In other situations, where a simple expanse of mucous membrane is spread out upon a surface of the body, as in the oesophagus, pharynx, mouth, nose and its sinuses, vagina, bladder, &c. (from all of which, however, there are numerous prolongations called follicles and glands, which shew this structure well,) a basement tissue such as that described has not been shown to exist. Its existence rests at present principally on analogy, and it is difficult to say whether it be not more or less modified. Certain of the peculiarities presented by these several parts depend on a modified form and greatly augmented mass of the epithelial element, but many also on varieties in the areolar and vascular tissues underlying the mucous tissue, and, properly speaking, forming no part of it. These will be treated of under the topographical descrip tion of the membrane.
Of the epithelium.—A very brief period has elapsed since it was universally held that most mucous membranes wanted epithelium, and their analogy with the skin was only maintained in this particular by a fancied resemblance drawn between epidermis and mucus. One of the principal results of microscopic observation, conducted with the improved modern instru ments, is that of Henle, proving not only that this structure is present throughout the mucous system, but that in most situations it is so abun dant as to constitute nearly the whole material of the tissue. This fact, as yet so novel, coupled with the discovery announced at the same time of the occurrence of a lining of analogous cha racter on all internal cavities, makes the study of this structure under its varied forms pecu liarly interesting and important. It will readily be conceived how wide a field is here opened to view, and how premature it would yet be to attempt to offer a general history of such a structure. The numerous questions presenting themselves on every side render this impossible; and if it were not so, the scope of the present article would oblige me to confine the descrip tion to those forms of epithelium met with in the mucous system. In acknowledging the great obligations I am under to Henle's admi rable paper on this subject, I may state that the following account has been written as much as possible from my own observations.
By the term epithelium is now meant a layer of particles or modified cells, furnished with nuclei and nucleoli, lining an internal surface of an organized body, and by their apposition and union constituting a kind of pavement.
A similar investment to an external surface is styled epidermis. Both these, in their ordinary forms, will be embraced by the following de scription.
Epithelium is an organized structure endowed with vitality. This is shewn by its form, the process of its growth, and the living properties it displays. Of these the most eminent is that of ciliary motion, which in all the higher animals is performed by cilia clothing the free surface of epithelial particles. But in very many situations, if not in all, the processes of nutrition carried on in the epithelial layer of the mucous system differ materially from those of other organic tissues ; the old elements, which in other cases are reconveyed into the blood, being here shed on the free surface of the membrane, and thus becoming at once eliminated from the system.
The epithelial particles preserve a greater resemblance to the form of the development cell than most other tissues. In many parts they continue to be truly cells throughout their existence, and in no instance is the nucleus, fiom which they have proceeded, absorbed.
In connection with a wide and varied range of function, these particles present numerous modifications of form, bulk, and texture, the leading features of which have been pourtrayed by Ilenle. The following arrangement, how ever, differs in several respects from that pro posed by him,* and is more in accordance with what I have myself observed. Founding it on the anatomical condition of the particles and on their office, I distinguish three varieties,— the lamellyhrm or scaly, the prismatic, and the spheroidal. These all run together by imper ceptible gradations. The particles may be also divided into non-ciliated and ciliated, the scaly being always bald, the prismatic and sphe roidal in some situations furnished with cilia.
Of the lamelliform or scaly variety.—This consists of broad flattened particles (or scales, properly so called), having an angular outline (caused by their lateral apposition) and a nucleus, which is generally eccentric. These scales form layers of extremely. variable thick ness. They are generally, however, super imposed in great numbers over one another, as in the mouth, fauces, and esophagus of the human subject, where they constitute the opaque defensive investment so visible to the eye in those parts.