VEINS OF FISHES. 467 



that those branches ramified in the renal tissue, like the portal 

 veins in the liver ; but my observations concur with those of 

 Meckel and Cuvier, 1 in showing that they rather receive or com- 

 municate with the renal veins in transitu in Osseous Fishes. In 

 the Lamprey the renal vein assumes the form of a cellular or 

 cavernous sinus, of a very dark colour, extending along the 

 mesial margin of the kidney, uniting with its fellow posteriorly, 

 and communicating by small orifices with the contiguous cardinal 

 vein. 



The visceral system of veins commences in Osseous Fishes by 

 the capillaries of the stomach and intestines, of the pancreatic 

 ca3ca and spleen, of the generative organs and air-bladder : these 

 by progressive union and reunion, constitute either a single trunk 

 which forms the portal arterial vein, fig. 308, L, of the liver ; or, 

 as in the Perch, a second trunk, the true homologue of the f in- 

 ferior vena cava ' which returns the blood from the genital organs 

 and air-bladder to the auricular sinus, without previous ramifica- 

 tion in the liver ; the portal trunk being formed only by the veins 

 of the alimentary canal and its appendages. The portal trunk is 

 single in the Ling, the Burbot, the Pope, the Eel, the Lamprey, 

 and the Plagiostomes ; but, in the Carp, where the lobes of the 

 liver interlace with the convolutions of the intestine, the veins of 

 this canal pass directly into the liver by several small branches, 

 which ramify therein without forming a portal trunk. 



In the Plagiostomes with the longitudinal spiral valve the main 

 root of the portal vein is concealed in the free, thickened, muscu- 

 lar margin of that valve : 2 the trunk of the intestinal vein is 

 lodged also in an internal fold of the mucous coat in the Lamprey : 

 in the Plagiostomes and Ganoids with transverse coils of the spiral 

 valve, the venous blood is collected into an external intestinal vein. 

 In the Paddle-fish this vein joins the vein of the spleen (fig. 276, 

 n), and then, with the duodenal, pancreatic, and gastric veins, 

 forms the portal trunk. 



Professors Eschricht and Mliller 3 found, in the Tunny, that 

 the veins of the stomach, intestine, pyloric appendages, and 

 spleen, respectively subdivided into numerous minute venules, 

 which interlaced with corresponding ' retia mirabilia ' of the 

 arterial branches sent from the cccliac axis to the same viscera, 

 and formed pyriform masses of vessels before entering the liver. 



In a few Osseous Fishes, as the Shad, some of the caudal 

 branches of the vertebral system of veins anastomose with the 



1 xxin. p. 381. 2 xcviii. p. 274. 3 ciil 



H H 2 



