THE HEART. 261 



D. Dissection of the Heart. 



Remove the floor of the mouth, with the heart and peri- 

 cardium, and pin it down in a dish under water, with the 

 ventral surface upwards. Open and examine the several 

 cavities of the heart in the order given below, washing out the 

 contained blood by a stream of water. 



Open the ventricle by a horizontal incision, and continue 

 the cut forwards along the right side of the conus arteriosus 

 to its anterior end. Turn the ventral portions of both ventricle 

 and conus over to the left side. 



1. The conus 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, thin and membranous at the edges. 



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



and a small central cavity. The opening from the 

 auricle is a transverse slit guarded by a two-lipped 

 valve, and is placed in the dorsal wall of the 

 ventricle near its hinder end, and slightly to the 

 left side. 



3. The auricle. 



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

 horizontal incisions 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 slit in the posterior wall 

 of the auricle ; and the transverse aperture leading 

 to the ventricle is in the ventral wall, and slightly 

 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 



