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Mucous Membranes and the skin are analogous in structure, somewhat similar in function, and sympathise closely with each other in health and in disease. Both are endowed with that peculiar degree of sensibility which enables them to bear with impunity the impressions of foreign bodies; and both are protected from the influence of these bodies by an inorganic covering; the cutis and rete mucosum by the epidermis; the corium of mucous membranes by a laminated epithelium. They are the seat of all excretions, and by them all substances are introduced from without into the system. The capillary portion of the vascular system appears to have somewhat the same arrangement in both; the distribution of blood to the mucous membranes being, however, more copious. At the commencement and extremity of the alimentary canal, they insensibly pass into each other by means of an intermediate structure, of which the prolabium may serve as an example. In particular circumstances, they change into each other, both in appearance and in function. Thus, in prolapsus of the gut or of the vagina, the discharge from the protruded mucous surface after a time subsides, the rugæ disappears, the membrane becomes thickened and indurated, and gradually assumes an appearance exactly resembling that of the skin. In natural paraphymosis, the delicate membrane which, in the healthy state of parts, lines the internal surface of the prepuce, becomes converted into a cuticular covering. In neglected and long-continued excoriation of the nates, the raw surface, which was at first tender and irritable, and discharged a serous fluid, becomes villous, less sensible, and discharges a fluid similar to a mucous secretion. In sinuses also of long duration, the secreting surface becomes changed, so as to resemble a mucous membrane, and the discharge, from being purulent or gleety, becomes mucous, or at least resembles a mucous fluid,

A mucous surface, when inflamed, has for a short time, perhaps, at first, its functions suspended; it then furnishes a secretion, increased in quantity, and but little changed in appearance from the healthy fluid; afterwards the discharge resembles purulent matter, and is termed muco-purulent. When, however, the inflammation is violent, the discharge becomes bloody, or is altogether suppressed, and the membrane is thickened. Inflammation of a mucous membrane is very apt to spread with great rapidity, in this respect resembling the corresponding affection of the skin. It is attended with a sense of itching, and a burning pain. This pain is much increased by the muscles surrounding the parts being thrown into action, as in expelling their contents, more especially if these be of an acrid quality. The membrane is thickened, and of a spongy appearance; its surface is red, and sometimes covered with flakes of lymph; occasionally it is much softened, and coated with a viscid adherent mucus; and it would appear, in many instances, that, in acute inflammation, the membrane is generally softened, whilst it becomes indurated from chronic inflammatory action. When the inflammation is violent, and consequently rapid, considerable quantities of lymph are effused either on the surface of the membrane, or into the submucous tissue: and the lymph subsequently becoming organised, the membrane is much thickened, and a contraction is the consequence. The functions of a part lined with mucous membrane are more or less deranged, in consequence of the vitiation of the secreted fluid.

In inflammation of this tissue, metastasis is also apt to occur, from one part of the membrane to another, and from the membrane to the external surface. Cynanche, for instance, often follows upon erysipelas of the face and scalp, and vice versâ.

The passages, the internal surfaces of which are invested by a mucous membrane, are those subservient to respiration, nutrition, generation, and the urinary secretions; in other words, the mucous surfaces are the Pneumogastric and the Genito-urinary. Their particular diseases will be treated of hereafter.