The excessive production over expansive "surfaces, both external and internal, of epidermis, with a normal form and aggregation of its elements, is often well exemplified, so far as the mucous membranes are concerned, in those of the oesophagus and vagina. There are, however, epidermidal luxuriations besides, marked by several peculiarities, such as site, circumscribed locality, unusual aggregation of elements. To these belong, also, the epithelial layers investing the various cysts.

(A.) As Epithelial Contents Of Encysted Tumors

The form of the cells is most commonly that of tessellated epithelium cells.

Upon the external skin these tumors manifest themselves as luxuriant new growths, sometimes overspreading a wide surface, sometimes limited to a smaller space, occasionally as cyst-like developments of cutaneous follicles with their excretory ducts. These growths not rarely attain to a considerable circumference, and are distinguished by a peculiar anomalous arrangement of their elements, as also by, on the one side a retarded, on the other an excessive, horny character of the elementary cells.

(B.) Clavus

Clavus, a local accumulation of epidermis-cells, of a conical shape with the apex pointing to the interior of the papillary body, with a superimposed disposition of the cells not deviating from the normal.

(C.) Warts

Of these there are sundry varieties. The most ordinary consists of cornified epidermis forming a sheath-like receptacle of considerable thickness for the hypertrophied cutaneous papillae. Others are marked by the elongated fibrous arrangement of very luxuriating cells, as polyedrical, edged cylinders in parallel array, some of which show imperfect cornification. They have a fibro-villous, appearance, are humid, and readily broken up by pressure into fibres and their elements. Their cells are devoid of nuclei, and in only a few instances cornified.

(D.) Ichthyosis

The higher grades alone concern us here, the epidermis covering a papillary body, proportionately hypertrophied, luxuriates into polyedrical tessellae, cylinders, and disks. The disposition of the cells, at least in the cylinder form, is a fibrous one, parallel to its length. The degree of cornification is not in every case the same.

(E.) Horn

Horn - cornu cutaneum - a very common, for the most part dingybrown, longitudinally ribbed, more or less curved, cylindrical or conical horn-growth, springing from a cutaneous follicle of cyst-like development. It attains now and then to several inches in length. Its structure is seemingly fibrous. The cornification of the cells is very marked. It affects parts abounding in follicles, or hairy surfaces and their vicinity; for example, the forehead, the neighborhood of the pubes, and again, the back and the upper extremities.

All these growths are in their nature innocent.

Besides these, however, there occur upon the common integuments, as also upon the mucous membranes, growths which, although often extirpated with a favorable result, occasionally prove malignant and assimilate in all respects to cancer. Their elementary cells repeat the form of the non-cornified, nucleated, tessellated epithelium-cell, not rarely with a fibre-like prolongation, and whose secondary arrangement often displays the areolar type, or else represents in fibrils a moist velvety growth, similar to hypertrophied cutaneous papillae.

Anomalous hair occurs in various shapes with reference to the form, color, length, and thickness of the hair-cylinder.

Besides their appearance at unusual points of the external integuments, especially upon pigment naevi, we have to advert to -

(A.) Hair Contained Within Encysted Tumors

This is commonly mingled with fat and epithelium. It is extremely frequent in the fatty cysts of the ovaries, but is also found in those of the omentum, of the cutis, of the subcutaneous areolar tissue, and even of the lungs. In these cysts it is often found to pervade the fat, as with a felt growing out of variously-sized patches, closely resembling the cutaneous texture, from the inner surface of the sac. Its development is here seen to be entirely identical with that of hair upon the common integuments.

(B.) Hair Upon Mucous Membranes

It has been detected upon various mucous membranes, including even the conjunctiva of the eye.

Very small, partly microscopical, hairs are sometimes mixed up with the contents of encysted tumors, - with cholesteatoma. Those said to occur in the different secretions, the urine for example, are evidently derived from a mucous membrane.