The walls of the small intestine consist of similar layers to those of the stomach, but with sonic remarkable alterations. Like the stomach, the small intestine has muscular layers by whose contraction food in the intestine is propelled onwards. On opening a part of the small intestine and floating it out in water, the inner coat — the mucous membrane — is seen to be thrown into transverse folds, which are called valvular con niventes, by which the internal surface of the small bowel is increased. The surface is found to have a velvety feel ing, clue to innu merable very fine projections termed villi. On examin ing the surface with a microscope these projections take the form of finger shaped processes from the mucous membrane. They are represented in Fig. 106 (a). A very highly magnified view of a single villus is given in Fig. 107, where it is repre sented as covered with a layer of columnar cells, nucleated. The centre of the projection is formed of very delicate tissue, containing a net work of small blood-vessels (b and c). Besides these, in the very centre is seen a larger vessel (d) with a blind extremity. This is the begin ning of a lacteal vessel, so called because it does not contain blood, but a milk-like fluid (lac, milk) obtained from the food in the canal of the intestine. In fact the lacteal is a vessel for absorbing, sucking up, certain of the nourishing parts of the food and conveying them away to other communicating vessels, by which they may finally be poured into the current of blood. This will be referred to agai n in speak ing of absorption. Apart from these projections from the mucous surface, the small intestine presents appearances not unlike those of the stomach. Thus, buried in the substance of the mucous layer, and opening on the surface at the bases of the villi, is a series of tubular glands (Fig. 106, b), lined with columnar cells. They are called Lieberkiihn's glands, after the anatomist who first described them ; and they pour out the intestinal juice. The bases of the glands rest on a fibrous tissue (c, Fig. 106) between them and the muscular layers of the bowel (d and e). These glands are found throughout the length of the small and large intestines ; the villi, how ever, diminish greatly in number in the lower parts of the small, and are quite absent in the large, intestine. Towards the beginning of the duodenum a few glands, in clusters like grapes, are found. They are called Brii nner's glands, but their function is not known. Embedded in the mucous membrane, also, are found groups of little glands, which are in the shape of closed sacs, with no duct or other means of communi cation with the cavity of the intestine. The closed sacs contain cells and blood-vessels, and are of the size of millet seeds. They are found in patches —Peyer's patches—specially towards the lower end of the small intestine, but are also found singly here and there scattered over every part of the intestine.
The Large Intestine begins in the right iliac region. The small intestine joins it at right angles, and not precisely at its extremity, so that a blind end projects beyond the place of junction—the ceecum (Latin, ccecunt, blind). From the place of junction the large intes tine passes upwards on the right side, as the !Mending colon, till the under surface of the liver is reached, where it turns and proceeds across to the left side, as the transverse colon, below the stomach. Having reached the left side it turns downwards as the descending colon to the sigmoid bend and the rectum (Fig. 99, p. 189). The large intestine is altogether 5 or 6 feet long. It is much wider than the imall intestine. The longitudinal muscular fibres tre collected into three bundles, which, being shorter than the canal itself, produce a series A pouches or bulgings in the wall, represented in Fig. 99. These pouches delay the progress A the remains of the food, and so permit the nourishing materials to be completely removed. The large intestine possesses no folds like the Small, and no villi. It has, however, the glands A Lieberkiihn in its mucous coat. Glands of Peyer are also found in the large intestine. The termination of the large intestine at the anus has been already noted on p. 189.
Attached like a tag to the blind extremity A the large bowel is the appendix, having in miniatpre, so to speak, the same structure as the rest of the bowel wall. Iu its centre is usually fine canal, communicating with the main canal of, the bowel, and at the free extremity of the appendix this very fine canal may be open.
Refer to Plates XII. and XIII. to see normal position of appendix from front and back view.
The of the, abdominal portion of the alimentary canal have rather a peculiar arrangement which it is desirable. to, note here. The stomach, intestines, spleen, pancreas, and the mesentery as well, all receive pure arterial blood from branches of the main artery that passes down along the front of the backbone. This blood circulates through the various organs iu tiny streantleta. In particular it has already been noted that it is from such arterial blood. supply that the glands in the stomach and in testines derive the material for their activity. The blood, after passing through the organs, is collected into veins. Thus the stomach has its own set of veins carrying the WO away from it ; the iutestiues have their set, the spleen and pancreas likewise. Ultimately, however, the veins from these different organs unite to form one large vessel, the portal vein, which passes to the liver. The liver thus, receives all the blood which, has previously circulated through the stomach. and. intestines, whickblood, as we shall see, is charged with nourishing material obtained from the food by the process mosia explained.on page 194.
The Liver must also be counted as a part of the digestive apparatus, siuce it forms the bile, one of the digestive juices. It is the largest gland in the body, and weighs from 50 to 60 ounces avoirdupois. It is placed just below the diaphragm and on the right side, as may be seen on reference to Fig. 104,h 198. It extends also across the middle line of the body towards the left side. Its front border reaches just below the border of the chest when a son is sitting or standing; but when the person lies the liver passes slightly up so as to he completely under cover of the ribs, except for a small portion beyond the lower end of the breast-bone, In women, by tight lacing, the liver is often permanently displaced, forced downwards out of cover of the ribs. This causes crowding in the abdomen and pelvis, and may serve to displace other organs, notably the womb. When a small piece of liver is examined under a microscope it is found to sist mainly of large sided cells, containing a large nucleus and a nucleolus. The cells are represented in Fig. 108. The protoplasm of which they consist is very granular, and frequently hibits a large number of minute bright oil globules. The cells are faintly yellow in colour, and measure the of an inch in diameter. They are disposed in groups or masses, each little mass being called a lobule. When a single lobule is examined it appears to be of an irregularly circular shape, and the cells are ranged in it in rows which seem to radiate from the centre to the circumference. of the lobule, It is the lobules that give the liver its coarse granular appearance when torn. It has been mentioned above that the portal vein comes to the liver carrying blood full of nourishing material obtained from the stomach and tines. When this vein reaches the liver it breaks up into branches, which pass into the substance of the organ, giving off smaller and smaller branches as they go. The smallest branches of the portal vein reach the circumference of the lobules, and from this surrounding vessel fine streams of blood pass inwards among the cells to the centre of the lobule, where they into one. vessel, a branch of what is now called the. hepatic vein (hepar, the liver). The. central vessels, of the lobules unite to duce larger and larger veins till one vessel is formed which carries all the blood away from the liver upwards towards the right side of the heart. The portal vein is thus formed by a set of capillary (hair-like) blood-vessels in the stomach, intestines, &c., and splits up into a similar set in the liver, which in their turn give rise to the hepatic vein. It is to be noted that neither of these vessels carries arterial blood. Yet the liver is not without its supply of such pure blood, brought to it by an artery --the hepatic artery.—which enters the liver and distributes its blood, not to the cells, but to the connective tissue of the bile-ducts and. vessels in the organ, the blood afterwards finding its way, like that of the portal vein, into the hepatic vein.