THE HEART. 447 



2. The ventricles. 



Gut across the ventricles transversely, about the middle of 

 their length. Note the shapes of the cavities, and the thick- 

 ness of their walls. 



a. The right ventricle wraps round the right side of 



the left ventricle. Its outer wall is compara- 

 tively thin, and its cavity is crescentic in trans- 

 verse section. 

 i. The right auriculo-ventrieular valve is a mus- 

 cular flap, projecting into the ventricular 

 cavity, on the right side of the auriculo-ven- 

 trieular aperture. It is connected by chordse 

 tendinese with the wall of the ventricle. 

 ii. The aperture of the pulmonary artery is at the 

 extreme anterior end and left side of the 

 ventricle. 



Pass a seeker from the ventricle along the pulmonary 

 artery, and lay this open with scissors. 



a. The semilunar valves are three crescentic 

 pocket-like flaps, guarding the entrance to 

 the artery. 



b. The left ventricle has very thick spongy walls, ex- 



cept at the apex, where it is thin. Its cavity is 



somewhat irregular in shape, but is more or less 



circular in transverse section. 



i. The left aurienlo-ventricular or mitral valve is 



formed by two membranous flaps, which 



proj ect into the ventricle. Their hinder borders 



are attached by chordae tendineae to muscular 



processes of the ventricular wall, the mus- 



culi papillares. 



ii. The aperture of the aorta is at the anterior 

 end of the ventricle, and at its right side. 

 In the natural position of the parts, it lies 

 dorsal to the aperture of the pulmonary 

 artery. 



