THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 579 



lateral anastomoses with the cardinal veins. At the heart it opens 

 into the sinus venosus. 



From this primitive form of the venous system (fig. 321 A) is 

 derived the ultimate condition in Man. There are three changes 

 which are conspicuous in this connection. (1) The veins empty 

 directly into the atrium instead of a venous sinus. (2) The sym- 

 metrical arrangement in the region of the Cuvierian ducts and the 

 jugular and cardinal veins gives place to an unsymmetrical arrange- 



Fig. 321. Diagram of the development of the venous system of the body. 



dc, Ductus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subclavia ; vh, v. hepatica 

 revehens ; U, v. umbilicalis ; ci (ci 2 ), v. cava inferior ; ca (ca l , ca 2 , ca 3 ), v. cardinalis ; 

 ilcd, ilcs, v. iliaca communis dextra, sinistra ; ad, as, v. anonynia brachiocephalica dextra, 

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

 v. cava superior sinistra ; cc, v. coronaria cordis ; az, v. azygos ; hz (hs*), v. hemiazygos ; 

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



ment accompanied by a degeneration or stunting of some of the 

 chief trunks. (3) With the development of the liver there is formed 

 a special portal system. 



The alteration first mentioned is accomplished by the incorporation 

 of the sinus venosus in the atrium. At first enclosed in the septum 

 transversum, the sinus elevates itself above the upper surface of the 

 latter, from which it detaches itself, and comes to lie as an 

 appendage to the atrium in the anterior trunk-cavity. Finally it 

 fuses completely with the heart and furnishes the smooth region of 

 the atrial wall, which is destitute of the pectinate muscles (His). 



