THE VENOUS SYSTEM. 651 



but they need not be specified in detail. The vertebral arteries 

 usually arise in close connection with the subclavians, but in 

 Birds they arise from the common carotids. 



BIBLIOGRAPHY of the Arterial System. 



(496) H. Rathke. " Ueb. d. Entwick. d. Arterien \v. bei d. Saugethiere von 

 d. Bogen d. Aorta ausgehen." Mliller's Archiv, 1843. 



(497) II. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier." 

 Dcnkschriften d. k. Akad. IVien, Vol. xm. 1857. 



Vide also His (No. 232) and general works on Vertebrate Embryology. 



The Venous System. 



The venous system, as it is found in the embryos of Fishes, 

 consists in its earliest condition of a single large trunk, which 

 traverses the splanchnic mesoblast investing the part of the 

 alimentary tract behind the heart. This trunk is directly con- 

 tinuous in front with the heart, and underlies the alimentary 

 canal through both its praeanal and postanal sections. It is 

 shewn in section in fig. 367, v, and may be called the sub- 

 intestinal vein. This vein has been found in the embryos of 

 Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs 

 parallel to the dorsal aorta above, into which it is sometimes 

 continued behind (Teleostei, Ganoidei, etc.). 



In Elasmobranch embryos the subintestinal vein terminates, 

 as may be gathered from sections (fig. 368, v.cau}, shortly before 

 the end of the tail. The same series of sections also shews that 

 at the cloaca, where the gut enlarges and comes in contact with 

 the skin, this vein bifurcates, the two branches uniting into a 

 single vein both in front of and behind the cloaca. 



In most Fishes the anterior part of this vein atrophies, the 

 caudal section alone remaining, but the anterior section of it 

 persists in the fold of the intestine in Petromyzon, and also 

 remains in the spiral valve of some Elasmobranchii. In 

 Amphioxus, moreover, it forms, as in the embryos of higher 

 types, the main venous trunk, though even here it is usually 

 broken up into two or three parallel vessels. 



It no doubt represents one of the primitive longitudinal trunks of the 

 vermiform ancestors of the Chordata. The heart and the branchial artery 

 constitute a specially modified anterior continuation of this vein. The 



