COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 227 



one again divides in two, making a total of three afferent branchial arteries, one 

 to each of the gills. Trace each one into the gill, removing the skin from the 

 gill. At the entrance into the gill the first afferent branchial artery gives off 

 an external carotid artery which turns medially, running beside the branchial 

 artery, and then branches into the floor of the mouth. Within the gill each 

 branchial artery sends up a loop which branches among the gill filaments, from 

 which other branches collect into a loop on the other side of the gill, this loop 

 joining the branchial artery again. In addition to the two loops, a short con- 

 necting branch runs through the base of each gill. 



Next turn back the flap, previously formed, of the floor of the mouth and 

 pharyngeal cavities. Extend the incision posteriorly along the left side of the 

 esophagus. Strip off the mucous membrane from the roof of the mouth and 

 pharyngeal cavities. In the roof will be ssen a pair of large vessels, the roots 

 or radices of the aorta, which pass obliquely posteriorly and unite. Trace the 

 right root laterally into the visceral arches dissecting off the mucous membrane 

 from the latter. Locate from the inside the branchial arteries exposed in the 

 preceding paragraph and note their emergence from the dorsal ends of the visceral 

 arches as the efferent branchial arteries. The second and third efferent branchial 

 arteries unite as they emerge from the visceral arches, thus forming two efferent 

 branchial arteries on each side. From the common vessel formed by the union 

 of the second and third efferent branchial arteries an artery arises which passes 

 posteriorly. Trace it into the pleuroperitoneal cavity and note that it courses 

 along the dorsal or attached side of the lung. It is the pulmonary artery. From 

 the first efferent branchial artery very near the place where it joins the second 

 and third arises the internal carotid artery which passes forward on the roof of 

 the mouth. Accompanying the internal carotid artery is the internal jugular 

 (anterior cardinal) vein. The efferent branchial arteries medial to the origin 

 of the pulmonary and internal carotid arteries unite to form the root of the aorta 

 on each side. From this springs the vertebral artery which passes at once dorsally 

 into the skull. The two roots of the aorta then pass obliquely caudad and 

 unite in the mid-dorsal line to form the large dorsal aorta. 



Draw the afferent and efferent branchial arteries and their branches. 



The afferent and efferent branchial arteries are as in the elasmobranchs, the ventral and 

 dorsal halves, respectively, of the aortic arches. Besides the network in the gills, each arch 

 forms a complete semicircle by means of the direct connecting branch in the base of the gill. 

 The three aortic arches present in Nectwrus correspond to the third, fourth, and sixth arches 

 of the original six. Note the origin of the external carotid from the ventral part, of the internal 

 carotid from the dorsal part of the third aortic arch, and of a new vessel, the pulmonary artery, 

 from the last or sixth arch (see Fig. 58C, p. 267). 



7. The branches of the dorsal aorta.— Trace the dorsal aorta posteriorly 

 into the pleuroperitoneal cavity. It runs in the median dorsal line. Immedi- 

 ately beyond its origin it gives off a subclavian artery on each side. Trace one 



