The mucous lining of the whole alimentary canal below the cardia is the largest and best marked example of what I have termed the compound mucous membrane, being com posed of vertical tubes which are truly glands, opening on the general surface. That of the small intestine presents villi also. This entire membrane is very soft and easily torn, because its chief mass consists of an epithelium, the particles of which adhere but slightly either to one another or to the basement membrane, and are everywhere disposed in a single layer. There is moreover scarcely any areolar tissue between its involutions, which have, therefore, little besides the vascular web to sustain them. The submucous areolar tissue is in considerable abundance between the mucous and the muscular coats. (See STOMACH and INTESTINAL CANAL.) The lining membrane of the hepatic and pan creatic ducts is simple, and its epithelium of the prismatic variety.
In the.genito-urinary tract, the epithelium presents every variety. The fossa naviculariss is clothed with small, flat, or roundish scales, the rest of the urethra with a single series of prismatic particles. The cells of the prostate are lined with spheroidal epithelium, the vasa deferentia with prisms. In the vesicular semi nales there is a pavement of somewhat flat tened granules, and also in Cowper's glands. In the bladder, ureters, and pelves of the kid neys, the epithelium is in the form of longish cells intermediate between the spheroidal and the prismatic varieties. The nymphw, clitoris, hymen, and vagina are covered with scaly epi thelium, and this has been noticed by Henle in cases where theliymen has been entire. Within the neck of the uterus the epithelium becomes prismatic and clothed with cilia, and so con tinues over the surface of the uterus and Fal lopian tubes, and even for some distance over the outer surface of their fimbriated extremities. Beyond this it merges gradually into the com pressed cells of the serous membrane. The lining membrane of the Fallopian as well as that of the uterus, is of a compound nature, especially during gestation, and consists of tubules arranged vertically to the general sur face. It is to be observed that the cilia only clothe the general surface, and that the epithe lium lining the tubules is spheroidal, or inter mediate between that and the prismatic. It is a form of the glandular variety, and bears no cilia.
Of the glands.—The varieties apparent in these organs also may be explained by an ex amination of the modifications and modes of aggregation of the elementary tissues already mentioned. It may be said, in general terms, that the glands are characterized by their solid form, by the great preponderance of their epi thelial and vascular tissues, and by the small quantity of their areolar tissue. It is rare for this last to invest every individual involution of the mucous surface in the interior of a gland; but it usually gives a common covering to the whole organ, as well as less complete ones to those subdivisions of it, termed lobes or lo bules, which result from the mode of distri bution of the bloodvessels and duct, and are designed for the purposes of package or pro tection.
Such an investment is usually termed the proper coat or capsule of a gland, and seems to correspond most nearly with the submucous areolar tissue of the compound mucous mem brane, as, for example, that of the intestinal canal.
The propriety of these remarks will appear, on a particular application of them. As I be fore entered somewhat in detail into the internal composition of the liver, it may now be se lected for illustration. The epithelium, which in the gall-bladder and larger ducts is of the prismatic kind, becomes bulky and of a flat tened spheroidal form, in the lobules. It there also acquires a peculiar character, viz. nume rous minute globules of an oily or fatty nature, disseminated within the substance of each par ticle. The basement tissue seems to cease, and on an examination of a thin section of the lobule under a high power of the microscope, its chief bulk appears to consist of epithelium. There is scarcely a trace of areolar tissue to be anywhere detected. Even the coats of the capacious capillary bloodvessels, in the close meshes of which the ultimate ramifications of the bile ducts are situated, are with difficulty seen, and are of extreme delicacy. The sub mucous areolar tissue of the hepatic ducts, with which the whole of the contiguous cap sule of Glisson should be associated, cannot, when arrived at the lobules, be followed into their interior. It can only be distinguished in very slender quantity, giving them a partial in vestment, on those aspects which share in form ing the portal and hepatic-venous canals, and where, in the angles of union between three or more lobules, a terminal twig of the portal vein runs up to open on all sides into their capillary plexus. No lobule is isolated from the rest by a complete capsule, but commu nicates immediately by its capillary network, with those near it. The intralobular vein has a similar want of areolar tissue around it; and thus the main mass of the lobule, and of the whole liver, consists of epithelium and a plexus of capillaries. Those lobules, however, which contribute to form the general surface of the organ have an additional and dense covering of areolar tissue on that surface : a covering, which has the same relation to the mucous element, as that on the portal aspect; which is continuous with the capsule of Glisson at nu merous points; and which is here developed as a membrane of support, as a nidus for a lymphatic rete, and as a foundation for the peritoneal tunic, that it sustains.