EPITHELIUM EPIDERMIS, ETC. 235 



sure, dried up if externally situated, and which are agglutinated to 

 each other by an intercellular substance difficult of detection. Be- 

 sides this last " problematical substance" (Lehmanri), there are three 

 distinct elements of the cells: 1, the substance of the cell-mem- 

 branes, which constitutes the principal portion of all these tissues; 

 and, which is almost insoluble in alkalies ; 2, the cell-contents, which, 

 with the nucleus, are more readily soluble in alkalies; and 3, the 

 granular matters which are wholly insoluble in alkali. The last 

 remain after the entire solution of some of these tissues, and by 

 no means consist entirely of fat. 



They all contain a considerable amount of unoxidized sulphur, 

 and usually about one per cent, of mineral substances in all. 



SECTION I. 

 EPITHELIUM (EPIDERMIS, ETC.). 



Every free surface of the body is covered by one or more layers 

 of cells, constituting an epithelium. 1 Epithelium, therefore, enters 

 into the structure at every point of the skin, and of serous and 

 mucous membranes, forming the outermost (i. e. farthest from the 

 vessels), of the three layers of which they are alike composed. 

 The next layer underneath the epithelium is the basement-mem- 

 brane, already described (p. Ill), and the innermost, the corium. 

 Viewed in its histological relations, therefore, epithelium may be 

 defined to be a continuous expansion of cells ; consisting of one or more 

 strata developed upon and completely covering a basement-membrane. 

 A single layer of cells constitutes a simple, and two or more layers 

 a compound epithelium. 



The epithelium of the skin is usually called epidermis or cuticle. 

 These terms, however, include only the outer layers of the cuta- 

 neous epithelium, as will be shown. The epithelia of the skin and 

 of mucous membrane, are of course continuous where these mem- 

 branes are so ; as at the mouth, nostrils, anus, &c. 



Epithelial cells present no original peculiarities of form and con- 

 tents. They consist of cell- wall, contained fluid, granules, nuclei, and 

 nucleoli (p. 114). On some portions of the mucous membrane, how- 

 ever, they assume a conoidal or elongated (cylindrical) form ; while 

 on serous membranes the contact of the opposed surfaces gives 

 them a very flat form, allowing but a small amount of fluid con- 



1 From M upon, and flnXJj the nipple it being very apparent on this part. 



