THE HEAKT 323 



ii. The left auriculo- ventricular aperture is a 

 large circular funnel-like opening, leading into 

 the left ventricle. 



2. The ventricles. 



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

 the apex ; .note the shape of the cavities. 



The cavity of the right ventricle is crescentic in 

 section: its wall is thin, embracing and half-sur- 

 rounding 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 wall of the right ventricle, 

 by longitudinal 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 by three 

 membranous flaps attached round its margin. 

 The free 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 nmsculi papil- 

 lares. 



ii. The orifice of the pulmonary artery is at the 

 left anterior angle of the ventricle, and is 

 guarded by three pocket-like semilunar valves, 

 the free edges of which are directed away 

 from the ventricle. 



Note the position of the valves from above ; slit up the 

 pulmonary artery between two of them, and examine them. 



