THE THORAX 1015 



Left Subcardinal. — ^The left subcardinal vein, in the cephalic direction, 

 has no connection with the liver. The part above the level of the great 

 transverse pre-aortic subcardinal anastomosis persists as the left suprarenal 

 vein ; and its lower part, below the level of the anastomosis just named, 

 atrophies. 



Development ol the Inferior Vena Cava. 



The inferior vena cava, as regards its development, consists of t^vo divisions 

 — lower, or postrenal, and upper, or prerenal. 



As previously stated, the posterior cardined tind subcardinal veins of the 

 right side are connected by the great transverse pre-aortic anastomotic vessel 

 below the root of the superior mesenteric artery, and at the level where the 

 right renal vein opens into the right posterior cardinal vein. The part of 

 the right posterior cardinal below this anastomotic vessel comes into line 

 with the part of the right subcardinal above it, and so one channel is formed. 



The postrenal division of the inferior vena cava is formed by the lower 

 portion of the right posterior cardinal vein — that is to say, the portion below 

 the great transverse pre-aortic anastomotic vessel, which forms a part of the 

 left renal vein. 



The prerenal division is more compUcated in its development. As pre- 

 viously stated, the upper part of the right subcardinal vein becomes con- 

 nected with the hepatic veins by a fusion between the hepatic and subcardinal 

 sinusoids, dorsal to the liver and within the caval mesentery, a new channel 

 being thereby formed to the common hepatic vein and heart. (The common 

 hepatic vein is formed by the union of the right and left hepatic veins and 

 ductus venosus.) The prerenal division of the inferior vena cava is formed, 

 from below upwards, by (i) the upper portion of the right subcardinal vein — 

 that is to say, the portion above the great transverse pre-aortic anastomotic 

 vessel, (2) the fusion of hepatic and right subcardinal sinusoids within the 

 caval mesentery, (3) the right hepatic vein, and (4) the common hepatic vein, 

 which is connected with the heart. 



Summary of Development of Inferior Vena Cava. 



Postrenal Division = Portion of right posterior cardinal between right renal 

 and transverse iUac veins. 



1. Anastomotic connection between right posterior 

 cardinal and right subcardinal veins at level of 

 right renal vein. 



2. Upper portion of right subcardinal vein — that is to 

 say, the portion above the great transverse pre- 

 aortic anastomotic vessel. 



3. Fusion of hepatic and right subcardinal capilleiries 

 within the caval mesentery. 



4. Right hepatic vein. 



5. Common hepatic vein. 



Development of Renal Veins. — ^Each renal vein is originally a segmental 

 vein of the corresponding intermediate cell-mass, in which the renal organs are 

 developed. The right vessel has no other source, but the terminal part of 

 the left renal vein is formed by the great transverse pre-aortic anastomotic 

 vessel, which lies across the aorta below the root of the superior mesenteric 

 artery. 



Development of Suprarenal Veins. — Each vessel probably originates as a 

 segmental vein of the suprarenal area of the corresponding intermediate cell- 

 mass. The right vessel has no other source, but the terminal part of the left 

 vein s formed by the part of the left subcardinal vein above the great trans- 

 verse pre-aortic anastomotic vessel, which persists as the mesial part of the 

 left renal vein. 



Development of Lumbar Veins. — The right and left lumbar veins are origin- 

 ally the segmental veins of the lumbar region. The right vessels have no further 



Prerenal Division =■( 



