452 DISSECTION OF THE PIGEON. 



b. The left auricle. 



i. The openings of the pulmonary veins are in a 



small recess of the dorsal wall of the auricle. 



ii. The left auriculo-ventricular aperture is cir- 

 cular. 



2. The ventricles. 

 Cut 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-ventricular valve is formed 

 by two muscular flaps : one long and external, 

 the other short, meeting the first at a slight 

 angle and connected to the ventricular wall 

 at the base of the conus of origin of the 

 pulmonary artery. 



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. 



The semilunar valves are three crescentic 

 pocket-like flaps guarding the entrance to 

 the artery. 



b. The left ventricle has very thick spongy walls, 



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

 is somewhat circular in transverse section. 



i. The left auriculo-ventricular or mitral valve 

 is formed by three membranous flaps, which 

 project into the ventricle. Of these the two 

 outer form the antero- and postero-lateral 



