THE HEART. 361 



wide crescentic opening leading into the right 

 ventricle. 



b. The left auricle. The wall of the auricle is thin : 

 that of the auricular appendix is thicker, and is 

 marked by a network of muscular ridges. 



i. The apertures of the pulmonary veins. The 



veins of the two sides unite as they enter the 

 dorsal surface of the auricle. 



ii. The left auriculo-ventricular aperture is a large 

 circular funnel-like opening leading into the left 

 ventricle. 



2. The ventricles. 

 Cut across the ventricles about one-third of an inch from 

 the apex ; note the shapes of the cavities. 



The cavity of the right ventricle is crescentic in 

 section : its wall is comparatively thin, embracing 

 and half surrounding the left ventricle. 



The left ventricle has very thick walls and a 

 small irregular cavity. 



a. The right ventricle. 



Remove the auricles, and cut short the aorta and pulmonary 

 artery. Remove the outer wall of the right ventricle, by longi- 

 tudinal incisions through its dorsal and ventral walls, and a 

 transverse incision across its base a short distance behind the 

 anterior border. Avoid injury to the valve. 



i. The tricuspid valve guards the right auriculo- 

 ventricular aperture, and is formed of three 

 membranous flaps attached round its margin. 

 The borders of the flaps project into the 

 ventricle, and are connected by tendinous 

 strings, the chordae tendineae, to^muscular 

 processes of the wall of the ventricle, the 

 larger of which are known as musculi papil- 

 lares. 



