324 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



a.c.v. 



p.c.u.. 



iriJb. 



of stomach, part of pancreas) and the posterior mesenteric (rectal 

 gland). 



The main arterial trunks divide up into finer and finer branches and 

 the terminal twigs lead into the network of capillaries which traverses 



all the living tissues of the body 

 and serves to bring the circulat- 

 ing blood into intimate relation 

 with the living protoplasm. From 

 the network of capillaries the 

 blood is drained off and returned 

 to the heart by the system of 

 vessels known as veins. The 

 venous system of the adult Dog- 

 fish is of greater morphological 

 importance than the arterial, for 

 in its general plan it shows an 

 arrangement which is clearly re- 

 peated in the young embryonic 

 stages of the higher vertebrates 

 and which may therefore safely 

 be regarded as very primitive. 



The sinus venosus is continued 

 outwards on each side as a large 

 vein — the duct of Cuvier (Fig. 

 136, i.C), named after Cuvier the 

 great French pioneer of Compara- 

 tive Anatomy. The duct of 

 Cuvier conveys to the heart the 

 blood from a large anterior car- 

 dinal vein {a.c.v) which extends 

 back from the head region dorsal 

 to the gill-clefts, and a posterior 

 cardinal ve^n (^.c.v) which drains 

 the blood forwards from the 

 kidney. By far the greater part 

 of the blood circulating through the capillary network of the kidney 

 comes to it by the renal portal {r.p), the two renal portals being formed 

 by the bifurcation of the caudal vein (c.v) or main vein of the tail 

 region which lies immediately underneath the caudal artery (Fig. 132, 

 p. 316, V). 



The blood from the intestine reaches the heart by a special set of 



Fig. 136. 



Diagram illustrating the arrangement of the 

 main venoas trunks in a Dogfish (ScvWiwm). 

 «.c.v, Anterior cardinal ; c.v, caudal ; d.C, duct of 

 Cuvier ; i.j, inferior jugular ; i/ni, intestine ; U, liver; 

 op, opisthonephros ; p.c.v, posterior cardinal ; 

 p.v, hepatic portal ; r.p, renal portal ; s.v, sinus 

 venosus ; sc, subclavian. The hepatic veins are 

 indicated as two simple vessels passing from liver 

 to sinus venosus, their dilatation and approximation 

 not being shown. 



