204 THE ELASMOBRANCH FISHES 



CIRCULATORY SYSTEM OF ELASMOBRANCHS IN GENERAL 



Blood which has been distributed by the arteries to tlie capillaries of the 

 tissues is collected and returned to the heart by the veins. The veins, as we 

 have said, differ as a rule from the arteries in possessing thinner walls. In sec- 

 tion this is seen to be due especially to a lack in the muscular layer. A back- 

 ward flow of blood, which in the arteries is prevented by the muscular elas- 

 ticity of the larger proximal arteries and by the rhythmic action of the heart, 

 is prevented in the veins by valves. These valves are present at irregular in- 

 tervals throughout the course of some of the veins and are especially marked 

 at the junction of principal trunks, as for example at the entrance of the an- 

 terior cardinal to the duct of Cuvier (v., fig. 188). The valves are formed by 

 the lining of the veins as loose crescentic folds, the concavity of which is 

 directed toward the heart. These permit a free course of the blood toward the 

 heart but prevent its backward flow by filling with blood and thus blocking 

 the lumen. 



The Elasmobranch veins frequently become greatly enlarged sinuses. An 

 incision through the postcardinal sinus shows that its walls, though similar in 

 other respects to those of the veins, differ from them especially in two ways. 

 In the first place if they possess any musculature it is exceedingly thin, and 

 secondly they have, passing from wall to wall, numerous tendinous supporting 

 cords. Many such enlarged sinuses are present in the Elasmobranclis espe- 

 cially in the region of the head and in the proximal part of the veins near 

 the heart. 



Veins 



For convenience of description the veins of the Elasmobranchs in general may 

 be grouped into seven systems as follows: (1) those which return blood from 

 the head, the anterior cardinal sj'stem; (2) veins which bring blood from the 

 caudal region to the kidnej', the renal portal system ; ( 3 ) the vessels draining 

 the kidneys along the dorsal body wall, the posterior cardinals; (4) the veins 

 which carry the blood from the digestive tract and its appendages to the liver, 

 the hepatic portal system; (5) those veins which return blood from the ex- 

 tremities and sides, the lateral abdominal system; (6) a system of veins drain- 

 ing the walls of the heart; and (7) the cutaneous veins or veins of the skin. 



AXTEEIOR CARDINAL SYSTEM 



The anterior cardinal system consists of the anterior cardinal or jugular veins 

 and the inferior jugulars together with their tributaries. The anterior cardinal 

 vein {a.c.s., fig. 190), like that of Heptanchus, passes from the orbital sinus 

 {o.s.) back over the branchial basket. The orbital sinus receives the anterior 

 facial (a.f.v.) or orbitonasal and the anterior cerebral veins {a.c.v., fig. 191), 

 together with certain cutaneous veins of the head. Right and left orbital sinuses 



