THE HEART. 351 



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. Th 



veins of the two sides unite as they enter the 

 dorsal surface of the auricle. 



ii. The left auriculo-ventricular aperture is 

 large circular funnel-like opening, leading 

 into the left ventricle. 



2. The ventricles. 



Cittt 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 pul- 

 monary artery. Remove the outer luall of the right ventricle, 

 by longitudinal incisions through its dorsal andventraliocMs, 

 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 chordse tendinese, to muscular 

 processes of the wall of the ventricle, the 

 larger of which are known as musculi papil- 

 lares. 



