434 



ZOOLOGY 



SECT. 



is an ear-like auricular appendix, the inner surface of which is. 

 raised up into numerous cords of muscular fibres, the musculi 

 pectinati. A membranous fold, the remnant of the foetal Eustackian 

 valve, extends from the opening of the post-caval forwards* towards 

 the auricular septum. The opening of the left pre-caval is bounded 

 behind by a crescentic fold, the valve of Thebesius. On the septum 

 is an oval area where the partition is thinner than elsewhere ; this- 

 is the fossa ovalis (f. ov.) ; it marks the position of an aperture, the 

 foramen ovale, in the foetus. The crescentic upper rim of the 

 aperture is known as the annulus ovalis. The cavity of the right 

 auricle communicates with that of the right ventricle by the wide 

 right auriculo-ventricular opening. This is guarded by a valve, 



the tricuspid (tri. v.\ com- 

 posed of three membran- 

 ous lobes or cusps, so 

 arranged and attached 

 that while they flap back 

 against the walls of the 

 ventricle to allow the 

 passage of blood from the 

 auricle to the ventricle,' 

 they meet together across 

 the aperture so as to close 

 the passage when the ven- 

 tricle contracts. The lobes 

 of the valve are attached to 

 muscular processes of the 

 wall of the ventricle, the 

 musculi papillares(m. pap. \ 

 by means of tendinous 

 threads called the chorda? 

 tendinece. The right ven- 

 tricle, much thicker than 

 the auricle, forms the right side of the conical apical portion, but 

 does not extend quite to the apex. Its walls are raised up into 

 muscular ridges called columnar carnece. It gives off in front, at 

 its left anterior angle, the pulmonary artery, the entrance to which 

 is guarded by three pouch-like semi-lunar valves (sem. v.). 



The left auricle, like the right, is provided with an auricular- 

 appendix. Into its cavity on its dorsal aspect open together the 

 right and left pulmonary veins. A large left auriculo-ventricular 

 opening leads from the cavity of the left auricle into that of the 

 left ventricle : this is guarded by a valve, the mitral, consisting of 

 two membranous lobes or cusps with chords tendineoe and muscuJi 

 papillares. In the walls of the ventricle are columnae carneas rather 

 more strongly developed than in the right. At the basal (anterior) 

 end of the left ventricle is the opening of the aorta, guarded by 



FIG. 1027. Lepus cuniculus. Heart, seen from 

 the right side, the walls of the right auricle and 

 right ventricle partly removed so as to expose the 

 cavities, ao. aorta ; /. ov. fossa ovalis ; I. pr. c. open- 

 ing of left pre-caval ; m. pap. musculi papillares ; 

 pt. c. post-caval ; pi. c'. opening of post-caval ;r.pr.c. 

 right pre-caval ; r. pul. right pulmonary artery ; 

 gem. v. semi-lunar valves ; tri. r. tricuspid valve. 



