Of the mucous membranes.—These hold an intermediate place between the skin and the true glands. They blend insensibly with the former at the different orifices of the body, and may, under favourable conditions, become so modified as to assume the appearance of skin. The change then wrought is nothing more, however, than an increased deposit of epithelial scales, with an absence of the natural moisture; and it may be doubted whether a transforma tion of this kind could occur in a mucous membrane of which the epithelium was not of the scaly variety. On the other hand certain parts of the membranes usually termed mu cous are nothing less than real glands arranged in a membranous form.
The mouth, pharynx, cesophagus, the vagina and vaginal surface of the uterus, are the parts whose lining membrane most nearly resembles the skin. Their most remarkable feature is the thickness of their covering of epithelial scales, provided for their protection against foreign contact and pressure, and in connection with this the existence of numerous glands opening upon them for the lubrication of their surface. Many of these glands correspond with the sweat-glands of the skin in being similarly scattered under the surface. Such are the buccal and all the small glands allied to them, which, in particular, resemble the largely deve loped sweat-glands of the axilla. The only difference between them is in the mode of in volution of the secreting membrane, which in the former is cellulated, in the latter tubular. These portions of the mucous membranes also approach the skin by the denseness of their submucous areolar tissue.
In the pharynx it is only that part of the lining membrane below the posterior arches of the palate, or that exposed to friction during deglutition, that has the dermoid characters now described : all above is more delicate, is clothed with ciliated epithelial prisms, and be longs physiologically to the nasal or respiratory tract. The lower or buccal surface of the soft palate differs in a similar way from the upper.
The lining membrane of the Eustachian tubes and tympana is very delicate, none of the elementary tissues predominating. The epithelium is in a single layer of prisms clothed with cilia. The submucous areolar tissue is in very small quantity, and the vascular network consists of little more than a simple plane ex pansion. In the nose, the epithelium, accord ing to Ilenle, is scaly on the septum and on the abe for some way within the nostrils. here also there are hairs—an advance towards the characters of the skin; beyond this it is every where ciliated, even within the bony sinuses.
The membrane covering these sinuses is of extreme tenuity, and presents the elementary tissues all in a simple form. That covering the pendulous parts of the spongy bones, on the contrary, has long been noted for its great thickness—a character due to neither of the elements of the mucous tissue itself, but to the extraordinary size of the submucous vessels. Both arteries and veins are large, but especially the latter, which here form a plexus imme diately beneath the surface, and not separated from it by any considerable quantity of dense areolar tissue. Hence the facility with which these vessels give way externally when dis tended with blood. The lining of the nose has been sometimes called a fibre-mucous mem brane, from its close connection with the pe riosteum. The periosteum in the sinuses is extremely delicate, in consequence of the te nuity of the bony lamince it invests; and it would perhaps be impossible to separate it there from the submucous areolar tissue. The globe and cornea are covered with scaly epithe lium, of which the particles are smaller towards, the folds of the eyelids,* where they gradually/ become prismatic, and along the tarsal borders clothed with cilia, so small as to be only recog nizable a short time after death. The conjunc tiva of the lower lid is very minutely villous. At the pharyngeal orifice of the glottis, the epithelium becomes ciliated and continues so along the trachea and bronchial ramifications as far as the air-cells, but, according to my own observations, the cilia there cease, and the epithelium changes its character to a remark able.variety of the glandular form. In the air passages, as formerly described, the submu cons areolar tissue presents a remarkable mo dification, and is closely joined to the peri cliondrium of the inner surface of the cartilages. It is worthy of remark that the glands with which the tracheal portion of the membrane is furnished, are not placed, like the buccal, duodenal, and other similar glands, immedi ately subjacent to the mucous membrane, but on the posterior surface of the tracheal is muscle, which is pierced by their ducts. This peculiar arrangement would seem to be accounted for by the deviation from the ordinary form which the submucous areolar tissue here presents, and which renders it ill adapted to give to these irregular-shaped bodies that loose investment which they everywhere possess, and which therefore appears necessary to them.