EPITHELIUM. 247 



its removal. Nitrate of silver blackens the epidermis and renders 

 opaque the epithelium of mucous membranes, but destroys nothing 

 beneath them. It is therefore not a caustic, in any scientific sense. 

 The epidermis is separated from the corium by a blister, and by 

 exudations underneath it from other causes as in all vesicular skin 

 diseases. 



Pathological Conditions of Epithelium. 



1. Epidermic and epithelial tumors (epithelioma) are of very fre- 

 quent occurrence. Warts (verrucae) and callosities of the skin, espe- 

 cially corns (clavi), are minor instances of this group. In the case 

 of warts, however, the papillae as well as the epidermis become 

 hypertrophied. The wart-like naevi materni, ichthyosis, and ele- 

 phantiasis Arabum, also belong to this class, though this last is not 

 limited to the epidermis alone. 



2. Condylomata (more properly termed papillomata), mucous tu- 

 bercles, and similar vegetations, apt to form around the orifices of 

 mucous canals from the irritation of syphilitic or other discharges, 

 belong also to this class. Fig. 153, B, shows one of these vegetations 



Fig. 153. 



Epithelial new formations. A. Papilloma highly magnified. B. Epithelial tumor from lip. (Lebert.) 



as figured by Lebert; it being a papilla formed by a layer of closely 

 imbricated epithelial scales, the deeper portions consisting of less 

 flattened cells, or nuclei in an amorphous blastema, and extending 

 to the corium of the skin. Horns are also epidermic productions, 

 and sometimes appear on the human body. They originate in the 

 sebaceous follicles, whose epithelium, throVn off in abundance and 

 together with fatty secretion, forms a conical mass which protrudes 

 from the skin (usually of the head or of the forehead), sometimes 

 even to the length of six inches. 



3. Epithelial cancer should be distinguished from mere epithe- 

 lioma ; the former being doubtless malignant, though not so certain 

 to affect the lymphatic glands, and the body generally, as the other 

 forms of cancer. It occurs on the skin and mucous membrane, the 



