xi HEART 521 



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

 the right auricle ; 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 precavals 

 and postcaval enter the right auricle directly (Figs. 135 and 

 140). 



The line of separation between the two ventricles can be 

 seen externally as an oblique 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 cavity of which is crescentic in transverse 

 section (Fig. 138), while its walls are much thinner than those 

 of the left ventricle, as it has only 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 the auricular septum is a thin, oval area, the fossa ovalis (Fig. 

 I 39if- ov }> which in the embryo is perforated and so allows the blood 

 from the body to pass directly into the right auricle without going to 

 the lungs, which are not, of course, functional until 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. 139, tri. v) of three flaps : the valves are attached by 

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

 tending into the corresponding ventricles. Attached to their 

 edges are tendinous cords arising from conical elevations 

 of the ventricular walls known as papillary muscles, which 

 are much larger in the left ventricle than in the right 



