THE PIGEON. 



241 



under water, and note carefully the relations of 

 the great vessels already seen, especially those of 

 the pulmonary veins : then remove the outer walls 

 of both auricles, wash out the contained blood, 

 and make out : 



258. The septum auricularum, a thin muscular parti- 

 tion separating the auricles from one another. 



a.ao 



r.vn. 



FIG. 56. Columba livia. The heart, viewed from the dorsal 

 aspect (nat. size). 



a.ao, arch of aorta: br.a, brachial artery: br.v, brachial vein : c.c, 

 common carotid ; ju, jugular ; l.au, left auricle: l.p.a, left pulmonary 

 artery: l.prc, left precaval : l.vn, left ventricle: r.au, right auricle: 

 r.au', its smooth-walled dorsal portion receiving the pre- and post- 

 cavals : r.p,a, right pulmonary artery: r.prc, right precaval: r.vn, 

 right ventricle. 



259. The entrance of the right precaval, in the anterior 

 wall of the right auricle. 



260, A tunnel-like passage in the dorsal region of the 

 same chamber, leading into the left precaval. 



R 



