VEINS 



227 



small veins from the mesentery, which are united with one another across the front 

 of the aorta. These unite with one another on each side to form a pair of longitudinal 

 anastomoses (subcardinal veins, Lewis), running parallel to the cardinal veins and 

 united with^them at both ends as well as by many small veins along their course 

 (fig. 282). The cross-connexion between the two subcardinal veins next becomes 

 limited to one large anastomosis below the superior mesenteric artery (fig. 283). The 

 right subcardinal now acquires a secondary connexion with the hepatic veins on 



t;. cartiina/is 



liver 

 v. caca inferior 



FIG. 282. DEVELOPMENT OF THE INFERIOR VENA CAVA IN THE 'RABBIT-EMBRYO (FIRST STAGE). 



(After Lewis.) 



the dorsal aspect of the liver, and as the blood comes to take this new short cut 

 to the heart the portion in front of the anastomosis becomes rapidly enlarged. 



According to Hochstetter's account ' (with which that of Lewis here followed otherwise agrees 

 in every essential respect), this connexion is established by a new channel which arises from the 

 common hepatic (original right vitelline) vein, and first appears as a small vessel which passes 

 downwards, through a coalomic fold which is named the caval mesentery along the mesial 

 surface of the Wolffian body, where it forms the vein called by Lewis the right subcardinal. 

 A similar vessel appears simultaneously on the left side, and the two are connected by the 

 anastomosis already mentioned. 



The right subcardinal, thus enlarged, becomes a portion of the vena cova, while 

 the disconnected anterior part of the left subcardinal probably persists as the left 

 suprarenal vein. The portions of both subcardinals behind the cross-anastomosis 

 diminish in size and disappear as blood-channels ; the corresponding sections 

 of the two cardinal veins, which are connected with one another across the middle 



1 For references, see Hochstetter's article in Hertwig's Handbuch III. Th. ii. and iii. p. 161 seq. 

 More recent papers are Lewis (rabbit), Amer. Jour, of Anat. i. ; Dexter (cat), ibid. i. ; Miller (bird), 

 ihid. ii. ; Bonne (rabbit and sheep), Jour. d'Anat. et 'de la Phys. xxxix. 1904 ; Soulie et Bonne (mole), 

 ibid. xli. 1906. 



Q2 



