462 ELASMOBRANCHII [CH. 



mesenteric artery supplies the greater part of the intestine and 

 sends branches to the reproductive organs. The lieno-gastric 

 supplies the posterior part of the stomach, the spleen and part 

 of the pancreas. The posterior mesenteric supplies the rectal 

 gland. After giving branches to the genital organs, kidneys and 

 pelvic fins, the aorta continues its course into the tail as the caudal 

 artery. From the two most anterior branchial loops a pair of vessels 

 arise running forward in the dorsal wall of the pharynx and at the 

 same time converging. These are the common carotid arteries, 

 which supply blood to the head. Each divides into two main 

 branches, an external carotid, which pierces the floor of the 

 orbit and supplies the eye and the jaw, and an internal carotid, 

 which pierces the floor of the skull near the middle line and supplies 

 the brain. The pseudobranch on the front wall of the spiracle 

 receives its blood from the hyoidean artery which, branching 

 from the loop surrounding the first gill- sac, runs forward in the 

 roof of the mouth parallel with the common carotid artery and 

 eventually joins the internal carotid. In the venous system the 

 anterior portion of the subintestinal vein is represented by 

 a pair of hepatic veins returning the blood from the liver, 

 opening into the sinus venosus close to the middle line, whilst 

 the posterior portion has dwindled to a small vein embedded 

 between the folds of the spiral valve ; this however is joined by 

 branches from the sides of the intestinal wall to form the main trunk 

 of the portal vein. Both anterior and posterior cardinal 

 veins are represented by wide, somewhat irregular spaces. Each 

 anterior cardinal has an expansion called the orbital sinus which 

 surrounds the eye. The two orbital sinuses communicate by an 

 interorbital canal tunnelled in the base of the skull. The blood 

 from the ventral sides of the gill-sacs and pharynx is returned to 

 the ductus Cuvieri by a pair of independent trunks called the 

 jugular veins. These are each connected with the anterior cardinal 

 vein of its side by the hyoidean vein lying in a groove on the 

 hyomandibular cartilage (Fig. 213). The blood from the tail is 

 returned by a median caudal vein lying beneath the caudal artery 

 and like it enclosed between the centra and the united ventral ends 

 of the haemal arches. At the level of the posterior end of the 

 kidneys the caudal vein divides into the two renal portal veins 

 lying on the outer edges of the kidneys. These veins, as has been 

 already explained (see p. 433), are the hinder portions of the 

 posterior cardinal veins which break up into the renal portal system 

 of capillaries. These filter amongst the kidney tubules and reunite 



