THE HEAET 237 



D. Dissection of the Heart. 



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

 cardvu/m. Pin it down in a dish under water, with the ventral 

 swrface 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 ahout 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, nearly globular, 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. 



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

 a horizontal incision along its lateral and anterior borders, turn 

 the ventral wall back, 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 ; 

 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 siau-auricular aperture is a median 



