XII.] VENOUS SYSTEM. 409 



In some Mammals both subclavians spring from 

 a trunk common to them and the carotids (arteria 

 anonyma) ; or as in Man and some other Mammals, 

 the left one arises from the systemic aorta just beyond 

 the carotids. Various frirther modifications in the origin 

 of the subclavians are found in Mammalia, but they 

 need not be specified in detail. The vertebral arteries 

 arise in close connection with the subclavians, whereas 

 in Birds they arise from the common carotids. 



The venous system. In Mammals the same venous 

 trunks are developed in the embryo as in Birds (Fig. 

 138 A). The anterior cardinals or external jugulars 

 form the primitive veins of the anterior part of the 

 body, and the internal jugulars and anterior vertebrals 

 are subsequently formed. The subclavians (Fig. 138 

 A, s), developed on the formation of the anterior hmbs, 

 also pour their blood into these primitive trunks. In 

 the lower Mammalia (Monotremata, MarsupiaUa, Insec- 

 tivora, some Rodentia, etc.) the two ductus Cuvieri 

 remain as the two superior venae cavse, but more usually 

 an anastomosis arises between the right and left in- 

 nominate veins, and eventually the whole of the blood 

 of the left superior cava is carried to the right side, and 

 there is left only a single superior cava (Fig. 138 B and 

 C). A small rudiment of the left superior cava remains 

 however as the sinios coronarius and receives the coronary 

 vein from the heart (Figs. 138 C, cor and 139 cs). 



The posterior cardinal veins form at first the only 

 veins receiving the blood from the posterior part of the 

 trunk and kidneys ; and on the development of the hind 

 limbs receive the blood from them also. 



An unpaired vena cava inferior becomes eventually 



