996 OF GENERATION. 



</, </), receiving the blood from the head, and a pair of posterior trunks (&, &'), 

 formed by the confluence of the veins of the trunk, Wolffian bodies, &c. ; the 

 former are persistent as the jugular veins ; the latter remain separate in most 

 Fishes, where they are designated the cardinal veins; but in Man (as in warm- 

 blooded Vertebrata generally) they are only represented by the venae azygos, 

 major and minor, 1 which coalesce into a common trunk for a considerable part 

 of their length. One of the anterior trunks and one of the posterior unite on 

 either side, to form a canal which is known as the Ductus Cuvieri ; and the 

 ducts of the two sides coalesce to form a shorter main canal, which enters the 

 auricle, at that time an undivided cavity. This common canal is absorbed into 

 the auricle at an early period, in all Vertebrata above Fishes; and after the 

 septum auriculorum is formed, the two Cuvierian ducts separately enter the right 

 auricle. This arrangement is persistent in Birds and the inferior Mammals, in 

 which we find two Venae Cavae superiores, entering the right auricle separately ; 

 but in the higher Mammalia and in Man, the left duct is obliterated and the 

 right alone remains as the single Vena Cava superior, a transverse communicating 

 branch being formed, to bring to it the blood of the left side. 3 The double 

 Vena Cava sometimes presents itself as a monstrosity in the Human subject. 

 As the anterior extremities are developed, the subclavian veins are formed to 

 return the blood from them; and these discharge themselves into the jugulars. 

 The Omphalo-Mesenteric vein ( 999), which is another primitive trunk common 

 to all Vertebrata, is formed by the confluence of the veins of the yelk-bag and 

 intestinal canal, and passes by itself, with the two Cuvierian ducts, into the 

 auricle. The upper part of this remains to constitute the upper part of the 

 Inferior Cava (Figs. 266, 267, j), the lower portion of which arises between, the 

 Wolffian bodies, and originally enters the omphalo-mesenteric vein above the 

 liver. When the liver is formed, the omphalo-mesenteric vein becomes connected 

 with it both by afferent and by efferent trunks, the former remaining as the 

 Vena Portae, and the latter as the Hepatic vein ; and after giving off the former 

 trunks, the omphalo-mesenteric vein is itself obliterated, so that all the blood 

 which it conveys passes through the liver. The Inferior Cava, which receives 

 the hepatic vein, is gradually enlarged by the reception of most of the veins 

 from the inferior part of the trunk and the lower extremities, and the vena 

 azygos is reduced in the same proportion ; in some rare cases of abnormal forma- 

 tion, however, the vena cava fails to be developed, and then the blood from the 

 lower parts of the body is conveyed to the superior cava through the system of 

 the vena azygos. The Umbilical Vein is to be regarded as a product of the 

 combination of the veins of the allantois with an anterior vein of the abdominal 

 parietes; it being probably through this latter channel that it comes to discharge 

 itself into the vena portae, which lies in a part of the body very distant from 

 that at which the allantois was developed. As the omphalo-mesenteric vein 

 diminishes in size, the umbilical vein increases, becoming the chief source of 

 supply to the vena portae ; and it also forms an anastomosis with the inferior 

 cava, which constitutes the Ductus Venosus. 



1004. The following is the course of the Circulation in the mature Fretus : 

 The fluid brought from the placenta by the umbilical vein is partly conveyed at 

 once to the ascending Cava by means of the ductus venosus, and partly flows 

 through the vena portae into the liver, whence it reaches the ascending Cava by 

 the hepatic vein. Having thus been transmitted through the two great depu- 



1 See Miiller's " Verleichende Anatomic der Myxinoiden," Berlin, 1840. 



2 The stages of this transformation have been particularly well made out by Mr. Mar- 

 shall, in his elaborate Memoir " On the Development of the Great Anterior Veins in Man 

 and Mammalia" (" Phil. Trans*.," 1850); and he has further shown that some vestiges of 

 the original arrangement may be traced even in the normal condition of the venous system 

 in the adult. 



