xm PHYLUM CHORDATA 91 



the sinus venosus. The blood from the tail returns by a caudal 

 vein (cd. v.), lying immediately below the caudal artery in the 

 haemal canal of the caudal vertebrae (Fig. 769, D). On reaching 

 the coelome the caudal vein forks horizontally, and the two 

 branches either become directly continuous with the cardinals 

 or pass one to each kidney under the name of the renal portal 

 veins (Fig. 781, r. p. v.). In the kidneys they break up into 

 capillaries (Fig. 782, K), their blood mingling with that brought 

 by the renal arteries and being finally discharged into the 

 cardinals by the renal veins (Fig. 781, r.v. ). Thus the blood from 

 the tail may either return directly to the heart in the normal 

 manner or may go by way of the capillaries of the kidneys. 

 In the latter case there is said to be a renal portal system, the 

 essential characteristic of which is that the kidney has a double 

 blood-supply, one of pure blood from the renal artery, and one of 

 impure blood from the renal portal vein ; in other words, it has 

 two afferent vessels, an artery and a vein, and the latter is further 

 distinguished by the fact that it both begins and ends in 

 capillaries instead of beginning in capillaries and ending in a vein 

 of higher order. 



The blood from the gonads is returned to the cardinals by 

 veins called spermatic (sp. v.) in the male, ovarian in the female. 

 That from the paired fins takes, in what appears to be the most 

 typical case, a somewhat curious course. On each side of the 

 body there is a lateral vein (lat. v.), running in the body-wall and 

 following the course of the embryonic ridge between the pectoral 

 and pelvic fins. It receives, anteriorly, a subclavian vein (scl. v.) 

 from the pectoral fin, and posteriorly an iliac vein (il. v.) from the 

 pelvic fin, and in front pours its blood into the precaval. 



The veins from the stomach, intestine, spleen, and pancreas join 

 to form a large hepatic portal vein (h. p. v.), which passes to the 

 liver and there breaks up into capillaries, its blood mingling with 

 that brought to the liver by the hepatic artery (h. a.), a branch of 

 the cceliac. Thus the liver has a double blood-supply, receiving 

 oxygenated blood by the hepatic artery, and non-oxygenated but 

 food-laden blood by the hepatic portal vein (Fig. 782, L). In 

 this way we have a hepatic portal system resembling the renal 

 portal system both in the double blood-supply, and in the fact 

 that the afferent vein terminates, as it originates, in capillaries. 

 After circulating through the liver the blood is poured, by hepatic 

 veins (h. v.), into the sinus venosus. The hepatic, unlike the renal 

 portal system, is of universal occurrence in the Craniata. 



In the embryo there is a sub-intestinal vein, corresponding with 

 that of Amphioxus, and lying beneath the intestine and the post- 

 anal gut. Its posterior portion becomes the caudal vein of the 

 adult, its anterior portion one of the factors of the hepatic portal 

 vein. 



