282 



AMPHIBIA 



Circulation Is incomplete, since three kinds of blood are found, 

 arterial, venous, and mixed. In many animals arrangements exist 

 which nearly separate venous from arterial blood. 



When the pericardium is opened on the ventral side, the following 

 parts come into view: (i) The conical ventricle with its apex pointing 

 backward. The ventricle has very thick muscular walls and ap- 



Truncus arteriosus 



Ouricle 



flight auricle—— 



3pira/ vo/ve— 

 Bulbus cordis 



Dorsaf otric 

 venfricuior \fal\/e 



VerttricJe 



Opening into pulmonary vein 

 Opening into sinus venosus 



Septum 



Riqht olrio 



ventricular valve 



■Muscle ridges 



Fig. 152. Internal view of the heart of the frog. (Modified from Kerr. Courtesy 



of Macmillan and Co., Ltd.) 



pears paler than the rest. (2) The thin-walled auricles lie imme- 

 diately in front of the ventricle; they are separated internally by a 

 septum. (3) The truncus arteriosus, a cylindrical body arising 

 from the right anterior border of the ventricle, and running obliquely 

 across the auricles. It divides into two trunks which soon give rise 

 to arteries — common carotid, systemic and pulmoCutaneous. (4) 

 The sinus venosus, a thin-walled sac, dorsal to the ventricle and 

 behind the auricle; it receives two precavae and the large postcava. 



