The further course of the primary carotid and of the right subclavian will also be described in connection with the arterial system. The auricles are very large in proportion to the ventricles, though their relative sizes will, of course, vary with the amount of contained blood. [Illustration: FIG. 59. The veins of the posterior region of the Florida alligator. The postcaval system and its associated veins are shown in the main figure; the hepatic portal system is shown in the smaller figure to the left.--For lettering, see pages 224-25.] THE VENOUS SYSTEM _The Posterior Vena Cava and its Branches._ The _postcava_, Fig. 59, pc, as noted above, is a wide, thin-walled vessel seen extending across the short space between the anterior face of the right lobe of the liver and the sinus venosus. As was also noted above, the hepatic veins, vh,--at any rate that from the left lobe of the liver,--enter the postcava so close to the heart that they may be considered to have one or more distinct openings into the sinus venosus. Followed caudad, the postcava may be traced through the large right lobe of the liver, from which it receives several branches. Emerging from the posterior border of the liver, it is seen to extend caudad, in the median line, as a rather inconspicuous vessel that receives blood from the reproductive organs and the kidneys that lie close on either side of it. The _hepatic portal_ vein, h, has the usual distribution for that vessel. Entering the liver in the neighborhood of the bile duct, it receives first (_i.e._, nearest the liver) a small branch from the pancreas, pv; near the pancreatic are one or two branches from the stomach, g, and a branch from the spleen, sp. A short distance caudad to these vessels are two or three mesenteric veins, m, leading from the mesentery and small intestine. Caudad to the mesenterics, the portal system may be seen as a vein of diminished caliber, i, leading from the posterior part of the small intestine and from the large intestine. The connection mentioned by Bronn between the rectal branch of the portal vein and the caudal vein could not be demonstrated. After entering the liver, the portal, of course, breaks up into capillaries, and the blood thus distributed is re-collected by the capillaries of the hepatic veins above mentioned. The _internal epigastric_ veins, ep, are, perhaps, the most conspicuous vessels of the postcaval system. When the ventral abdominal wall of the animal is removed, they may be seen extending forward from the pelvic region, on each side of the body, to enter the posterior edge of the liver. The epigastric of the right side enters the large or right lobe of the liver, where it breaks up into capillaries; the left epigastric sends its main branch into the left lobe of the liver, but also sends a branch over to enter the right lobe.