CIRCULATORY ORGANS. 



279 



the two cardinal veins, an anterior cardinal (superior jugular) and 

 a postcardinal vein (fig. 285), which belong to the dorsal half of 

 the body. The superior jugular comes from the head, 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 nephric 

 system, and keep pace with its development backward, so that they 

 eventually reach the loop which the caudal and subintestinal vein 



FIG. 285. Developing anterior veins of Scyttium embryo, 26 mm. long; after Grosser. 

 b l - 6 , veins of the visceral arches; cd, Cuverian duct; h, vein of hyoid arch; ij, inferior jugu- 

 lar; m t vein of mandibular arch; os, orbital sinus; sv, segmental veins; vca, vcp, pre-*and 

 post-cavas; III-X, cranial nerves; 2-8, spinal nerves. 



makes in passing around the anus. They run just above the dorsal 

 side of the ccelom and dorsal to the nephridial arteries (p. 275). 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. 

 As the mesonephroi (see Excretory Organs) reach the hinder end of the 

 ccelom, the caudal vein loses its primitive connection with the subintesti- 

 nal vein and becomes connected with a pair of vessels, the subcardinal 

 veins, which develop between the mesonephroi and ventral to the nephrid- 

 ial arteries (fig. 286, B). The blood from the tail now goes through 

 the subcardinals and from them into the excretory organs, passing 

 through a system of capillaries, to be gathered again in the postcardinals 



