232 THE DOG-FISH 



Trace the cardiac aorta forwards from the conus arteriosus, 

 and follow its branches on the left side to the gills, removing 

 as much of the muscles of the under surface of the head and 

 of the gills as is necessary to expose the vessels thoroughly. 



a. The cardiac aorta is a direct continuation forwards 



of the conus arteriosus. It is a median vessel 

 lying beneath the floor of the mouth and pharynx, 

 and deeply placed in the muscles of the under 

 surface of the head. It is little more than an 

 inch in length, and gives off laterally the five 

 pairs of branchial arteries. 



b. The branchial arteries arise in pairs from the 



cardiac aorta, and carry the blood from it to the 

 gills. The two hindmost pairs arise close together, 

 or sometimes by a common trunk, immediately 

 in front of the pericardial cavity. The third pair 

 arise about a quarter of an inch further forwards. 

 At its anterior end, about mid>vay between the 

 pericardial cavity and the mouth, the cardiac 

 aorta divides into right and left branches, each of 

 which again divides into the two anterior bran- 

 chial arteries of its side. 



The first or most anterior branchial artery 

 runs along the outer border of the hyoid arch, 

 extending nearly to its dorsal end, and giving off 

 branches to the giU borne by it. 



The four hinder branchial arteries follow 

 similar courses along the outer borders of the 

 first four branchial arches, extending almost to 

 their dorsal ends, and giving off anterior and pos- 

 terior branches to the gills borne by these arches. 



2. The efferent branchial vessels. 



The efferent branchial vessels can readily be injected from 

 the caudal artery in the following manner. Cut across the tail 

 about a couple of inches behind the cloaca. Find the caudal 

 artery in the hcemal arches of the vertebral column, lying 

 immediately below the vertebral centra, and dorsal to the 



