Between the conus arteriosus (_f_) and the pulmonary artery are three pocket-like =semilunar valves= (Fig. 117, _g_), one ventral, one dextral, and one sinistral. Between each valve and the wall of the pulmonary artery there is an enlargement of the cavity of the artery, one of the =pulmonary sinuses= (or sinuses of Valsalva). Beyond the sinuses the pulmonary artery divides into right and left branches (Fig. 116, _j_). 3. The =left auricle= (atrium sinistrum) (Fig. 115, _d_; Fig. 116, _b_). The auricular appendage (Fig. 115, _d_) is bent at right angles and its apex turned caudad. The cavity is thus irregular. Muscular trabeculæ occur only in the auricular appendage. When the auricular septum is examined by transmitted light the position of the fossa ovalis is indicated at about the middle of the septum as seen from the left side. Ventrad of it is a fold of the septal wall. The =pulmonary veins= enter the dorsal wall of the auricle in three groups (Fig. 116, _g_, _h_, _i_). Each group opens into a more or less pronounced sinus or extension of the auricular cavity. The sinuses may be called, on account of their position, dorsal (_g_), sinistral (_i_), and dextral (_h_). 4. The =left ventricle= (Fig. 115, _b_) occupies rather more than the left half of the base of the heart. Its walls are two or three times as thick as those of the right ventricle. Internally its walls present a few muscular bands, comparable to the trabeculæ of the left ventricle. There are two very large columnæ carneæ, one dorsad and one ventrad. At the cranial end it communicates near the lateral wall with the left auricle by the auriculoventricular opening, and near the septum with the aorta. The =bicuspid valve= (or mitral valve), which guards the auriculoventricular opening, consists of two flaps, one septal and one lateral. Their chordæ tendineæ, which come from their free borders as well as from their outer surfaces, are attached to the columnæ carneæ, chordæ passing from both valves to each columna carnea. The opening into the aorta is guarded by three =aortic semilunar= valves, each of which partly conceals an =aortic sinus= (or sinus of Valsalva). One valve is dorsal, one sinistral, and one dextral. In the dextral sinus is the opening of one of the =coronary arteries=, which carry blood to the walls of the heart. =Pericardium.=