Development of the Venous Drainage of the Equine Hypophysis Cerebri

The development of the venous drainage from the equine hypophysis cerebri was investigated in embryos and fetuses ca. 22nd to 270th day of gestation. The early venous drainage was represented by the right and left retrohypophysial veins which were tributaries of the primary head veins. In later development an anastomosis was formed between the contralateral retrohypophysial veins. This anastomosis was named the retrohypophysial sinus which was identical with the caudal intercavernous sinus of the postnatal animals. The development of the rostral intercavernous sinus was not observed. The right and left cavernous sinuses developed secondarily from the retrohypophysial sinus. The venous drainage was directed rostrolaterad into the ophthalmic veins. In addition to the rostrolateral route, a caudoventral route, represented by the right and left ventral petrosal sinuses, developed from the retrohypophysial sinus relatively late in the fetal period. Both the rostrolateral and caudoventral routes joined eventually the external jugular system basically resembling postnatal conditions.