510 MANUAL OF ZOOLOGY SECT, xil 



than elsewhere ; this is the fossa ovalis, it marks the 

 position of an aperture, the foramen ovale, 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 valve, the tricuspid, com- 

 posed of three membranous 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 ventricle 

 contracts. The lobes of the valve are attached to muscular 

 processes of the wall of the ventricle, the musculi papillares^ 

 by means of tendinous threads called the chorda tendinetz. 

 The right ventricle, 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 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. 



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

 auricular appendix. Into this 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 chordae tendineae and musculi papillares. In 

 the walls of the ventricle are columnar carneae 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 three semi-lunar valves similar to those 

 at the entrance of the pulmonary artery. The coronary 

 arteries, which supply the muscular substance of the heart, 



