THE ELASMOBRANCH FISHES 



191 



and then runs to the rectal gland and the surrounding parts of the digestive 

 tract, where it l)reaks up into a network of vessels. Unlike Heptanchus, how- 

 ever, it does not usuallj' anastomose with the dorsal intestinal artery. 



Occasionally median vessels arise from the dorsal aorta posterior to the 

 origin of the inferior mesenteric. They are characterized by passing directly 

 downward to the region of the rectum and, occasionally, to the oviducts. Such 

 vessels may be present in Acanthias and one or several of them may be present 

 in Raja. These vessels have been held by 

 Howes (1891) to represent the true inferior 

 mesenteric comparable to that in mammals. 



Paired Branches op Aorta 



SUBCLAVIANS AN^D ILIACS 



The subclavians of the more specialized Se- 

 lachii are similar in position to the same ar- 

 teries in Heptanchus (s.cl., fig. 157) and 

 Hexanchus (fig. 169). Considerable varia- 

 tion in the origin of the subclavians may oc- 

 cur. As a rule they are given otf from the 

 dorsal aorta at the region between the third 

 and fourth pairs of efferent arteries, but 

 they may arise farther back at the base of 

 the fourth pair of efferents {Dasyatis, fig. 

 162, s.cl.). While their origin is usually sym- 

 metrical, sometimes right and left arteries, 



as Monroe (1785) long ago claimed for the ray, arise asymmetrically. Such a 

 condition is found in Dasyatis (fig. 177b). 



The subclavians (fig. 166a) are usually much stronger vessels than are those 

 of Heptanchus where they take little blood from the dorsal aorta. In some 

 specimens of Heptanchus twigs given oflf from the subclavians indicate that 

 the blood is flowing toward the dorsal aorta. In other types, as the subclavians 

 pass outward along the pectoral girdle they give off important branches which 

 differ considerably in the sharks and the rays. Usually in both, after the dorso- 

 lateral {a.cll., fig. 170c) is given off it divides into a relatively large branch 

 which passes forward across the scapula, and another which passes backward 

 between the bundles of muscles. The brachial leaves the subclavian and enters 

 the foramen of the pectoral girdle. As it passes through the girdle it fol- 

 lows the ventral canal and consequently enters the fin on the ventral side 

 (Squalus). 



In the fin of Acanthias the brachial artery separates into a median pterygial 

 {m.p., fig. 178) and a lateral pterygial artery (l.p.). In the rays the brachial 

 separates into a strong branch to the propterygium and another to the meta- 

 pterygium. From these branches numerous smaller arteries run to the radial 

 muscles of the fin. 



Fig. 179. Eelations of rectal artery, 

 Squahis sucJclii. (C. E. Bird, orig.) 



d.a., dorsal aorta; fm., femoral 

 artery ; il., iliac artery ; i.rn., inferior 

 mesenteric ; r., rectal artery ; r.g., 

 rectal gland. 



