In the auricular septum ventrad of the opening of the inferior vena cava is seen a faintly marked smooth oval depression, the =fossa ovalis=. When the auricular septum is examined by transmitted light it is seen to be thinner over the fossa ovalis. There is an opening, =foramen ovale=, at this point in fœtal life, so that the blood of the inferior vena cava then passes directly from the right auricle to the left auricle. Caudad the cavity of the right auricle communicates with that of the ventricle by the large oval auriculoventricular opening, which is guarded by the tricuspid valves (Fig. 117). 2. The =right ventricle= (ventriculus dexter) (Figs. 115 and 116, _a_) does not reach quite to the apex of the heart, so that it makes up less than one-half of its ventricular portion. It makes a half-spiral turn about the left ventricle, from its lateral side at the apex toward its ventral side at the base, where it ends in the pulmonary artery (Fig. 115, _e_). Its wall is very thin as compared with that of the left ventricle. Its cavity (Fig. 117) is nearly flat on its medial side, convex on its lateral side. It communicates with the auricle craniodorsad. Cranioventrad the cavity is narrower and turns around toward the ventral side of the heart and opens into the pulmonary artery. That portion of the ventricle between the auriculoventricular opening and the pulmonary artery is the =conus arteriosus= (Fig. 115, _e_; Fig. 117, _f_). Internally the wall presents many muscular =trabeculæ= (Fig. 117, _a_) of various sizes. These are more numerous over the ventricular wall than on the septum. They do not occur on the conus (_f_). Surrounding the auriculoventricular opening is the =tricuspid valve= (Fig. 117, _d_, _d′_, _d″_). It consists of three flaps. One of these (_d′_) is septal (i.e., it lies against the septum), while of the two others which do not lie against the septum, one is dorsal (_d″_) and the other ventral. Each flap is thin, semicircular, and membranous, and is attached to the border of the auriculoventricular opening by the diameter of the semicircle. The free border of each is attached to the wall of the heart by numerous delicate tendinous bands, the =chordæ tendineæ= (_c_), some of which are attached to the lower face of each valve. The chordæ tendineæ of the septal valve (or most of them) are attached at their opposite ends to the septum directly, while those of the dorsal and ventral flaps are attached to the ends of three or more band-like muscles, =columnæ= (or trabeculæ) =carneæ= (_b_), which are fixed by their opposite ends to the ventricular wall. [Illustration: FIG. 117.--HEART, WITH RIGHT VENTRICLE LAID OPEN TO SHOW THE TRICUSPID VALVE. _a_, trabeculæ; _b_, columnæ carneæ; _c_, chordæ tendineæ; _d_, _d′_, _d″_, the three flaps of the tricuspid valve; _e_, aorta; _f_, conus arteriosus, laid open; _g_, semilunar valves of the pulmonary artery.]