COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 217 



constitute the coronary arteries. In the spiny dogfish, the coronary artery 

 arises on each side from the ventral end of the loop around the third gill cleft 

 (counting the spiracle as the first). It gives off a branch anteriorly and then 

 proceeds posteriorly to the anterior end of the pericardial chamber. Here it 

 forks, giving one branch to the conus arteriosus and other chambers of the heart 

 and the other to the walls of the pericardial cavity. The latter branch after a 

 short distance again passes to the floor of the pharyngeal cavity and is distributed 

 to the esophagus. In the smooth dogfish the coronary artery is formed by paired 

 branches from the ventral ends of the loops around the third and fourth gill 

 clefts. These branches pass to the median line and join to form a single vessel 

 which penetrates the pericardial cavity and runs along the ventral surface of 

 the conus arteriosus where it branches to the conus and ventricle. From the 

 branches arising from the ends of the loops of the fourth gill clefts a vessel runs 

 posteriorly on each side in the dorsal wall of the pericardial cavity and into the 

 walls of the esophagus. In the skate the disposition of the coronary arteries is 

 somewhat irregular. Vessels arise from or near the ventral ends of the loops 

 around the fourth and fifth gill slits. The former passes forward along the floor 

 of the mouth cavity; the latter gives rise to the anterior coronary arteries which 

 run along the conus arteriosus to the heart walls. In the posterior part of the 

 pericardial cavity are the posterior coronary arteries, running along the sinus 

 venosus. These originate from the subclavian artery which will be identified 

 later. 



Turn again to the roof of the mouth. From the dorsal end of the loop 

 around the second gill slit a vessel runs forward. It is the common carotid artery. 

 After a short distance it bends toward the median line. At this bend it gives 

 off a branch, the external carotid artery, which will be found by gently shaving 

 away the cartilage at this point. Beyond this branch the main artery, now 

 known as the internal carotid artery, passes to the median line where it joins its 

 fellow of the opposite side; the vessel thus formed penetrates the cartilage of 

 the skull. In the dogfishes slender vessels connect the first efferent branchial 

 arteries with the common carotid artery; in the spiny dogfish these connecting 

 branches are paired all of the way, while in the smooth species the vessel arises 

 unpaired from the median line and subsequently forks. 



Clear away all tissue from the pretrematic branch of the first efferent 

 branchial artery. From about the middle of the pretrematic branch a vessel 

 arises and passing forward very soon turns sharply dorsally and disappears. 

 This is the hyoidean artery. Turn the animal dorsal side up and remove the 

 skin around and posterior to the spiracle. Pick up the hyoidean artery again 

 below the spiracle. It runs on the inner side of a white band (hyomandibular 

 nerve) which is located just posterior to the spiracle. Follow the hyoidean 

 artery to the walls of the spiracle and note its branches to the rudimentary gill 

 iu the spiracle. (In the skate there are numerous branches to adjacent muscles 



