THE HEART. 277 
musculi pectinati, which branch and unite into a network, are 
most abundant on its dorsal wall at the right.. The axes of 
the two vene cavze 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 depres- 
sion, 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 
foetal life, so that the blood of the inferior vena cava then 
passes directly from the right auricle to the left auricle. Cau- 
dad 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, @) 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, ¢). 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. Cranio- 
ventrad 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 trabecule (Fig. 117, @) 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 auriculo- 
ventricular opening is the tricuspid valve (Fig. 117, d, 
d’,d'’). It consists of three flaps. One of these (@’) is septal 
