270 CHORD ATE ANATOMY 



Immediately in front of the conus the truncus divides into four pairs of 

 aortic arches, the third to the sixth of the original series. In the Dipnoi 

 pulmonary arteries make their first appearance in the vertebrate series as 

 posterior branches of the last pair of aortic arches. As another novelty 

 in fishes, the right postcardinal vein degenerates and a new vein, the 

 postcava, drains most of the posterior part of the body. The caudal vein 

 in this group bifurcates into the left postcardinal and the postcaval veins. 

 The ihac veins, as they leave the pelvic fins, divide into pelvic and renal 

 portal veins. The two pelvic veins unite to form a median abdominal 

 vein. Venous blood from the fins may thus reach the heart either by 

 way of a capillary network in the mesonephroi or by the abdominal vein. 

 The efferent renal veins drain into the postcava and into the left post- 

 cardinal veins. Thus in the appearance of an atrial septum, pulmonary 

 arteries and veins, and a postcaval vein, the Dipnoi make notable 

 advances towards the circulatory system of the higher vertebrates. The 

 differences between the dipnoan and amphibian circulation are slight. 



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 amphibians while in 

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

 changes in circulation which occur within the group is the abandonment 

 by the higher amphibians of the capillary branchial network character- 

 istic of fishes. In the perennibranch amphibians 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 amphibians. Impure blood from the 



