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 venae cavae 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 avale, 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. 1 17). 



2. The right ventricle (ventriculus dexter) (Figs. 115 and 

 1 1 6, 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. 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, /). Internally the wall presents many 

 muscular trabeculae (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 (_/"). Surrounding the auriculo- 

 ventricular opening is the tricuspid valve (Fig. 117, d, 

 d' , d"}. It consists of three flaps. One of these (d') is septal 



