The hepatic portal vein is formed by the veins of the œsophagus, stomach, and intestines. After traversing the capillary vessels of the liver, the revehent hepatic veins unite to form an extensive hepatic sinus before entering the heart.]
The following main facts are to be noted in the venous system of the Selachian (Fig. 262):
=1. There are Two Portal Systems.= (_a_) _Renal Portal System._--The caudal vein divides near the vent into two branches which course along the lateral border of the kidneys, sending _afferent_ or _advehent_ veins into the organ. The blood traverses the renal capillaries and is gathered together by the _efferent_ or _revehent_ veins, which empty into median paired vessels, the posterior cardinals.
(_b_) _Hepatic Portal System._--The veins of the digestive tract and appendages unite to form a hepatic portal vein. The blood after traversing the capillary system of the liver is collected by hepatic veins, which form a dilated hepatic sinus emptying into the sinus venosus of the heart.
2. The middle segment of the intestine, presenting a spiral valve in the interior, gives rise to a vein emptying into the portal vein which corresponds to the subintestinal vitelline vein of the mammalian embryo (Fig. 202).
3. The posterior cardinal veins, also greatly dilated and forming the posterior cardinal sinus, join, near the heart, the veins returning blood from the head, the anterior cardinal or jugular, to form a transversely directed trunk, the duct of Cuvier, which empties into the sinus venosus at the auricular extremity of the heart. Into the duct of Cuvier empties on each side a _lateral vein_ returning the blood from the body walls. This vein can be considered, for our present purpose, as representing in general the abdominal vein of amphibians and reptiles, and the umbilical vein of the mammalian embryo.
The adult selachian venous system is therefore to be considered as illustrating the following conditions above encountered in our study of the embryology of the mammalian venous system.
1. The heart illustrates excellently the stage in the mammalian development, in which auricular and ventricular segments have differentiated, but before the division of the cavities into a pulmonary and systemic portion by the development of the auricular and ventricular septa and the division of the arterial trunk into pulmonary artery and aorta.