xii PHYLUM CHORDATA 505 



the left, and the base forwards. Like that of the Pigeon, it 

 contains right and left auricles and right and left ventricles, 

 the right and left sides of the heart having their cavities 

 completely separated off from one another by inter-auricular 

 and inter-ventricular partitions. 



Into the right auricle open three large veins the right 

 (r.pr. c.) and left (l.pr. c.} precaval veins and the single post- 

 caval (pt. c.) the first into the anterior part, the second into 

 the left-hand side of the posterior portion, and the third 

 into the dorsal region. Projecting forwards from the right 

 auricle is an ear-like auricular appendix. On the auricular 

 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 ovate, in the foetus. 

 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 tricuspid valve (tri. v.), 

 composed of three membranous lobes or cusps, so ar- 

 ranged 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 ventricle 

 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 ventricle, much thicker than the auricle, 

 forms the right side of the conical apical portion of the 

 heart, but does not extend quite to the apex. Its walls are 

 raised up into muscular ridges called columns 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 (sent. v.). 



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



