Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/244

226 226 DIGESTIVE ORGANS the second lumbar vertebra, where it becomes continuous with the jejunum. The duodenum is distinguished from the rest of the small intestine by having the ducts of the liver and pancreas opening into its canal, by containing in its wall a collection of compound racemose glands, named the glands of Brunner, and by being developed from the primitive fore-gut, and not, like the jejunum and ileum, from the primitive middle gut. Like the stomach, it should be regarded as a distinct segment of the alimentary canal. The Jejunum and Ileum form by far the longest part of the small intestine, and are not separated from each other by any sharp line of demarcation the upper two- fifths being called jejunum, on account of its being usually empty after death, the lower three-fifths being termed ileum, from its convoluted arrangement. They occupy the umbilical, hypogastric, right and left iliac regions of the abdomen, in which they are arranged in a series of coils or convolutions ; one or two coils of the ileum sometimes lie in the cavity of the pelvis, between the bladder and rectum. The coils are attached to the posterior wall of the abdomen, along a line from the body of -the first lumbar vertebra to the right sacro-iliac joint, by the fold of peritoneum called the mesentery. Owing to the extent of the mesentery, the coils of the jejunum and ileum can be freely moved about in the abdominal cavity, so that they are apt to be dis placed from their natural position, and, when a rupture occurs, to become the most usual contents of the hernial sac. The lower end of the ileum passes into the right iliac fossa, where it becomes continuous with the large intestine, at the junction of the ccecum and ascending colon. Though the line of demarcation between jejunum and ileurn is an arbitrary one, yet the upper end of the jejunum may be distinguished from the lower end of the ileum by being wider, and having a thicker mucous membrane, in which the folds called valvuloe conniventes are larger and more numerous. Structure of the Small Intestine. The wall of the small intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, submucous, and mucous coats. The serous or external coat, derived from the peritoneum, forms a complete investment for the jejunum and ileum, and is continuous with the mesentery along a line of attachment, named the mesenteric border of the intestine ; but the serous covering of the duodenum is incomplete. The muscular coat consists of non-striped fibres arranged in two layers from without inwards. The outer layer consists of longitudinal fasciculi, which form a thin layer parallel to the long axis of the intestine. The inner layer consists of circular fasciculi arranged around the gut transverse to its long axis ; this layer is thicker, stronger, and more highly coloured than the longitudinal layer. By the contraction of the muscular coat, the peristaltic or vermicular movement is produced, which propels the ingested materials along the intestine. The sulnnucous coat lies immediately subjacent to the circular layer of the muscular coat, and consists of areolar connective tissue ; in it the blood-vessels ramify before they pass into the mucous membrane. The mucous or internal coat is a soft, velvety-looking membrane, which lines the wall of the small intestine, and possesses a complex appearance and structure. The inner surface is not smooth, but is thrown into strongly-marked, transverse folds, the valvulae conniventes, which are not obliterated during distension of the gut. They are very numerous in the duodenum and jejunum, but then decrease in size and numbers, until at the lower end of the ileum they have disappeared. Each valvula consists of a fold of the mucous membrane with its submucous coat. Owin^ to their presence, the extent of the mucous surface is much greater than if it were a plane-surfaced membrane. In its more minute structure the mucous coat may be regarded as composed of numerous projecting bodies, a glandular layer, and a muscular layer. The projecting bodies are the intestinal Villi, which jut out into the lumen of the intestine from the free surface of the mucous membrane, not only of the valvulae, but of the intermediate surface. They are delicate, minute processes, varying in length from a fourth to half a line, and in number amount to several millions. They are best examined when the mucous surface is placed in water or spirit, when they may be seen with the naked eye, or, still better, with a pocket lens ; when the chyle-vessels or blood-vessels are injected, they become erected, and stand out more prominently from the surface. They vary in form, being filiform, or cylindrical, or conical, or club-shaped, or leaf-shaped. They are more numerous in the duodenum and jejunum than in the ileum, and to their presence is due the velvety appearance of the mucous surface. They are not found elsewhere than in the small intestine. As they are the parts of the mucous membrane directly concerned in the absorption of the chyle, their structure is interesting and important. Each villus is invested by a cap of epithelium continuous with the general epithelial covering of the mucous membrane. The epithelium con sists of a single layer of columnar cells, compactly arranged side by side. Scattered amidst the columnar cells are cells which possess the form of microscopic goblets, and are named goblet cells. The free end of each goblet cell appears to have an open mouth on the surface of the villus, through which a mucus-like substance exudes. Various opinions have been expressed as to the nature of these goblet cells. Some regard them as special struc tures engaged either in the absorption of chyle, or the secretion of mucus ; others look upon them as merely modifications of the columnar epithelium ; whilst others again consider them to be post-mortem productions, due to the swelling out of the columnar epithelium by the imbibition of fluid. There can be no doubt, however, that they are not specially concerned in the absorption of chyle, as cells of the same character are found in the respiratory mucous membrane, and on other surfaces, where the absorption of chyle does not take place. The sub-epithelial tissue of a villus forms its matrix or basis substance, and consists of the sub-epithelial connec tive tissue of the mucous membrane. When thin sections through a villus are examined, the matrix is seen to ba Fio. 4. A. transverse section tnrotipn ar. intestinal villni, shoving : Its : epi thelial investment and the matrix of lyniphoid tissue; c, columnar epithelium; ff, goblet-shaped cell; /, lacteal; r, r, lymphoid retiform tissue; t&amp;gt;, r, trans versely divided blood-vessels. B, free ends of columnar epithelium, witn mouths of four goblet-shaped cells. X 300. composed of a delicate retiform tissue, which forms a net work, in the meshes of which numbers of colourless lymphoid corpuscles are imbedded. These cells were described and figured by Goodsir, as the absorbing cells or vesicles of the villus. In the axis of the villus one, or perhaps two, minute lacteals or chyle vessels are situated, which serve as rootlets of origin of the lacteal division of the lymph vascular system. The lacteal is a capillary tube,