THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 581 



return of the blood to the heart. In consequence of this the 

 proximal end of the right cava becomes much larger, the left, on 

 the contrary, proportionately smaller. Finally, there is a complete 

 wasting away of the latter blood course (fig. 322 C ess) as far as the 

 terminal part (cc), which is lodged in the coronal groove. This part 

 remains open, because the cardiac veins convey blood to it, and is 

 now distinguished as sinus coronarius. 



A process in many respects similar to this is repeated in the case 



Fig. 322. Diagram of the development of the venous system of the body. 



<Zc, Dxictus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subolavia; vh, v. hcpatica 

 revehens; U, v. umbilicalis ; ci (ci 2 ), v. cava inferior; ca (ca l , ca 2 , ca'), v. cardinalis ; 

 ilcd, Hcs, v. iliaca comnmnis dextra, sinistra ; ad, as, v. anonyma brachiocephalica dextra, 

 sinistra; cs, v. cava superior; csd, v. cava superior dextra; ess, rudimentary portion of 

 v. cava superior sinistra ; cc, v. coronaria cordis ; az. v. azygos ; hz (/iz 1 ), v. hemiazygos ; 

 He, v. iliaca externa; Hi, v. iliaca interna; r, v. renalis. 



of the cardinal veins (fig. 322 A ca). The latter collect the blood 

 from the primitive kidneys and the posterior wall of the trunk, from 

 the pelvic cavity and the posterior limbs. From the pelvic cavity 

 they receive the venae hypogastricse (Hi), and from the limbs the 

 v. iliacse externoe (He) and their continuation, the v. crurales. In this 

 way the cardinal veins are at first, as in Fishes, the chief collecting 

 trunks of the lower half of the body. Subsequently, however, they 

 diminish in importance, since the inferior vena cava becomes the 

 main collecting trunk instead of them. 



It is only within the last few years that the development of the 



