill the auricle and run over. If you look at the membrane carefully as it comes bulging up, you will notice that it is made up of three pieces joined together as is shown in Fig. 9 (_lv._ 1, _lv._ 2, _lv._ 3). These three pieces form the valve between the right auricle and ventricle, called the =tricuspid=, or three-peaked valve. Why it is so called you will understand if you lay open the right ventricle by cutting with a pair of scissors from the auricle into the ventricle along the side of the heart, or by cutting away the front of the ventricle as has been done in Fig. 8. You will then see that the valve is made up of three little triangular flaps, which grow together round the opening with their points hanging down into the cavity of the ventricle (Fig. 10, _t.v._) They do not, however, hang quite loosely. You will notice fastened to the sides of the flaps, thin delicate threads, the other ends of which are fastened to the sides of the ventricle, and often to little fleshy projections called papillary muscles (Fig. 8, _P.P._)