VEINS 



311 



dorsal to the gill clefts and brings blood from the more dorsal regions. 

 Since the inferior jugulars are found only in fishes and salamanders, 

 the anterior cardinal is usually called simply the jugular and that 

 usage will be followed here. 



The postcardinals are closely related in development to the 

 excretory system, and keep pace with its development backward/ so 



Fig. 331. — Scheme of development of the principal veins, a, anus; az, azj'gos 

 major; c, coronary vein; ca, caudal vein; cd, Cuvierian duct; ei, external iliac; g, gonads; 

 ge, genital (spermatic, ovarian) vein; h, hepatic veins; ha, hemiazygos; hi, heart; i, 

 ischiadic; y, jugular; li, left innominate; mn, min, meso- and metanephroi; om, omphalo- 

 mesenterics; p, postcava; pc, postcardinal; pn, pronephros; pr, precava; r, renal; ri, 

 right innominate; s, subclavian; sc, subcardinal; M,subintestinal; 5tc, superior intercostal 



In A the early condition with paired omphalomesenterics and subintestinals, the 

 postcardinals extending back as far as the pronephroi. B, mesonephroi developed and 

 with them the subcardinals and the beginnmg of the postcava; one omphalomesenteric 

 lost and subintestinals and caudals beginning to fuse; the intestinal vessels omitted in 

 the later figures. C, postcava has joined sinus and postcardinals have reached caudals; 

 D, amniote, appearance of metanephroi (true kidneys) with obsolescence of mesonephroi; 

 the postcardinals lose connexion with caudal, their place being taken by the backward 

 extension of the subcardinals; formation of cross connexions between jugulars and 

 between postcardinals of the two sides. E, breaking up of postcardinals and disappear- 

 ance of left Cuvierian duct, the other being called the precava. 



Postcaval elements crosslined; subcardinal, dotted; other veins black. 



that they eventually reach the loop which the caudal and subintes- 

 tinal vein makes in passing around the anus. They run just above 

 the dorsal side of the coelom and dorsal to the nephridial arteries 

 (p. 307). (iT They are preeminently the blood-drainage system of the 

 early excretory organs and they retain that function throughout life 

 in the lower vertebrates^^ 



Closely associated with the postcardinals are the subcardinals. 



