THE PIGEON. 243 



267. The convex right face of the septum ventri- 

 culorum, or partition between the ventricles, projecting so 

 much into the cavity of the right ventricle as to make it 

 crescentic in section. 



268. The right auriculo-ventricular valve, a large 

 flap of muscle, attached partly to the outer edge of the 

 auriculo-ventricular aperture, partly to the outer wall of the 

 ventricle, and depending into the cavity of the latter. 



269. The origin of the pulmonary artery from the left side 

 of the anterior end of the ventricle : at the base of the artery 

 the three pocket-shaped pulmonary semilunar valves. 



270. The columnae carneae, fleshy ridges into which 

 the wall of the ventricle is raised. 



XLII. Remove the outer wall of the left ventricle, and 

 note : 



271. The great thickness of its wall in comparison with 

 that of the right ventricle. 



272. The concave left side of the septum ventriculorum. 



273. The two membranous flaps of the mitral valve, 

 connected by delicate tendons, the chordae tendineae, 

 with small conical elevations of the ventricular wall, the 

 musculi papillares. 



274. The aperture of the aorta, guarded by three pocket- 

 shaped aortic semilunar valves. 



XLIII. Note in the body-cavity, now the heart is 

 removed : 



275. The backward continuation of the gullet, dorsal to 

 the trachea, to join the proventriculus. 



276. The remains of the intermediate ( 160, 169) and posterior 

 ( J 59) air-sacs, the ventral walls of which will have been removed ; 

 they are best seen under water : the anterior-intermediate air-sac 

 covers the greater part of the ventral surface of the lung, the posterior- 

 intermediate lies immediately posterior to the lung : in the antero- 

 dorsal region of the posterior-intermediate and posterior sacs are seen 



R 2 



