THE SKIN 541 



Hence this is distinguished as the horny layer (stratum 

 corneum). 



In the living subject the superficial layers of the 

 epidermis become separated from the lower layers and 

 the dermis, when friction or other irritation produces a 

 "blister." Fluid is poured out from the vessels of the 

 derma, and, accumulating between the upper and lower 

 layers of the epidermis, detaches the latter. 



The epidermis is constantly growing upon the deep 

 or dermic side in such a manner that the horny layer 

 is continually being shed and replaced. The "scurf" 

 which collects between the haii's and on the whole surface 

 of the body, and is removed by our daily brushing and 

 washing, is nothing but shed ejjidermis. When a limb 

 has been bandaged up and left undisturbed for weeks, as 

 in case of a fracture, the shed epidermis collects on the 

 surface of the skin in the shape of scales and flakes, which 

 break up into a fine white powder when rubbed. Thus 

 we "shed our skins" just as snakes do, only that 

 the snake sheds all his dead epidermis as a coherent 

 sheet at once, while we shed ours bit by bit, and hour 

 by hour. 



What is the nature of the process by which the 

 epidermis is continually removed ? 



If a little of the epidermic scurf is mixed with water 

 and examined under a power magnifying 300 or 400 

 diameters, it will seem to consist of nothing but irre- 

 gular particles of very various sizes and with no definite 

 structure. But if a little caustic potash or soda is 

 previously added to the water the appearance will be 

 changed. The caustic alkali causes the horny substance 

 to swell up and become transparent ; and this is now 

 seen to consist of minute separable plates, some of which 

 contain a rounded body in the interior of the plate, 

 though in many this is no longer recognisable. In fact, 

 so far as their form is concerned, these bodies have the 

 character of nucleated cells, in which the protoplasmic 



