662 



THE VEINS OF MAMMALIA. 



blood from the posterior part of the trunk and kidneys ; and on the 

 development of the hind limbs receive the blood from them also. 



As in the types already described 

 an unpaired vena cava inferior becomes 

 eventually developed, and gradually 

 carries off a larger and larger portion 

 of the blood originally returned by the 

 posterior cardinals. It unites with the 

 common stem of the allanloic and 

 vitelline veins in front of the liver. 



At a later period a pair of trunks 

 is established bringing the blood from 

 the posterior part of the cardinal veins 

 and the crural veins directly into the 

 vena cava inferior (fig. 374, //). These 

 vessels, whose development has not 

 been adequately investigated, form the 

 common iliac veins, while the posterior 

 ends of the cardinal veins which join 

 them become the hypogastric veins (fig. 

 374, hy}. Owing to the development of 

 the common iliac veins there is no renal 

 portal system like that of the Reptilia 

 and Amphibia. 



Posterior vertebral veins, similar to 

 those of Reptilia and Birds, are estab- 

 lished in connection with the intercostal 

 and lumbar veins, and unite anteriorly 

 with the front part of the posterior 

 cardinal veins (fig. 373 A) 1 . 



On the formation of the posterior vertebral veins, and as the inferior 

 vena cava becomes more important, the middle part of the posterior car- 

 dinals becomes completely aborted (fig. 374, c), the anterior and posterior 

 parts still persisting, the former as the continuations of the posterior 

 vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins 



FIG. 374. DIAGRAM OF THE CHIEF 

 VENOUS TRUNKS OF MAN. (From 

 Gegenbaur.) 



fs. vena cava superior ; s. sub- 

 clavian vein ; ji. internal jugular ; jc. 

 external jugular ; az. azygos vein ; ha. 

 hemiazygos vein ; c. dotted line shew- 

 ing previous position of cardinal veins ; 

 ci. vena cava inferior ; r. renal veins ; 

 il. iliac ; hy. hypogastric veins ; Ji. 

 hepatic veins. 



The dotted lines shew the position 

 of embryonic vessels aborted in the 

 adult. 



Though in a few Mammalia both the posterior vertebrals persist, a 

 transverse connection is usually established between them, and the one (the 

 right) becoming the more important constitutes the azygos vein (fig. 374, as), 

 the persisting part of the left forming the hemiazygos vein (ha). 



The remainder of the venous system is formed in the embryo of the 

 vitelline and allantoic veins, the former being eventually joined by the 

 mesenteric vein so as to constitute the portal vein. 



1 Rathke, as mentioned above, holds that in the Snake the front part of the 

 posterior cardinals completely aborts. Further investigations are required to shew 

 whether there really is a difference between Mammalia and Reptilia in this matter. 



