582 EMBRYOLOGY. 



inferior vena cava has been (by Hochstetter) explained. According 

 to his investigations there are to be distinguished two tracts which 

 are of different oiigin, a shorter anterior and a longer posterior. 

 The former, as previously mentioned, makes its appearance as an 

 inconspicuous vessel on the right side of the aorta in the tissue 

 between the two primitive kidneys (fig. 322 A and B ci) ; the latter, 

 on the contrary, is developed subsequently out of the posterior region 

 of the right cardinal vein (fig. 322 £ ci ^). The anterior, inde- 

 pendently arising part of the inferior vena cava, soon after its 

 establishment, unites with the two cardinal veins by means of 

 transverse branches in the vicinity of the vena renalis (?•). In con- 

 sequence of this increase of drainage territory, it scon increases con- 

 siderably in calibre, and since it presents more favorable conditions 

 for the conveyance of blood from the lower half of the body than 

 the upper portion of the cardinal veins does, it finally becomes the 

 chief conduit. 



If the stage thus far described were to become the permanent 

 condition (fig. 322 B), we should have an inferior vena cava, which 

 forks in the region of the renal veins (r) into two parallel trunks, 

 that descend at both sides of the aorta to the pelvis. Such cases, as 

 is known, are found among the varieties of the venous system ; they 

 are derived from the previously described stages of development as 

 malformations by arrested growth. However, they are only rarely 

 observed, for in the normal course of development there is established 

 at an early period an asymmetry between the lower portions of the 

 two cardinal veins, from the moment, indeed, when they have united 

 themselves to the lower part of the inferior vena cava by means of 

 anastomoses. The right portion acquires a preponderance, becomes 

 enlarged, and finally alone persists (fig. 322 B, C), whereas the left 

 lags behind in growth and withers. This results from two conditions. 

 First, the right cardinal vein (ci^) lies more in the direct prolongation 

 of the vena cava inferior than does the left, and thus occupies a 

 more favorable situation ; secondly, there is formed in the pelvic 

 region an anastomosis (ilcs) between the two cardinal veins, which 

 conducts the blood of the left hypogastrica and the left iliaca externa 

 and cruralis to the right side. Owing to this anastomosis, which 

 becomes the vena iliaca communis sinistra, the portion of the left 

 cardinal vein lying between the renal veins and the pelvis (fig. 322 

 C ca^) is rendered functionless, and with the degeneration of the 

 primitive kidney disappears. The right cardinal vein has now 

 become for a lertain distance a direct continuation of the inferior 



