[Illustration: FIG. 188.—Renal portal circulation in the Eel (_Anguilla vulgaris_). _c.v_, Caudal vein; _i.v_, intestinal vein; _l.p.c_, _r.p.c_, left and right posterior cardinal veins; _p.v_, hepatic portal vein; _R_, kidney; _r.p.v_, _r.p.v′_, renal portal veins; _sg.v_, segmental veins; _x_, arch-like anastomoses between the renal portal and hepatic portal veins; _y_, vein from the urinary bladder. (From Jourdain.)] In the Cod (_Gadus morrhua_) the caudal vein divides into two branches. The larger right vein retains its direct continuity {321}with the corresponding posterior cardinal; the left, on the contrary, has ceased to be continuous with the greatly reduced left posterior cardinal and forms a renal portal vein, the distribution of which is, however, restricted to the hinder portion of the left kidney (Fig. 190). As in _Amiurus_, a branch of the caudal vein forms one of the tributaries of the hepatic portal vein. In the Cod it would therefore seem that only a relatively small proportion of the caudal blood flows through the imperfectly developed renal portal system, the bulk of it traversing the right posterior cardinal and passing directly to the heart, leaving, nevertheless, a modicum for transmission to the liver. Finally, it may be mentioned that in some Teleosts the caudal vein retains its embryonic continuity with one, usually the right, posterior cardinal, without giving off a renal portal affluent, as in the Perch (_Perca fluviatilis_); or, after division, with both posterior cardinals, as in the Lump-sucker (_Cyclopterus lumpus_). In such instances as these no portion of the caudal blood traverses the kidneys, and if a renal portal system exists at all, the only true renal portal veins are the adjacent segmental veins, which transmit venous blood directly to the kidneys, instead of first uniting with renal portal branches of the caudal vein as in the Tench and the Eel. [Illustration: FIG. 189.—Renal portal system in the Tench (_Tinca vulgaris_). _d.c.v_, _v.c.v_, Dorsal and ventral caudal veins; _k_, kidney; _l.p.c_, _r.p.c_, left and right posterior cardinal veins; _p.v_, hepatic portal vein; _r.p.v_, renal portal vein; _sg.v_, _sg.v′_, segmental veins. (From Jourdain.)] Whatever may be the condition of the renal portal system, all the renal blood is eventually collected by renal veins and conveyed to the posterior cardinals, which are often connected by one or {322}by several transverse anastomoses (Fig. 190). In the region of the heart each posterior cardinal joins the corresponding anterior cardinal to form a short but wide Cuvierian duct, which finally opens into the sinus venosus. A subintestinal vein is present in the embryo (e.g. _Lepidosteus_, _Acipenser_, and some Teleosts),[363] but in the adult Teleostome its precaudal section is usually absorbed, or at all events ceases to be recognisable except, perhaps, as one of the minor tributaries of the hepatic portal vein.[364]