210 THE ELASMOBRANCH FISHES 



postcardinal sinuses, the two usuallj^ being freely intercommunicating. At the 

 middle of the postcardinals in the rays (fig. 194b) a spacious sinus (p.c.s.) is 

 formed which is prolonged backward by a narrower outpocket toward the 

 rectal gland. In most forms the thin walls of the sinuses are strengthened by 

 the unusual development of the trabeculae. 



The postcardinals may enter the posterior inner angle of the duct of Cuvier 

 a considerable distance from the entrance of the hepatic veins, as in Hep- 

 tancJms, or they may empty as in Mustelus (fig. 190a) . 



The first and most posterior branches received by the posterior cardinals 

 are the revehentes draining the kidneys; while the tributaries from the an- 

 terior region are usually the large genital sinus draining the gonads, and the 

 subscapular vessel coming from under the scapula. Between these two areas 

 and throughout the greater part of their course forward they receive seg- 

 mental veins from the body walls. The ventral branches of the segmentals 

 drain the blood from the interseptal spaces, and the dorsal branches the blood 

 from the deep musculature of the back. Into the dorsal rami the veins {vs.v., 

 fig. 181) from the spinal cord enter. 



In a type like Acanthias the vertel)rospinal veins leave the neural canal 

 through the foramina of the dorsal nerve roots. Within the canal each vein 

 divides into a dorsal ramus {d.r.v.) which drains the dorsal part of the cord 

 and a ventral ramus {v.r.v.) draining the ventral part. The ventral ramus is 

 also connected with a vena limit ans which extends longitudinally along the 

 ventral side of the cord. In Scyllmm the dorsal rami of each side of the cord 

 form plexuses of veins each of which is more or less united into a longitudinal 

 dorsolateral tract. In the rays, longitudinal tracts form a dorsal spinal vein 

 of large size which as we have seen joins the posterior cerebrals anteriorly. 



HEPATIC PORTAL SYSTEM 



Blood which has been distributed to the digestive tract by the coeliac axis and 

 the mesenteric arteries is returned from the tract by branches of the hepatic 

 portal system. Two such branches in both the sharks and the rays are of 

 special interest. These are the intraintestinal, including the anterior intestinal 

 vein, and the posterior intestinal or mesenteric veins. To these branches should 

 be added the gastric veins mentioned for Heptanchiis. 



The intraintestinal vein (see fig. 173b, i.v.) represents a part of the anterior 

 segment of the subintestinal vein of the embryo. It was discovered first on the 

 free margin of the scroll valve of Zygaena where it is of so large a size that 

 Duvernoy (1833) described it as a "venous heart." Such, however, is not its 

 nature. As it emerges from the anterior end of the valvular intestine it is 

 continued as the anterior intestinal vein. 



The anterior intestinal vein (see p. 184, fig. 173b, a. i.v., and figs. 174 and 

 175) is usually well developed in the sharks, but is small or relatively insig- 

 nificant in the rays. As it continues forward from the intraintestinal vein it 

 is joined by the ventral intestinal which arises on the ventral side of the 

 valvular intestine, and receives annular branches from the attached side of 



