EPIDERMIS. 247 



ART. X. EPIDERMIS. 



THE entire of the external surface of the body is invested 

 with a membrane which has been denominated epidermis, 

 formed of superimposed layers of nucleated cells (see Plate 

 XXIV.^r. 3.), and the number of which layers is greatest 

 in those situations in which the membrane is subject to the 

 most pressure, as in the palms of the hands and soles of the 

 feet, in which the epidermis sometimes attains the thickness 

 of the y 1 ^ or even the ^ of an inch.* 



The form, structure, and development of the cells com- 

 posing the epidermis are in every respect identical with those 

 of the tesselated variety of epithelium already described ; 

 thus, first, the younger and deeper-seated cells are spherical 

 in outline, and almost globular, while the older and more 

 superficial cells become irregular in form, expanded, thin, 

 and membranaceous ; secondly, like those of the tesselated 

 epithelium, the cells consist of a nucleus, cell wall, cavity, 

 and granules ; it is, however, worthy of remark, that the 

 nucleus, as well as the majority of the granules contained in 

 the epidermic cells, disappear at an earlier period of develop- 

 ment than do those of the epithelium, facts which will be 

 explained when the uses of the epidermis come to be treated 

 of; thirdly, the plan of development is the same in the two 

 cases, the cell wall being developed around the nucleus, 

 which is the part first formed. 



Thus far then there is a close correspondence between the 

 epidermis and epithelium, so close indeed as to make it ap- 

 parent that the two are but modifications of one and the 

 same structure. The chief respects in which it differs from 

 the epithelium are in the compact and firm union of the cells 



* Leeuwenhoek first observed that the epidermis was composed of 

 scales placed one against the other, and also that, after a certain lapse of 

 time, they were cast off and their place supplied by others. 



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