Fig. 269.—Iguana tuberculata, ♂. Genito-urinary tract, dorsal view, with renal-portal, postcardinal, and postcaval veins. (Columbia University Museum, No. 1862.)
The renal efferent system begins by a number of inter-renal anastomoses which unite along the mesal border of the right kidney into a large ascending trunk, while the corresponding vessel of the left side, starting from the same anastomosis, is considerably smaller (Figs. 266 and 269). Each of these vessels also receives blood from the testis, epididymis, vas deferens and adrenal body in the male, and from the ovary and oviduct in the female. They represent, in fact, the distal functional part of the right and left embryonic postcardinal vein. Just caudad of the left testis the vein of the left side crosses obliquely ventrad of the aorta and joins the right vessel to form the trunk of the postcava, which enters, immediately beyond the cephalic pole of the right testis, the prolonged caval lobe of the liver (Figs. 266 and 269). Ascending in the substance of this gland and receiving the afferent hepatic veins (Fig. 268), the vena cava emerges from the cephalic surface of the liver greatly enlarged and proceeds to the right auricle.
The abdominal vein divides below into two branches which pass caudad on each side of the bladder, receiving tributaries from the same, to the lateral border of the kidneys (Figs. 266 and 269). Here the vessel is connected by the transverse branch above described with the afferent renal portal system derived from the caudal vein. At the same point it receives the sciatic vein, the principal venous vessel of the posterior extremity. Above, the main abdominal vein, resulting from the union of the two branches referred to, ascends on the dorsal surface of the ventral abdominal wall, receiving a few twigs from the ventral mesogastrium within whose free caudal edge the vessel runs. Just before reaching the liver the abdominal vein turns dorsad on the caudal surface of the gland and joins the hepatic portal vein (Figs. 268 and [275]). Several accessory veins, two or three in number, belonging to the system of the abdominal vein, pass above this point from the ventral body wall between the layers of the ventral mesogastrium, to enter the liver separately on its convex ventral surface, above the fusion of the main abdominal vein with the portal vein. These additional branches on entering the liver join the portal system, forming a set of ventral accessory portal veins.
The hepatic portal vein derives its principal tributaries from the splenic, gastric, pancreatic and intestinal veins. One or two additional branches (accessory vertebral portal veins), as above stated, connect the system of the segmental and vertebral veins with the portal circulation, entering the liver separately. In like manner one or two gastric veins (accessory gastric portal veins) enter the dorsal aspect of the liver separately, passing from the stomach to the gland between the layers of the gastro-hepatic omentum ([Fig. 275]).
Compared with the development of the mammalian type, the venous system of Iguana serves to illustrate the stage in the history of the umbilical vein (represented by the abdominal vein of the reptile) in which the connection of the vessel with the portal vein has been formed and transmits the greater part of the blood returned by the umbilical vein to the liver, while the proximal segment above this point, originally continued into the sinus venosus, has begun to disappear, being, however, still represented by the vessels which, as accessory ventral portal veins, pass in the ventral mesogastrium, from the body wall to the liver.
It will be noted that all the hepatic portal blood, whether conducted by the main portal and abdominal vein, or by the accessory portal branches, traverses the capillary circulation of the liver before entering the postcava.
The vertebral and segmental venous system, representing the azygos veins of the mammalia, is very rudimentary ([Figs. 266] and [267]). The distal portions of the postcardinal veins form the efferent renal branches and the ascending trunks of the postcava.
The next segment of the vertebral veins appears as a trunk on the right side which enters the portal circulation. A second vein higher up is connected with both the gastric portal system and with the longitudinal chain of the vertebral veins. Finally a proximal venous branch on each side of the vertebral column, representing the upper portion of the postcardinal veins, receives the proximal segmental veins and empties into the subclavian vein ([Fig. 267]).