. The elasmobranch fishes. Chondrichthyes. 208 THE ELASMOBRANCH FISHES terior from this vessel in both Scyllium and Raja is the dorsal myelonal vein (my.). This vein is absent, however, in Acanthias (fig. 181). A ventral myelonal vein, arising posterior to the optic chiasma, drains the vascular sacs and passes on down the cord. The hyoidean vein in Mustelus enlarges ventrall}' into the hyoid sinus. Each sinus (, figs. 190 and 193) is triangular and of large size, the base of the triangle extending from the tip of the lower jaw in front to back of the first afferent artery. In Acanthias the
. The elasmobranch fishes. Chondrichthyes. 208 THE ELASMOBRANCH FISHES terior from this vessel in both Scyllium and Raja is the dorsal myelonal vein (my.). This vein is absent, however, in Acanthias (fig. 181). A ventral myelonal vein, arising posterior to the optic chiasma, drains the vascular sacs and passes on down the cord. The hyoidean vein in Mustelus enlarges ventrall}' into the hyoid sinus. Each sinus (, figs. 190 and 193) is triangular and of large size, the base of the triangle extending from the tip of the lower jaw in front to back of the first afferent artery. In Acanthias the fore- most of the communicating vessels be- tween right and left hyoidean veins forms the hyoidean sinus. In Car- charias and Raja a thyroidean sinus of considerable size is formed in the mid- line ventral to the thyroid gland (Fer- guson, 1911). From the posteroventral angle of the hyoidean sinus, the inferior jugular vein passes backward to enter the duct of Cuvier (figs. 190 and 193). On its way it receives the ventral nutrient veins (nu.) which are connected with the dorsal nutrients of the anterior car- dinal sinus. EENAL PORTAL SYSTEM In the embryonic condition a su])intes- tinal vein extends from the tail to the heart. It later separates into an ante- rior and a posterior part, the anterior part becoming the line of the hepatic portal system and the posterior part that of the renal portal system now under consideration. The caudal vein, as the basis of the renal portal system of the adult, ex- tends from the tip of the caudal fin through the haemal canal to the cloaca. It represents the stem of a Y, the arms of which pass to the sides of the cloaca as the renal portal veins (, fig. 194a) . Each renal portal continues forward and upward dorsal to and along the lateral margin of the kidneys, giving to each numerous advehentes. In its course as a single median vessel, the caudal receives dorsal and ven- tral segmental veins on each side, which are of large size at the
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