218 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



and only very small branches to the walls of the spiracle.) Now turn the animal 

 ventral side up, remove the mucous membrane between the spiracle and the 

 upper jaw, and shave away the cartilage about halfway between the upper jaw 

 and the common carotid artery An artery of moderate size will be revealed. 

 Trace it toward the spiracle and note that it is the continuation of the hyoidean 

 artery already seen, formed by the reunion of the branches in the spiracular 

 gill. It passes into the skull ventral to the external carotid artery, and is then 

 known as the ventral carotid artery. On tracing it into the skull by scraping 

 away the cartilage, the ventral carotid artery will be found to join the internal 

 carotid. 



The efferent branchial arteries pass to the median dorsal line of the roof of 

 the pharyngeal cavity. In the skate the first and second join to one, and shortly 

 posterior to this junction a vertebral artery arises on each side and passes into the 

 cartilage of the skull, where it is distributed to the brain and spinal cord. The 

 efferent branchial arteries join in pairs in the median dorsal line and form a 

 large trunk, the dorsal aorta, which passes into the pleuroperitoneal cavity. 



Draw, showing efferent branchial arteries, their branches, and their distri- 

 bution to the gills. 



We are now in a position to compare the head arteries of the elasmobranch with the 

 primitive plan explained at the beginning of this section. The afferent branchial arteries are 

 the ventral halves, the efferent branchial arteries the dorsal halves of the aortic arches. These 

 arches connect the ventral aorta springing from the heart with the dorsal aorta, and run in 

 the walls of the pharynx, one to each visceral arch. In elasmobranch and other fishes the 

 central part of each aortic arch breaks up into a number of smaller vessels and capillaries 

 running in the gills. Theoretically and in vertebrate embryos there are six aortic arches. In 

 elasmobranchs there are five ventrally and four dorsally. The first is lacking on the ventral 

 side and also absent on the dorsal side; but the second aortic arch is imperfectly represented 

 by the hyoidean artery which supplies the spiracle. The four efferent branchial arteries pres- 

 ent in their full development in elasmobranchs are, then, the third, the fourth, fifth, and sixth 

 aortic arches. It should be noted that the carotid artery springs from the third aortic arch, a 

 condition universal among vertebrates. The first aortic arch is missing in all adult vertebrates 

 (except cyclostomes) , and above fishes the second has also vanished. 



7. The dorsal aorta and its branches. Separate the esophagus from the body 

 wall on the left side and follow the dorsal aorta posteriorly. From the dorsal 

 aorta between the points where the third and fourth pairs of efferent branchial 

 arteries unite with it a subdavian artery is given off on each side. Trace the left 

 one into the pectoral fin. It proceeds obliquely caudad and laterad passing on 

 the dorsal wall of the large bag formed by the posterior cardinal sinus. (In the 

 skate it gives off the posterior coronary artery and passes internal to a large white 

 band, the nerve of the pectoral fin. This may be cut through.) At the lateral 

 boundary of the posterior cardinal sinus, the subclavian artery gives rise to the 

 small lateral artery which branches into the body wall and usually proceeds pos- 

 teriorly along the body wall in a position on a level with the lateral line. Farther 



