Of the spheroidal variety (see figs. 273 to 277).—In this the particles are of a rounded form, though generally somewhat flattened where they touch. They are always thick, from the substance they contain. It is this variety that constitutes the chief mass of the secreting glands, and hence it might not impro perly be styled glandular. It corresponds with the prismatic variety, in its usually constitu ting in the glands a single layer, and in the predominance, from the first, of its substance over its membrane. In the glands, indeed, the membrane can seldom be discerned at all, and the substance surrounding the nucleus, though more bulky, has the same finely mottled cha racter already noticed in the prisms. In other situations the cell-membrane is persistent, but even then it never flattens into a scale. This variety presents in the different glands nume rous modifications, which have not yet been studied with the accuracy they merit. It is difficult to reject the belief that it is intimately concerned in the glandular function, and varies in correspondence with it.
To the preceding summary account of these three principal kinds of epithelium much might be added respecting the intermediate forms. This, however, does not appear to be required in so general a description. The spheroidal and the prismatic are seen blended in the speci men I have figured from the human inembrana tympani (fig. 281).
Of the non-ciliated and ciliated epithelium.— The true scaly variety appears never to be clothed with cilia. The prismatic epithelium is that which most commonly bears these vibra tile organs. They are placed on the free extre mities of the prisms in the respiratory tract and in the uterus and Fallopian tubes. The true glandular epithelium is always without cilia. This is a general fact, and one of great import ance. But those varieties which seem interme diate between the spheroidal and the other two forms are often furnished with cilia ; of which examples may be seen in the Malpighian bodies of the kidney, in the mucous membrane of the frog's mouth, and in that of the human tympa num (fig. 281). In all cases the cilia, when they exist, are developed only on that aspect of the particles which forms a portion of the gene ral surface of the membrane.
It is as yet entirely unknown by what pro cess the cilia are produced and nourished ; whether the particles, with their cilia, are shed from time to time, and are succeeded by others, (as is most probable,) or whether the same organs remain, and merely change their com ponent elements. (On the subject of CILIA in general the reader is referred to Dr. Sharpey's excellent article.) Of the elementary tissues appended to the mucous system.—The two elementary tissues /low described may be considered as the more essential constituents of the mucous system, or as forming the simple mucous membrane. This simple mucous membrane envelopes the rest of the body. It contains within its own substance neither vessels nor nerves, but is, strictly speak ing, extra-vascular. By modifications, chiefly of the epithelial element, it is in itself capable of presenting great variety of appearance and properties in different situations. But in im mediate connection with its deep surface, that is, with the basement membrane, there are cer tain tissues common to almost every part of the frame, but here assuming a peculiar arrange ment and office, and by their diversities in various localities, occasioning the most compli cated varieties of outward form, of structure, and of function.
These appended tissues are minute blood vessels, a lymphatic network, nerves, and areo lar tissue.
It has been already stated that in many parts the simple mucous membrane, by its innume rable minute involutions over au extensive sur face, is formed into a compound membrane. Into the composition of this (of which a good example is afforded by that of the stomach) the appended tissues enter more or less largely, but they are likewise, in addition, generally spread out in great abundance as a layer underneath the compound membrane. This layer has been commonly termed submucous cellular membrane, (sometimes tunicit nervea,) in the case of in ternal surfaces, and cutis van or dermis in the case of the skin.
Bloodvmels. — These may be said to be universally present under the simple mucous membrane, with the exception perhaps of the cornea, where vessels, in the normal state, have not yet been demonstrated. The capillaries, in their simplest form, appear to be arranged as a plane network, such as that of the rectum of the frog (fig. 285). The interstices of this network vary much in size and shape in diffe rent localities. The most copious supply of blood distributed to any such membrane is that afforded to the air-cells of the lungs in all ani mals. Here this plane capillary plexus has areolm scarcely exceeding the diameter of the vessels themselves. Where the membrane they supply is folded, however irregularly, they follow its surface, and hence result many varie ties in their arrangement and inosculations. It even seems to be for the purpose of gaining a great freedom of inosculation between the ca pillaries that the extraordinary complexity has been given to many parts of the simple mem brane, especially in the secreting glands. For many foldings from somewhat distant parts of the membrane are there brought into imme diate proximity to one another, and are sup plied by the same or closely connected vessels. This is remarkably exemplified in the testis, kidney, and liver. The capillary system of all these, as well as of other solid glands, may be styled a solid plexus, being extended in every direction, and presenting areolte of nearly equal size in whatever plane a section of it be made. The liver presents the most perfect instance of such a solid plexus, and in it the vessels are of unusual dimensions, apparently to allow of the more free transit of the blood, which is here propelled feebly by the vas a tergo acting through the capillaries that form the portal vein. Though it has not been so described, I believe, from injections that I have made, that the whole organ is one such plexus, and that if it were possible to abstract from it all vessels larger than capillaries, and to leave these entire, all the lobules would still be connected together by capillary channels identical with those of which they themselves principally consist. Hence the lobules of the liver are not definitely bounded on all sides by a capsule of any kind, but here and there blend by continuity of sub stance with those adjoining them. The larger portion of their contour is, however, well de fined by the ultimate twigs of the portal vein, and of the ducts derived from the lobule, as so clearly proved by Mr. Kiernan in his well known paper.