DIGESTIVE ORGANS 227 which ends near the apex of the villas, as a dilated microscopic cul-de-sac. By its opposite extremity it becomes continuous with a plexus of lacteals in the sub-mucous coat. In the matrix substance, around the lacteal vessel ef the villus, is a layer of non-striped muscular fibre-cells, which is continuous with the general muscular layer of the mucous coat, and extends as far as the apex of the villus. By the contraction of this layer the chyle during absorption is propelled along the lacteal vessel. The villus also contains blood-vessels ; a small artery enters at its attached base, and terminates in a capillary plexus, situated in the peripheral part of the matrix, close to the cap of epithelium ; from the plexus a vein arises, which leaves the villus at its base, and joins the veins in the submucous coat. Various theories have been put forward to account for the mode of passage of the chyle, during digestion, from the lumen of the intestine into the lacteal vessels of the villi ; but the question cannot even yet be regarded as definitely settled. The appearance of a network of minute tubules within the matrix, extending from the epithelial investment to the lacteal, which Letzerich supposed to be the channels along which the chyle flowed, is doubtless produced by the arrangement of the strands of the fetiform tissue. There seems little doubt that both the cells of the epithelial investment and those of the retiform tissue of the matrix become distended with the particles of chyle previous to its passage into the lacteal. The view advanced by Schafer, that the corpuscles in the meshes of the retiform tissue may serve as carriers of the fatty particles of the chyle into the lacteals, is but another mode of expressing the function of these cells advocated thirty years ago by Goorlsir. The mucous membrane of the small intestine is abundantly provided with secreting glands, named the glands of Brunner and of Lieberkiihn. Brunner s glands are con fined to the duodenum ; they belong to the compound race mose group of glands, and resemble generally in struc ture the mucous and salivary glands. The minute lobules of these glands lie in the submucous coat, and the excre tory duct pierces the mucous membrane to open on the sur face. The wall of the duct is formed of connective tissue lined by columnar epithelium. The finest branches of the duct are continuous with the acini or gland-vesicles, and the gland-vesicles contain the secreting cells, which are col- Uinnar in form. A pleXUS of capillary blood-vessels is dis tributed outside the membrana propria of the gland-vesicles, and lymphatic vessels lie around the lobules. Into the duodenum, about the junction of its descending and horizontal portions, the duct of the pancreas, and the bile duct from the liver, open by a common orifice. These glands may be regarded, therefore, as accessory glands to this portion of the small intestine. The glands of Lieberkiihn are distributed throughout the whole length of the mucous coat of the small intestine. They are simple tubular glands, in shape like test tubes, glands of lirunner. V, intestinal villi ; L, layer of glands of Lieber kiihn; m m, muscularis musosa:; B, a Brunnei s gland, d, its excre tory duct; SM, submucous coat; M, muscular coat; v, a small artery X 40. which lie vertically in the mucous membrane, and form its proper glandular layer (figs. 5 and 6). The tubes are microscopic in size, vary in length from y^th to -^th of a line, and are sometimes closely set together, but in the locali ties where the solitary and Peyer s glands occur they are more widely separated. The glands open on the surface of the mucous membrane between the villi ; and the opposite end of the tubes is closed and rounded, and reaches close to the muscular layer of the mu- FlG - e. Horizontal section through the rrri ,. , , mucosa of the small intesthie, to COUS COat. Ineyare lined by show the glands of Lieberkiilm a layer of columnar epithe- fc^^tSZ^^PSS Hum cells, Continuous with the Tersely-divided blood-vessels. X 300. epithelial investment of the villi. The glands are separated from each other by retiform connective tissue, in the meshes of which colourless lymphoid corpuscles exist in consider able numbers ; the plexus of capillary blood-vessels, which is distributed outside the membrana propria of the gland tube, lies in this connective tissue. The connective tissue of the mucous coat is characterized generally by its retiform character, and by the diffusion of colourless lymphoid corpuscles in the meshwork. But in some parts of the mucosa these corpuscles, with their supporting framework of retiform tissue, are collected into distinct masses or follicles, visible to the naked eye, and known as the solitary and Peyer s glands or follicles. The solitary glands are scattered throughout the whole length of the intestinal mucous membrane. They are about the size of millet seeds, and vary in number and distinct ness in different individuals. They are globular or ovoid in form, and occasion a slight elevation of the mucous membrane. One pole of the gland lies next the free sur face of the mucous membrane, and is in relation to the columnar epithelium covering the mucosa, whilst the opposite pole rests on the submucous coat. Peyer s glands, or the agminated glands, consist of an aggregation of solitary glands or follicles, which are crowded together, so as to form distinct elongated patches, which may vary in length from inch to 3 or 4 inches. The long axis of each patch corresponds to the long axis of the intestine, and the patches are placed opposite to the mesenteric attachment of the bowel. Yilli either may FIG. 7. Vertical section through a Peyer s patch in the wall of the small in testine. V, the intestinal villi; L, the layer of Lieberkiihn s glands ; ni/,.tlie musculailB mucos; sm, the connective tissue of the submucous coot; P, tho follicles of a Pi yer s patch (the two to the right are completely divided from the cupola to the fcase; the two to the left are cut through to one side of the apex) ; aa, small arteries in the submucous coat, which enter the follicles of Peyer, an<i form c, a capillary network; Jl, muscular coat. Slightly magnified. or may not be situated on the surface of the patch, in the intervals between the individual follicles, but Lieberkiihnian glands are always found opening on the surface, and fre quently forming a ring of orifices around each follicle.
Peyer s patches are most abundant in the lower end of the