The veins connect the capillary vessels with the heart, one of their functions being, to bring back to the central organ of the cir culation the blood which has been distributed over all parts of the body : they are naturally divided into two classes.
1. The veins which concur to form the two vence cave, and which thus communicate with the right auricle of the heart ; these constitute the systemic venous system : and 2. The veins which commence in the ca pillaries of the lungs, and finally discharge their contents into the left auricle : these re present the pulmonary venous system.
These two classes of veins differ from each other in the nature of their contents, no less than in their modes of termination, for whilst the systemic veins contain dark-coloured, and essentially venous blood, the office of the pulmonary veins is to convey red or arterial blood from the pulmonary capillaries, where it has been re-oxygenated, to the left auricle of the heart. Valves are not found in the veins of the pulmonary system, whilst they are very abundant in many of the systemic veins.
The cardiac veins constitute a small system apart from that of the general venous system, for not having any communication with the venm cavm or with any of their branches, these veins open by a separate orifice into the right auricle of the heart.
Neither can the portal veins be included in any general description of the venous system. These veins emanate from the stomach, in testines, spleen, and pancreas, and unite to form one large vessel, the vena portce, which entering the liver, branches out in every di rection through that organ after the manner of an artery, and thus constitutes the system of the vena pothe.
From the capillaries of the portal vein in the liver, another series of veins is derived, which coalesce and form larger and larger trunks, like the systemic veins generally, and ultimately, to the number of three or four, issue from the liver at its thick margin, and join the inferior vena cava. These last-men tioned veins are termed hepatic veins, or venm cavm hepatica', and the function to which they are subservient in the economy, is that of re conveying into the general venous system, the blood which has been diverted to the liver by the ramifications of the portal vein.