xi THE HEART 521 



caJly absorbed into the ventricular portion of the heart, 

 and the latter into the right auricle ; so that the aorta 

 (together with the carotids) 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, 139, 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 

 avails (Fig. 139, /. or), which in the embryo is perforated and 

 so allows the blood from the body to pass directly into the 

 left 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, consist- 

 ing of two membranous flaps, that of the right, or 

 triciispid valve (Fig. 139, tri. v), of three flaps : the valves 

 are attached by their bases to the margins of the aper- 

 tures, their apices extending 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 (m. pap] '. these serve to 



PRACT. ZOOL. K K 



