326 COMPARATIVE ANATOMY 



FIG. 266. DIAGRAM OF THE VEINS OF AX ELASMOBRANCH. 



H, heart ; Duct. Cur, precaval sinus ; Card.ant (Jug), anterior cardinal (jugular) ; 

 the inferior jugular is seen nearer the middle line ; Subcl, subclavian ; Seit. 

 V, lateral vein, which arises from a venous network in the region of the 

 cloaca (Ven.Cl.J3), from one or more cutaneous veins of the tail (Cut. V), 

 from the veins of the body-walls, and from those of the pelvic fins 

 (HEV) ; Caud.v, caudal vein, which divides into two renal portals, A, A 1 , 

 at the posterior end of the kidneys (N) : from these arise the adve- 

 hent veins of the renal portal system (V.adv) ; V.rev, revehent renal 

 veins, from which the posterior cardinals (CV) arise; Card.V.S, cardinal 

 sinus, communicating with its fellow in the middle line ; V.port, hepatic 

 portal vein, receiving its blood from the intestine (ED), stomach (Mg), and 

 oesophagus (Oes.V), and anastomosing with the lateral vein posteriorly, and 

 with the cardinal sinus anteriorly ; Gen. V, genital veins ; L. V.S, hepatic 

 sinus ; Leb, liver. 



and giving off advehent vessels into the latter (Figs. 264, 265, IX 

 XII, 266-268). These divide up into capillaries, forming a renal 

 portal system, the capillaries again uniting to form revehent veins 

 which open into the posterior cardinals. Thus the typical condition 

 of the veins seen in adult Fishes is reached, and only a few of the 

 more important modifications can be mentioned here. 



In Cyclostomes and Elasmobranchs, the anterior part of the 

 subintestinal vein still persists as a small vessel running within 

 the spiral valve of the intestine. In the latter Order, many of the 

 veins (e.g., precavals, anterior and posterior cardinals, inferior jugulars, 

 hepatic and genital veins) enlarge to form capacious sinuses, and 

 a large lateral vein (Figs. 264, 266), running in the body-walls either 

 close to the skin or just external to the peritoneum, opens into each 

 precaval or posterior cardinal. This probably corresponds to the 

 vein of the primary lateral-fin folds (p. 104). 



A renal portal system is said to be absent in Cyclostomes, and 

 is inconstant and very variable amongst Ganoids and Teleosts : in 

 many instances the caudal vein communicates directly with one or 

 with both posterior cardinals, and in the former case the other 

 cardinal shows a tendency to become reduced in size : a similar 

 reduction occurs in many of the forms to be described next. 



Dipnoi. The chief point of interest as regards the veins of 

 Dipnoans (Fig. 267) is the presence of a large unpaired postcaval 

 vein, derived in part from the posterior cardinal, and comparable to 

 that of the Amphibia and Amniota. A renal portal system is 

 present, and the blood from the kidneys is collected into two veins 

 having the relations of posterior cardinals. Only the left of these 

 however, opens anteriorly into the corresponding precaval, the 

 right, which is much the larger of the two, passing along the 

 dorsal border of the liver to open independently into the sinus 

 venosus in the middle line. The renal portion of this vein is 

 evidently homologous with the corresponding part of the posterior 

 cardinal, the anterior portion of which can no longer be recognised. 



