248 THE DOG-FISH. 



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 midway 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 anterior branchial artery runs 

 along the outer border of the hyoid arch, extend- 

 ing nearly to its dorsal end, and giving off 

 branches to the gill borne by the arch. 



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 numerous anterior 

 and posterior branches to the gills borne by these 

 arches. 



2. The eflferent branchial vessels. 



The efferent braiichial vessels can easily be injected from 

 the caudal artery in the following manner. Gut 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 

 caudal vein. Inject the caudal artery with plaster of Paris, 

 coloured with carmine, and rapidly strained to remove any 

 particles large enough to plug the vessels. Lay the animal in 

 cold water for an hour. 



Insert one of the blades of a strong pair of scissors into the 

 mouth, and cut back horizontally along the left side through 

 the gill-arches, as far back as the last gill-arch ; from the 

 hinder end of this incision cut transversely across the floor of 

 the pharynx, behind the heart, to the corresponding point on 

 the opposite side. Turn the floor of the mouth, with the 

 heart, over to the right side. 



Wash the mouth and pharynx thoroughly. Dissect off the 

 mucous membrane from the roof of the mouth ; flnd the efferent 



