DEVELOPMENT OF THE VEINS. 



885 



arrangement (II) this anastomosis (As) becomes very large to form the left innominate vein, 

 while with the growth of the arms the subclavian veins increase (bb) ; and lastly, the calibre of 

 the jugular vein changes, the internal jugular (Ji) becoming very large, and the external jugular 

 (Ie) smaller. In some animals, e.g., the dog and rabbit, the large embryonic size is retained. 

 The part of the left superior cardinal vein, from the anastomosis downwards to the left duct of 

 Cuvier, disappears. The posterior cardinal veins divide in the pelvis into the hypogastric 

 (I, h) and external iliac (/,/). The inferior cava at first is very small (I, Vc), divides at the 

 entrance to the pelvis, and on each side 



goes into the point of division of the J # ]j [. Tc_. 



cardinal veins. There is also a trans- 

 verse ascending anastomosis between the 

 right and left cardinal veins. For the 

 final arrangement, the cava inferior (II, 

 Ci) dilates, and with it the hypogastric 

 and external iliac vein on each side. The 

 right cardinal vein remains very small 

 (Vena azygos, Az), and also the lower 

 part from the left one to the transverse 

 anastomosis. The latter itself also re- 

 mains very small ( Vena hemiazygos, Hz). 

 On the other hand, the upper part above 

 the anastomosis to the duct of Cuvier 

 disappears. Lastly, the common large 

 venous trunk is so absorbed into the wall 

 of the auricle ( V) that both venae cavae 

 have each a separate orifice (p. 876). 

 The embryonic condition of the veins 

 persists in fishes. 



Veins of the First and Second Circu- 

 lation, and Portal System* The two 

 omphalo-mesenteric veins (om, om x ) open 

 into the posterior or venous end of the 

 tubular heart (fig. 678, l,.H). The right 

 vein, however, disappears very soon. As 



soon as the allantois is formed, the two T } First appearance of the veins of the embryo, 

 umbilical veins join the truncus venosus Their transformations to form the final arrange- 

 (1, u, u x ). At first the omphalo-mesen- ment. 

 teric veins are larger than the umbilical 



veins ; at a later period this is reversed, and the right umbilical vein disappears. As soon as 

 veins are formed within the body proper of the embryo, the inferior cava also opens into the 



Fig. 677. 



II, 



om 



Fig. 678. 

 Development of the veins and portal system. H, heart ; E, L, right and left side of the body; 

 om, right omphalo-mesenteric vein ; om v left; u, right umbilical vein ; u x , left ; Ci, vena 

 cava inferior ; a, venae advehentes ; r, venae revehentes ; D, intestine : m, mesenteric 

 vein ; 4, I, splenic vein ; 2, I, liver. 



truncus venosus (2, Ci). Gradually, the umbilical vein (2, u x ) becomes the chief trunk, while 

 the small omphalo-mesenteric (2, om t ) carries little blood. 



Portal System. The umbilical and omphalo-mesenteric veins pass in part dim tly under the 



