142 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



226. The Epidermis covers the whole exterior surface of the body ; 

 not excepting the Conjunctiva of the eye, on which, however, it has 

 more the character of an Epithelium, and the Cornea, on which it par- 

 ticipates in the horny character of the Epidermic covering (Fig. 21). 

 The continuity is well seen in the cast skin or slough of the Snake ; in 

 which the covering of the front of the eye is found to be as perfectly 

 exuviated as that of any part of the body. The number of layers varies 

 greatly in different parts ; being usually found to be the greatest where 

 there is most pressure or friction. Thus on the soles of the feet, par- 

 ticularly at the heel and the ball of the great toe, the Epidermis is ex- 

 tremely thick ; and the palms of the hands of the labouring man are 

 distinguished by the increased density of their horny covering. It 

 would seem as if the irritation of the skin stimulated it to an increased 

 production of this substance. The. Epidermic membrane is pierced by 

 the excretory ducts of the sweat-glands, and of the sebaceous follicles, 

 which lie in the true skin and immediately beneath it ; or we should 

 rather say that it is continuous with the delicate epithelial lining of 

 these. The Nails may be considered as nothing more than an altered 

 form of Epidermis. When examined near their origin, they are found 

 to consist of cells which gradually dry into scales ; and these remain 

 coherent together. A new production is continually taking place in the 

 groove of the skin, in which the root of the nail is imbedded ; and pro- 

 bably also from the whole subjacent surface. 



227. The Epidermis, when analysed, is found to differ from the pro- 

 teine-compounds in its composition ; but not in any very striking de- 

 gree. The proportion of its elements is considered to be 48 Carbon, 

 39 Hydrogen, 7 Nitrogen, 17 Oxygen; and this corresponds exactly 

 with the composition of the substance of which Nails, Horn, Hair, and 

 Wool are constituted. It seems probable, however, that the cell-walls 

 are formed, as elsewhere, of Fibrine ; and that the horny matter is a 

 secretion in their interior, which is drawn from the elements of blood 

 during their growth and development. 



228. The Epidermis appears solely destined for the protection of the 

 true Skin; both from the mechanical injury and the pain which the 

 slightest abrasion would produce ; and from the irritating effects of ex- 

 posure to the external air and of changes of temperature. We perceive 

 the value of this protection, when the Epidermis has been accidentally 

 removed. It is very speedily replaced, however ; the increased deter- 

 mination of blood to the Skin, which is the consequence of the irrita- 

 tion, being favourable to the rapid production of Epidermic cells on its 

 surface. 



229. Mingled with the Epidermic cells we find others which secrete 

 colouring matter instead of horn ; these are termed Pigment-cells. 

 They are not readily distinguishable in the epidermis of the White 

 races, except in certain parts, such as the areola around the nipple, and 

 in freckles, naevi, &c. But they are very obvious, on account of their 

 dark hue, in the newer layers of the Epidermis of the Negro and other 

 coloured races ; and, like the true Epidermic cells, they dry up and 

 become flattened scales in their passage towards the surface, thus con- 

 stantly remaining dispersed through the Epidermis, and giving it a dark 



