266 



THE BIOLOGY OF THE FROG 



CHAP. 



CCZ 



'.CU 



The internal structure of the heart presents a complicated 

 and beautifully adapted mechanism for propelling the blood 

 always in one direction and keeping the pure and impure 

 blood separated. By removing the ventral wall of the 

 auricles, ventricles, and bulbus, most of the features of the 



internal structure may be 

 exposed to view. The 

 interauricular septum is 

 so situated that the right 

 auricle is much larger 

 than the left. In the 

 right auricle, close to the 

 septum, is the large 

 sinu-auricular aperture, 

 through which blood en- 

 ters from the sinus veno- 

 sus. It is a transverse 

 oval opening guarded by 

 valvular lips on the ante- 

 rior and posterior sides, 

 which prevent the blood 

 that has entered from the 

 sinus from being forced 

 back again when the 

 auricles contract. The 

 left auricle receives blood 

 from the pulmonary vein through a small opening near the 

 septum slightly anterior to the sinu-auricular aperture ; there 

 is no valve at this point, but since the vein perforates the wall 

 obliquely, the pressure caused by the contraction of the auricle 

 serves to close the opening and thus prevents the backward 

 flow of the blood. Both auricles empty into the ventricle 

 by a large opening, the auriculo-ventrkular aperture, which is 



pt.C 



Fig. 73. — Heart seen from the dorsal 

 side with the sinus venosus opened up. 

 ao, aortic trunk; ait, right auricle; au" , 

 left auricle; ca, carotid trunk; p. en, 

 pul mo-cutaneous trunk; pr.c, precaval 

 vein ; pt.c, postcaval vein ; p.v, pul- 

 monary vein ; s.v, sinus venosus ; v, 

 ventricle; va", sinu-auricular valves. 

 (After Howes.) 



