Chap, v.] HEART OF REPTILES. 199 



An indication of a division of tlie ventricle into 

 two parts is seen in the Dipnoi, but in the Amphibia 

 there is no septum ; in the uni- ventricular Reptiles 

 (that is, in all but Crocodiles) there is no complete 

 division of the cavity, but the muscular walls form 

 internal projections which are functionally of some 

 importance ; the most valuable of these is the 

 prominent fold which lies just beneath the entrance to 

 the pulmonary artery, and almost separates off the 

 part of the ventricular cavity which lies beneath it 

 from the rest of the cardiac chamber ; in consequence 

 of this being the region whence blood passes directly 

 to the lungs by the pulmonary arteries (PA), it is 

 known as the cavum pulmonale (Fig. 8G ; Cp\ 

 and it occupies the right extremity of the heart. As 

 the ventricle contracts, the blood in this cavum is 

 forced into the pulmonary artery, and as it is the 

 blood which has entered the ventricle from the right 

 auricle (RA), it is, of course, venous blood, or blood 

 that requires oxygenation. As the contraction con- 

 tinues, the wall of the ventricle and the edge of the 

 septum are brought closer together, so that the blood 

 which is the last to leave the cavity is prevented from 

 making its way into the cavum pulmonale ; this blood 

 is that from the left side of the heart, that is, from 

 the left auricle (LA) ; in other words, it is blood 

 which has just returned from the lungs, and requires 

 no further oxydation, and it passes altogether into 

 the systemic aortte. An inspection of the figure of 

 the heart will, however, show that some venous blood 

 must pass into the same vessels, so that the blood 

 in the systemic vessels of the tortoise is not pure 

 arterial blood, but is a mixture of partly oxygenated 

 blood and of blood that has already made a passage 

 through the body. 



In the Crocodilia, Birds, and Mammals there is a 

 complete interventricular septum., so that within the 



