COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 215 



revealed a large vessel, the ventral aorta, which continues forward from the conus 

 arteriosus. By dissecting carefully to the left side find the branches of the 

 ventral aorta. They are as follows : 



Dogfishes: There are three main pairs of branches, two of which subdivide 

 into two. The most posterior pair of branches arises just at the point where 

 the conus arteriosus passes into the ventral aorta. (At this point note the 

 coronary artery, paired in the spiny dogfish, single in the smooth species, passing 

 along the conus arteriosus onto the surface of the ventricle and to the walls of the 

 pericardial cavity. This artery should be preserved as far as possible.) Follow 

 the most posterior branch of the ventral aorta. It very shortly divides in two, 

 the posterior branch penetrating the interbranchial septum of the sixth visceral 

 arch, the anterior branch the septum of the fifth. The middle branch of the 

 ventral aorta arises shortly in front of the third branch and passes without 

 division into the interbranchial septum of the fourth visceral arch. After giving 

 off this branch the ventral aorta proceeds forward without branching to a point 

 just posterior to the lower jaw. Here it forks to form its anterior pair of branches . 

 Trace the left branch laterally. After some distance it forks, supplying the 

 second and third visceral arches. Trace any one of the branches of the ventral 

 aorta out into the interbranchial septum, slitting the septum. Note the small 

 branches from the artery into the gill filaments. The five pairs of branches of 

 the ventral aorta are named the afferent branchial arteries. How many gills 

 does each supply ? 



Skate: There are two main pairs of branches from the ventral aorta. The 

 posterior pair arises where the conus arterious passes into the ventral aorta. (At 

 this point note the coronary artery, paired, passing to the conus arteriosus and 

 to the visceral muscles. Preserve it as well as possible.) Follow out the posterior 

 branch of the ventral aorta. After some distance it subdivides into three branches 

 which pass to the fourth, fifth, and sixth visceral arches, penetrating the inter- 

 branchial septa. Trace them into the septum by slitting the septum, and note the 

 branches from each to the gill filaments of both demibranchs of the septum. Fol- 

 low the ventral aorta forward beyond the pair of posterior branches. It passes 

 without branching for some distance and then forks into right and left branches. 

 Follow the left branch. After a considerable distance it forks into two vessels, 

 which penetrate the interbranchial septa of the second and third visceral arches. 

 The five pairs of branches of the ventral aorta are named the afferent branchial 

 arteries. How many demibranchs does each supply ? 



In front of the anterior fork of the ventral aorta will be found some soft 

 brownish diffuse material, composing the thyroid gland. 



The afferent branchial arteries are the ventral halves of the aortic arches 

 which were previously described as present in the embryo. We noted that 

 there are six such arches in vertebrate embryos. Since the adult elasmobranch 



