242 ZOOTOMY. 



261. The aperture of the postcaval, in the postero- 

 external wall of the right auricle, and guarded by a valvular 

 fold of muscle, the Eustachian valve. 



262. The fossa ovalis, a thin place in the septum 

 auricularum, marking the position of the embryonic foramen 

 ovale by which the two auricles were placed in communica- 

 tion with one another : it is best seen by holding the septum 

 auricularum up to the light. 



263. A deep depression in the dorsal wall of the left 

 auricle, into which the pulmonary veins open. 



XL. Cut away both auricles, and all but the origins of 

 the aorta and pulmonary artery, and, looking at 

 the base of the ventricles, note : 



264. The rounded left auriculo-ventricular aperture, 

 guarded by the two membranous flaps of the mitral valve. 

 This and the other valves are best seen by filling the 

 ventricles with water and then squeezing them ; at every 

 squeeze the auriculo-ventricular valves close, when the 

 pressure is removed they open, and if a sufficient length of 

 the aorta and pulmonary artery is left, the semilunar valves 

 close. 



265. The crescentic right auriculo-ventricular aper- 

 ture, guarded on its outer or convex side by the large, 

 fleshy, right auriculo-ventricular valve (§ 268). 



266. The rounded apertures of the aorta and pulmonary 

 artery, each guarded by three membranous semilunar 

 valves (§ 269). 



XLI. Open the right ventricle by a V-shaped incision, 

 introducing the scissors into the cut end of the! 

 pulmonary artery, cutting obliquely backwards to 

 the apex of the ventricle, and then turning sharply 

 round and passing obliquely forwards, nearly to 

 the anterior border of the ventricle : note : 



