1. Defective And Excessive Development

Congenital deficiency of a mucous membrane involves deficiency of the apparatus which it composes, or which, as the expression is, it invests or lines: always, therefore, when the mucous membrane is absent, the whole apparatus is absent too. The only instance of acquired deficiency is a partial loss of substance, and it varies in character considerably according to its cause.

Preternatural development is sometimes an original anomaly, which may be exhibited in a congenital excess in the length and capacity of mucous canals and cavities, in the existence of unusual appendages and duplicatures, or in the unusual size of prolongations and folds which naturally exist in the membrane; Sometimes such an anomaly is acquired: it is exemplified in the similarity that exists between the surfaces of abscesses and fistulae, and mucous membranes; in other words, as Otto describes it, in the development of anormal cavities and canals, which, like normal mucous membranes, are connected, or about to be connected, with the surface of the body: it is further illustrated by the restitution of lost mucous membranes.

In regard to the former, whether the lining membrane of the abscess or fistula be composed of cellular tissue, of serous membrane, or of any other structure loosened in its texture so as to resemble cellular tissue, it is at first a granulating vascular layer closely connected with the subjacent structures; but afterwards it become a more distinct membrane, and may be isolated from them: its free surface may be smooth, or may be covered with shreddy appendages and prolongations. In its structure it has a general resemblance to mucous membrane; but inasmuch as it has few follicles and no actual villi, it is rather like those of simpler organization, such as the ducts of glands. Moreover, it exhibits pathological changes, which are very similar to those of normal mucous membranes: sometimes it is pale, and sometimes it is found injected, reddened, and swollen, just as is the case in acute or chronic inflammations of a natural membrane; polypus-like prolongations are formed upon it, and fungous growths of various kinds; and further, the cellular tissue beneath it becomes thickened and callous, etc. And just as mucous surfaces never unite together, except after some solution of their continuity, so also the allied anomalous mucous canals can only be closed by laying bare the tissues beneath them, either by laceration, or by compression carried to the extent of producing atrophy.

There is a difficulty in the regeneration of mucous membranes in their original form, proportioned to the complexity of structure of the membrane that has been lost, to the amount of its substance removed, to the extent to which the submucous tissues have been likewise destroyed, and lastly, to the change of texture which those tissues have undergone during the process by which the loss of substance was occasioned. Regeneration is extremely difficult, therefore, in several respects, but it is most so after deeply-extending ulceration. We shall again have, as we have already had, occasion, when considering the mucous membranes in particular, to observe several most interesting peculiarities in the mode of repairing losses of substance, especially those occasioned by ulceration. In general, the repair of a breach of substance on a mucous membrane is effected in the following manner: - The exposed submucous structures are first condensed to a serous or fibro-cellular (callous) tissue by a reactionary inflammation of more or less activity, and then are gradually covered by the adjacent mucous membrane, which is drawn in, and attenuated as it is drawn, from the margins towards the centre of the defective spot: no actual regeneration therefore, no new growth of mucous membrane, takes place. But occasionally the provisional serous membrane that covers the defect is converted into mucous membrane; and in the intestinal canal especially, when the typhous process has been limited by the submucous tissue, that tissue becomes developed at the middle of the ulcer, even to a villous mucous membrane. Extensive and deeper losses of substance are permanently replaced by a callous (cicatrix) tissue, that only occasionally obtains a smooth covering like serous membrane. And the cicatrix is of course more dense and thicker, and the mucous membrane upon it more firmly fixed to the submucous tissues, in proportion to the amount of damage those tissues have sustained, either at first from the loss of substance, or subsequently from the reactionary process which was called forth.

2. Deviations In The Size - Superficial Area - And In The Form Of Mucous Membranes

The anomalies which may be included under this general head, are the partial dilatations of mucous cavities and canals, and those which relate to the thickness of the membranes.

The former are the diverticuli spuria, as they are called, or herniae of mucous membrane. They occur chiefly in the intestinal tract, in the urinary bladder, and also, but less frequently, in the trachea and bronchi. The mucous membrane protrudes, in the form of a rounded, pear-shaped, or cylindrical, saccular appendage, through the separated fibres of the fleshy coats: the appendage is attached by a sort of neck, and the two cavities communicate with one another by a narrow opening, which at first is a mere fissure, or is lozenge-shaped, but afterwards becomes circular, and is bounded by a kind of sphincter.

The thickness of mucous membranes may be increased or diminished. Permanent increase of thickness is due not only to various changes of texture, but also to hypertrophy: diminished thickness is a result of atrophy. Either may involve the entire structure of the membrane, or may affect one of its component parts only, such as the follicular apparatus, or the papillae. Hypertrophy is for the most part produced by well-marked, and either repeated or continuous, states of irritation or of inflammation: it presents several degrees, and I shall treat of it further at a place where its development from these conditions can be more conveniently shown.

Atrophy very rarely occurs as a spontaneous affection in any mucous membrane: it must be distinguished from the softening of mucous tissue, which comes on after exudative processes. The membrane becomes more or less attenuated, and may be easily torn; its folds waste, or with the follicles and villi altogether disappear; its surface is pale and smooth, and glistens like a serous membrane. A similar attenuation is observed in the mucous membrane surrounding various extensive losses of substance, both during and after their repair. And there is yet another instance of the same condition, in which certain parts of the mucous system not only become extremely thin, but also undergo a change of texture: it is a consequence of the gradual and excessive distension which is produced by the accumulation of matters secreted during some occlusion of the cavity. This subject will be further considered.

3. Diseases Of Texture

Both acute and chronic diseases of mucous membrane are, as is well known, exceedingly frequent; and hence, as well as from the manifold connections which they maintain, both in their origin and consequences, with other systems and organs, they are diseases of great importance. For the most part, they are the result of that sensitiveness to all changes in the material components of the fluids, - whether immediate (primary), or produced through the medium of the nervous system, - as well as to all deviations from the proper evolution and distribution of the imponderable principles, which attaches to mucous membrane, as the most vascular of all structures, and the chief organ of absorption and secretion. Hence almost all acute (febrile), and many chronic, constitutional diseases establish themselves in various ways, the former rapidly, the latter gradually, upon these membranes. There are various processes, irritative and inflammatory, by which this is accomplished; but in the present chapter only the cardinal forms will be treated of, the catarrhal, the exudative, and the pustular; the other specific processes of the same class will be found described amongst the diseases of the separate portions of the mucous system.