THE HEART 233 



1. The conns arteriosus contains two sets of valves, each 



set consisting of three valves. 



The posterior set is just in front of the ventricle, 

 and the anterior and larger set about the middle of 

 the length of the conus. Each valve is thick in its 

 central part, thia and membranous at the sides. 



2. The ventricle has very thick, muscular and spongy walls, 



and a small, nearly globular, central cavity. The 

 opening from the auricle is a transverse slit guarded 

 by a two-lipped valve, and is placed ia the dorsal- 

 wall of the ventricle near its hinder end, and shghtly 

 to the left side. 



8. The auricle. 



Cut across the conus at its anterior end; open the auricle by 

 a horizontal incision along its lateral and anterior borders, turn 

 the ventral wall bach, and wash out the blood from the cavity. 



The walls of the auricle are thin, with radiating 

 muscular bands; the cavity is very large and tri- 

 angular, in shape. The opening leading from the 

 sinus venosus is a vertical sUt in the posterior wall ; 

 and the transverse aperture leading to the ventricle 

 is in the ventral wall, and sHghtly to the left of the 

 median plane. 



4. The sinus venosus has already been laid open. Its walls 

 are thin, with a very feebly developed muscular net- 

 work. The sinu-auricular aperture is a median 

 vertical slit in the anterior wall, to the left of the 

 ventral end of which is a small aperture leading 

 into the coronary sinns, which returns to the sinus 

 venosus the blood from the walls of the heart. 



VII. DISSECTION OF THE UEINAEY AND GENITAL SYSTEMS. 



These two systems have abeady been seen in part, but 

 may now be examined more fuUy. They are originally in- 

 dependent of each other, but in the course of development 



