286 COMPARATIVE ANATOMY. 



The blood from the right ventricle passes into the latter as well 

 as into the left aortic arch, and, according as the septum ventricu- 

 lorum is complete or incomplete, is either entirely venous (Croco- 

 diles) or mixed (other Reptiles, Fig. 229, C). 



The valves of the heart have undergone a considerable 

 reduction in Reptiles : at the origin both of the aorta and of the 

 pulmonary artery there is only a single row ; this is also the case 

 in all other Amniota. 



Birds and Mammals. In these, the atrial and ventricular 

 septa are always complete, and there is no longer any mixture of 

 the arterial and venous blood. The ventricles are much larger than 



FIG. 230A. HEART OF THE SWAN, WITH THE RIGHT VENTRICLE CUT ortx. 

 Vw, ventral wall of right ventricle turned on one side, thus putting the atrio-ven- 

 tricular valve which arises by two muscular folds, a and b on the stretch ; 

 f, point of insertion of these folds on the ventral wall of the ventricle : above c 

 is the atrio- ventricular aperture ; S, septum ventriculorum ; *, *, *, the three 

 semilunar valves of the pulmonary artery ; V, left ventricle. 



FIG 230n. TRANSVERSE SECTION THROUGH THE RIGHT (Vd) AND LEFT (Fir/) 

 VENTRICI/E OF Grus cintrca. 



S, septum ventriculorum. 



the atria, and their muscular walls are strongly developed and very 

 compact. This is particularly the case in the left ventricle, on the 

 inner wall of which the papillary muscles are well developed : 

 the left ventricle is partially surrounded by the right, the cavity 



