THE MAMMALIAN VENA CAVA POSTERIOR 29 



14 and 17). Also, it is to be observed in figures 28 and 29 

 that the internal spermatic arteries show certain variant 

 forms. It is of interest here to call attention to what is prob- 

 ably the earliest recorded observation of an atypical condi- 

 tion of the internal spermatic veins in adult man. In his 

 "De Humani Corporis Fabrica," published in 1543, Vesalius 

 shows a figure of the male urinogenital system of man in 

 which the left internal spermatic vein presents a double com- 

 munication with the cava and the left renal vein. The figure 

 published by Vesalius is reproduced in our figure 30. In 

 another figure, of the female urinogenital system, published 

 also in 1543, Vesalius shows a single right and a single left 

 sex vein opening, respectively, into the cava, caudal to the 

 renal level, and into the left renal vein. As this is the normal 

 condition in man, the multiple connection which Vesalius fig- 

 ured for the left sex vein must have been recognized by him 

 as atypical in form. 



The two remaining figures of this series, figures 31 and 32, 

 illustrate two different forms of atypical renal veins. 



Soon after emerging from the kidney, the left renal vein in 

 figure 31 (Fall.L, Froriep, '95) receives the left sex vein and 

 then extends, caudad and mesad, dorsal to the aorta, to open 

 into the cava, at the level of origin of the inferior mesenteric 

 artery. The distinguishing features of this variant left renal 

 vein are that it passes dorsal, instead of ventral, to the aorta 

 and joins the cava somewhat caudal to the usual level of the 

 renal veins. Except when a complete embryonic renal collar 

 persists in the adult (fig. 59), instances in which the left 

 renal vein passes dorsal to the aorta are associated with the 

 absence of the left subcardino-posterior cardinal anastomosis 

 (Siibc.Pc.Anast., fig. 1). In such cases, blood from the left 

 posterior cardinal (Pc.) and the left embryonic renal vein 

 (R.V.) is conveyed by the left side of the renal collar (Subc. 

 Sprc.Anast.) to the left supracardinal (Sprc.), which may 

 retain a connection witli the right supracardinal (cava), dor- 

 sal to the aorta (Int. Sprc.Anast.), at any level in the lumbar 

 region, between the kidneys and the common iliac veins. In 



