228 DIGESTIVE ORGANS ileum, but diminish in size and numbers in its upper end and in the jejunum, and are absent in the duodenum. These follicles are lymphoid organs, and are composed of lymphoid or adenoid tissue. The solitary and Peyer s glands, as is the case generally with the lymphoid organs, are more distinct and perfect in structure in infancy and childhood, than in adults or in advanced age. The muscular layer of the mucous membrane lies next to the submucous coat, and consists of non-striped fibres which lie parallel to the surface of the membrane. It passes into the substance of the villi, and lies around the closed end of the glands of Lieborkiihn, Of the blood-vessels of the small intestine, the arteries enter the wall of the jejunum and ileum at its attached or mesenteric border, and are branches from the arcades of the superior mesenteric artery. They run in the sub-serous tissue around the wall of the intestine ; then pierce the muscular coat and supply it ; they then enter the submucous coat, and a form a network from which branches pass into the mucous coat. The veins accompany the arteries, and form rootlets of the superior mesenteric vein. The lymph-vessels, or ladeals, may be traced into the wall of the intestine at the mesenteric border ; they form a net work in the muscular coat, and then enter the submucous coat, where they are very abundant ; from this submucous layer offshoots pass through the retiforni tissue, which lies between the Lieberkiihnian glands, into the villi. Where the solitary and Peyer s glands are situated, the lacteals, as Frey has pointed out, form a system of anastomosing vessels around the base and mesial part of each follicle. The nerves are derived from the plexuses of the sympathetic, which accompany the branches of the superior mesenteric artery. They form between the two layers of the muscular coat an important plexus, named, after its dis coverer, Auerbach s plexus, in which large stellate nerve- cells are intermingled with nerve-fibres, and a similar nervous plexus is found in the muscular coat of the other divisions of the alimentary canal. It supplies and regulates the movements of the muscular coat. Large The Large Intestine, though not nearly so long as the ntestine. small intestine, is of much greater diameter. It reaches from the end of the ileum to the orifice of the anus, and is divided into the coecum with the appendix vermiformis, the colon, and the rectum ; whilst the colon is subdivided into the ascending colon, the hepatic flexure, the transverse colon, the splenic flexure, the descending colon, and the sigmoid flexure. The Ccecum, the dilated commencement of the large intestine, lies below the ileum, and occupies the right iliac fossa. It forms a large cul-de-sac, closed in below, but communicating freely above with the ascending colon. Opening on the inner and posterior wall of the caecum is the appendix vermiformis, which is a slender hollow pro longation of the bowel, varying in length from 3 to G inches. It has the calibre of the titem of a common tobacco pipe, and ends in a free closed extremity, so that, like the cajcum, it is a cul-de-sac. It is not generally found in mammals, but is present in man, the oraug, certain lemurs, and the marsupial wombat. The Colon extends from the caecum to the rectum, and forms the longest part of the large intestine. The trans verse part of the colon lies immediately below the great curvature of the stomach, but owing to the length of the transverse meso-colon, which forms its peritoneal attach ment, it not unfrequently undergoes some change in its position, and may hang downwards towards the pelvis, or be elevated in front of the stomach, or thrown to the right or left side. The sigmoid flexure of the colon is situated in the left iliac fossa, but as the sifjmm l meso-colon, which forms its peritoneal attachment, is of some length, it is freely movable, and not unfrequently hangs into the pelvis, or even extends across into the right iliac fossa. The Rectum is the terminal segment of the large intestine, and extends from the sigmoid flexure to the orifice of the anus. It lies in the cavity of the pelvis. It commences opposite the left sacro-iliac joint, and passes at first obliquely downwards and to the right until it reaches the middle line of the sacrum ; secondly, it closely follows the curvature of the sacrum and coccyx, lying in relation to their anterior surface ; thirdly, when it reaches the tip of the coccyx its terminal or third part inclines downwards and backwards for about li inch to the anal orifice. The anus opens on the surface of the middle line of the perineum, midway between the two ischial tuberosities, and the skin surrounding the orifice is thin, and wrinkled when the opening is closed. Immediately beneath the skin is the sphincter ani externus muscle, which forms a thin layer of fasciculi, arranged in a series of ellipses around the orifice. The sphincter in its normal condition of con traction simply closes the opening, but, under the influence of the will, a more powerful contraction can be induced, so as to resist the entrance of foreign bodies into the rectum. The large intestine is arranged in the abdominal cavity in the form of an arch, the summit of which is the transverse colon, whilst the caecum and rectum are the right and left piers. Within the concavity of this arch the coils of the jejunum and ileum are situated. The large intestine is not, except in the rectum, a cylindriform tube, but is dilated into three parallel and longitudinal rows of sacculi, which rows are divided from each other by longitudinal muscular bands, whilst the sacculi in each row are separated externally by intermediate constrictions. In the rectum the sacculi have disappeared, and the intestine assumes a cylindrical form, but at its low r er end it dilates into a reservoir, in which the faeces accumulate prior to being excreted. At the junction of the large with the small intestine a valvular arrangement, termed the ileo-catcal or ileo-colic valve, is found. This valve is due to the peculiar manner in which the ileum opens into the large intestine. The opening is bounded by two semi-lunar folds, which project into the large bowel. These folds are the two seg ments of the valve ; one situated above the opening is the ileo-colic segment, the other, below the opening, the ileo- ccecal. The two segments become continuous with each other at the ends of the elongated opening, and are prolonged for some distance around the inner wall of tho large intestine as two prominent ridges, named the/rawa of the valve. The use of the ileo-ca;cal valve is to impede or prevent the reflux of the contents of the large into tho small intestine. When the caecum and colon are distended the fraena of the valve are put on the stretch, and the two segments are approximated, so that the opening is reduced to a mere slit, or even closed, if there is great distension of the bowel. Structure of the Large Intestine. The wall of the large intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, sub mucous, and mucous coats. The serous or external coat, derived from the peritoneum, forms a complete investment for. the flexures of the colon, the transverse colon, and the first part of the rectum, but not for the caecum, or the ascending and descending colon. The second part of the rectum has only a partial serous investment, and the third part has no serous coat. Nume rous pedunculated processes invested by the serous membrane, and containing lobules of fat, named appendices epiploicce, are attached to the large intestine.
The muscular coat consists of non-striped fibres arranged