THE VASCULAR SYSTEM 357 



systemic and carotid arches are connected with one another, as in urodeles, 

 by the dorsal aortae. In the crocodiles this connexion is lost, as in 

 mammals. The connexion between the postcava and the postcardinals 

 is lost in reptiles and the blood from the kidneys returns to the heart by 

 the postcava as in mammals. Both right and left common cardinals 

 (ductus Cuvieri) persist and bring blood from the head and anterior 

 limbs into the sinus venosus. Thence it passes to the right atrium. Blood 

 from the hind legs as in amphibia may return to the heart either by the 

 renal portal veins or by the lateral abdominal veins. 



Mammals. The complete division of the heart into a right venous 

 half and a left arterial half which was attained by reptiles is retained by" 

 mammals. In mammals, however, the sinus venosus merges into the 

 right atrium. In this region is located the sino -auricular node, a bundle 

 of muscular and connective tissue richly supplied with nerve fibers, which 

 is said to be the "pace-maker" of the heart-beat. Mammals have a 

 single ventral aorta. Of the paired systemic arches of amphibians and 

 reptiles only the left one persists. The renal portal system has disappeared 

 and with it the abdominal veins. The latter however form the transient 

 umbilical veins of the fetal circulation. The right and left iliac veins 

 establish connexions with the postcava by way of the posterior cardinals 

 and of their transverse anastomosis in the lumbar region. 



The postcava of mammals appears to be only in part homologous with 

 that of lower vertebrates. As is shown in Fig. 320, four distinct embryonic 

 vessels unite to form the mammalian postcava. These are the anterior 

 hepatic portion, the subcardinal anastomosis, the supracardinal veins 

 (in part), and the posterior portion of the right postcardinal vein. The 

 supracardinal veins seem to be mammalian novelties, arising in the embryo 

 dorsal to the post- and subcardinal veins. There is difference of opinion 

 in regard to the first appearance of subcardinal veins, whether they are 

 new in mammals or present in vertebrates from amphibia to man. Accord- 

 ing to McClure subcardinal veins are present in all vertebrates. 



The origin of the azygos and hemiazygos veins is also in doubt. Most 

 textbooks regard these vessels as persistent remnants of the postcardinal 

 veins together with their transverse anastomosis. The researches of 

 Huntington and McClure however indicate that the azygos and hemi- 

 azygos veins are limited to mammals and that they are in chief part 

 persistent portions of the supracardinal veins which appear to be mammal- 

 ian novelties. The renal veins of mammals are not the homologs of the 

 renal veins of lower vertebrates but are new vessels formed from the 

 intersubcardinal anastomosis. 



In the lower mammals both common cardinals occur as in lower 

 vertebrates. In the higher mammals and man, however, a transverse 

 anastomosis between the precardinals or jugulars is converted into the 



