DIGESTION, ORGANS AND PROCESS OF.
This important gland first shows itself in the form of yellowish-brown cells in the polyps, and gradually becomes more concentrated and developed in the echinoderms• annelids, nudo branchiate gastropods. insects, crustaceans, air breathing mollusks, cephalopods, fishes. reptiles, birds, and mammals. The physiological anat omy of the liver may be briefly stated as fol lows: The lobules previously mentioned are about of an inch in diameter and of ovoid shape. They are surrounded by a plexus of blood-vessels, nerves, and ramifications of the hepatic duet, comprising what arc called the interlobular vessels. These are all inclosisl by a sheath which is a prolongation of the prop r coat of the liver (capsule of Glisson), but at tac•hed loosely by areolar tissue. This sheath follows the vessels to the subdivisions within the interlobular spaces (spaces between the Itlailes). hut does not extend to the capillary vessels within the lobules. In a few animals. as tin pig and polar bear, the lobular structure can be seen with the naked eve, but in nmn and most In mals it. cannot. The lobules are intimately con nected with each other• branches of the inter lobular vesels being each distributed to several of the lobules. Any one lobules however. may his considered as representing the physiologist aria 1.0111W of the whole liver, and the study of its anatomy and functions will answer for the study of the 'whole gland. The lobules receive blood at their surfaces from the capillary terminations of the portal vein. these vessels having received 1 accumulation of the bile, when its flow into the intestine is interrupted, as it is always found full after a long fast. and empty when digestion is going on. That the gall-bladder is not an essential appendix to the liver is shown by the fact that it is absent in many genera of mam mals.
It was formerly believed that the liver served merely for the separation of the binary secretion from the blood; hut there is now abundant eri the terminations of the hepatic artery before passing into the lobules. It is very important to
bear in mind this peculiarity of distribution, which is often overlooked. The branches of each hepatic vein, the vessel which carries the blood from the liver to the ascending great vein (as cending vena cava), by which it is returned to the heart and lungs. have their origin within the Their capillary extremities arise from the capillary ramifications of the portal vein. and, passing toward the centre of the lob ule, converge into three or four radicles, which, uniting at the centre, form the introlobulur veins, which are the commencement of the hepatic veins. These intralobular veins, which are in the centre of each lobule, are from to of an inch in diameter, and they follow the long axis of the lobule, receiving vessels in their course till they empty into larger vessels situated at the base of the lobules, called by Kiernan sub-lobular veins. They collect the blood from all parts of the liver, and, increasing in size by union with one another, they at last form the three hepatic veins which discharge the• blood from the liver into the ascending vena cava. Now, these hepatic veins are a long way from the influence of the heart's action, lying as they do between the portal circulation and the veins going to the heart; but a provision has been made to assist in the pro pulsion of their contents, and they are supplied with a muscular coat, composed of unstriated muscular fibres. The most essential elements of the lobule, or of the liver, are the hepatic cells, which are the true secreting elements of the gland. They are minute, polygonal-shaped bodies about of an inch in their longest and in their shortest diameter, having one nucleus, or sometimes two nuclei, with some granular matter. (See CELL.) They are surrounded by an independent network of extremely minute ves sels of an inch in diameter, of uniform size throughout, called the biliary capillaries, and in which the bile first makes its appearance.