The cavity (including that of the auricular appendage) is somewhat egg-shaped, with its long axis transverse. The musculi pectinati, which branch and unite into a network, are most abundant on its dorsal wall at the right. The axes of the two venæ cavæ if produced into the cavity of the auricle would meet one another at its centre and nearly at right angles. Caudad of the opening of the inferior vena cava is seen the slit-like opening of the coronary sinus, guarded craniad by the semilunar valve of the coronary sinus (or valve of Thebesius). The coronary sinus receives blood from coronary veins, which collect it from the walls of the heart.

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.

[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.

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.