504 THE RABBIT CHAP. 



into the ventricular portion of the heart, and the latter into 

 the right auricle ;j so that the aorta — together with the caro- 

 tids — and the pulmonary artery now arise directly from 

 the left and right ventricles respectively, and the pre- 

 cavals and postcaval enter the right auricle independently 

 (Figs. I 25 and 130). 



The line of separation between the two ventricles can be 

 seen externally as an oblicjue depression extending from the 

 base of the heart backwards and to the right, but not 

 reaching the apex, which is formed by the left ventricle only. 

 The small, irregular cavity of the latter is enclosed by very 

 thick muscular walls, and is partly surrounded by the right 

 Ventricle, the cavitv of which is crescentic in transverse 

 section (Fig. 1 28), while its walls are much thinner than those 

 of the left ventricle, as it has onh- to pump the blood to the 

 lungs. The auricles have thin walls : each is produced into 

 a little flap or appendix which envelops the base of the cor- 

 responding ventricle, and the walls of which are strengthened 

 by a network of muscular bands. 



In Lhu auricular septum i.s a thin, o\al area, \\'\t fossa oralis [Y\'^. 

 \z^, f. 01')^ which in the cn"vi.)r)u i.s perforated, and so allows the lilood 

 from the body to pass directly into the right auricle without gouig t" 

 the lungs, which are not, of course, functional tnitil the animal is born. 



The two auriculo-ventricular apertures are guarded by 

 valves that of the left side, or mitral valve, consisting of 

 two membranous flaps, that of the right, or tricuspid valve 

 (Fig. \ 21), tri. V) of three flaps : the valves are attached by 

 their bases to the margins of the apertures, their apices ex- 

 lending into the corresponding ventricles. Attached to their 

 edges are tendinous cords arising from conicttl elevations 

 of the ventricular walls known as papillary muscles, which 

 are niucli larger in the left \entricle than in the right 



