THE VASCULAR SYSTEM 355 



Amphibia. In the amphibians the connexion of the sinus venosus is 

 shifted to the right atrium while the pulmonary veins connect with the 

 left atrium. The two atria are divided by a septum which is usually 

 perforate in urodeles. There is however little mixing of impure and 

 pure blood in the atria. In the undivided ventricle some mixing of the 

 two kinds of blood does occur. A spiral septum in the truncus arteriosus 

 shunts the venous blood from the right side of the ventricle chiefly into 

 the pulmonary arteries while that which passes to the dorsal aorta and 

 systemic arteries is mostly aerated blood. Of the six original aortic arches 

 of the embryo the last four persist in some adult amphibia while in others 

 only the third and fourth arches persist. One of the most important 

 changes in circulation which occurs within the group is the abandonment 

 by the higher amphibia of the capillary branchial network character- 

 istic of fishes. In the perennibranch amphibia most of the blood in the 

 aortic arches short-circuits the gills, and with the loss of gills in the anura 

 the aortic arches form direct connexions between ventral and dorsal 

 aortae. In the anura as in most amniotes that portion of the dorsal 

 aortae between the carotid (third) and systemic (fourth) arches degener- 

 ates. In urodeles as in dipnoi the pulmonary arteries form posterior 

 branches of the sixth aortic arch while in the anura the connexion by a 

 ductus arteriosus with the dorsal aortae is lost as in mammals. Several 

 splanchnic arteries convey blood from the dorsal aorta to the intestine. 

 In anura however they are reduced to three, celiac, anterior, and posterior 

 mesenteric arteries. In urodeles three veins drain the mesonephroi, the 

 right and left postcardinal veins and the postcava. The connexion of the 

 iliac veins with the renal portal or advehent veins which made its appear- 

 ance in dipnoi is also present in amphibia. Impure blood from the hind- 

 legs may thus return to the heart either through the mesonephroi, or by 

 the abdominal vein. The increased flexure of the heart brings the atria 

 anterior as well as dorsal to the ventricle. (Fig. 296) 



Reptiles. The reptilian vascular system strikingly resembles that 

 of amphibia. The main arteries and veins are homologous in the two 

 groups. The chief differences appear in the heart and truncus arteriosus. 

 The ventricle is partly divided by a septum in lower reptiles and more 

 or less completely divided into the crocodiles and alligators. Conse- 

 quently pure and impure blood are separated in the two sides of the heart 

 as in mammals. A peculiarity of the reptilian circulation, however, is 

 manifested in the triple spHtting of the truncus arteriosus. Three 

 arteries instead of the two characteristic of mammals leave the heart. 

 One of these is the pulmonary artery carrying venous blood from the 

 right ventricle to the lungs. The remaining two vessels are the systemic 

 arteries, one of which comes from the right, and the other from the left, 

 ventricle. Soon after leaving the heart each artery crosses to the opposite 



