THE VENOUS SYSTEM. o35 



both subclavians spring from the right side. In most other types 

 the origin of the subclavians is carried upwards, so that they usually 

 spring from a trunk common to them and the carotids (arteria 

 anonyma) (Birds and some Mammals); or the left one, as in Man and 

 some other Mammals, arises from the systemic aorta just beyond 

 the carotids. Various further modifications in the origin of the sub- 

 clavians of the same general nature are found in Mammalia, 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 uf the Arterial Si/stem. 



(496) H. Rathke. "Ueb. d. Entwick. d. Arterieii w. bei d. Ssiugetbiere von d. 

 Bogen d. Aorta ausgelien." MuUer's Archil', 1843. 



(497) H. Eathke. " Untersuchungeu lib. d. Aortenwurzeln d. Saurier." Denk- 

 schriften d. k. Akcid. Wie)i, Vol. xiii. 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, con- 

 sists 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 continuous in front with the 

 heart, and underlies the alimentary canal through both its prff'anal 

 and po.stanal sections. It is shewn in section in fig. 367, v, and may be 

 called the subintestinal 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 some- 

 times continued behind (Teleostei, Ganoidei, etc.). 



In Elasmo branch 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 imiting into a single vein both in front 

 of and behind the cloaca. 



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

 Ejection 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 Cliordata. The heart and the branchial artery 

 constitute a specially modified anterior continuation of this vein. The 

 dilated portal sinus of Myxine is probably also part of it; and if this is 



