628 Comparative Morphology of Chordates 



the right auricle into the dorsal aorta, is not seriously detrimental, but 

 it would be intolerable in a bird whose high temperature and high 

 output of muscular energy require a blood having maximum content 

 of oxygen. 



Abnormalities of the blood-vessels are frequent, affecting even 

 the main trunks. Among the many and varied abnormalities which 

 have been found in man are numerous cases in which the right aortic 

 trunk has been retained instead of the left, but still opening into the 

 left ventricle. In these cases the basal region of the left subclavian is 

 presumably a persisting remnant of the left aortic trunk. In rare 

 human cases both right and left aortic trunks persist, both opening 

 into the left ventricle. 



In their embryonic history and their relations in the adult condi- 

 tion, the heart, and the arteries derived from the aortic arches in 

 mammals, differ in such manner and to such a degree from those of 

 birds as to put a great phylogenetic gap between the two Classes. It is 

 true that, in minor details of their arrangement, blood-vessels are 

 notably erratic. Normally a cat has a single artery passing from the 

 dorsal aorta into each kidney, but among the many cats dissected in 

 laboratories are frequently found individuals having two or three 

 arteries passing to one kidney, and the number may not be the same on 

 the two sides of the body. But the differences in the relations of the 

 ventricles to the main arterial trunks leading out from them are of 

 radical nature and involve the basic symmetry and relations of a 

 large group of most important vessels. While abnormalities more or 

 less commonly occur, each Class of Amniota has its characteristic 

 pattern to which the very great majority of its members strictly 

 adhere. The avian pattern is essentially like that in all modern reptiles 

 and, in its details, most closely similar to that of Crocodilia. The 

 mammalian pattern is radically different from that of any known 

 modern reptile. Concerning the blood-vessels of extinct reptiles we are 

 completely ignorant and doubtless must always remain so. 



The available facts point to the conclusion that, early in the history 

 of Reptilia, a cleavage in the group occurred. Along one line, or several 

 related lines, was developed the vascular pattern which we now find 

 in all modern reptiles and, with mere omission of the left aortic trunk, 

 also in birds. Along another line developed the pattern found in modern 

 mammals. There is convincing evidence that such a cleavage did occur. 

 Those early reptiles of the theromorph type, with their synapsid skull, 

 secondary bony palate, modified jaw-joint and heterodont teeth, had 

 already diverged far from the main reptilian stock which went on to 

 give rise to the dominant reptiles of the Mesozoic and the several 



