42 THE THORACIC VISCERA, BUCCAL CAVITY, 
opening of the inferior cava, lying between it and the 
opening of the auricle into the ventricle. Follow 
its course back along the groove between the auricle 
and the ventricle. 
d. THe AURICULO-VENTRICULAR ORiFiczE. The 
crescentic opening leading into the right ventricle. 
e. Tue Lerr AuRICLE has thin walls like the right. 
The walls of the appendix are thicker and marked in- 
ternally by muscular ridges. 
f. THe Lerr AURICULO-VENTRICULAR ORIFICE. The 
circular opening leading into the left ventricle. 
2. The: Ventricles. 
Cut across the two ventricles at some distance from 
the apex—in the lower third of the right ventricle. 
The cavity of the right ventricle has a crescentic 
shape ; the wall is relatively thin. The cavity of the 
left ventricle is circular ; its walls are quite thick. The 
ventricular septum is very thick and arched, with the 
convexity projecting into the cavity of the right ven- 
tricle: The cavity of the: left ventricle extends into 
the apex, while that of the right ventricle terminates 
some distance anterior to it. 
Remove the auricles close to the base of the ven- 
tricles, and cut short the aortaand pulmonary artery. 
A good view of the relative positions of the four open- 
zngs into the ventricles rs thus obtained. 
a. Tue ITricuspip VALVE Oo! the right ventricle: 
It guards the right auriculo-ventricular orifice, and is 
formed by three membranous flaps attached round 
the margin of the orifice. The free borders project 
into the ventricle, and are connected to its wall by 
tendinous cords, the chord@e tendinee@, which end in 
papillary elevations of the ventricular wall known as 
the papellary muscles. Some of the chordz tendinez 
