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 
Fic. 285.—Developing anterior veins of Scyllium embryo, 26 mm. long; after Grosser. 
b 1-5, veins of the visceral arches; cd, Cuverian duct; /, vein of hyoid arch; 2), inferior jugu- 
lar; m, vein of mandibular arch; os, orbital sinus; sv, segmental veins; vca, vcp, pre- and 
post-cavas; JIJ—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 
coelom, 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 
