EXCRETION. 



495 



At the bottom of the follicle is a small papilla, or projection of true 

 skin, and it is by the production and outgrowth of epidermal cells from 

 the surface of this papilla that the hair is formed. The inner wall of 

 the follicle is lined by epidermal cells continuous with those covering 



Fig. 814. Surface of a white hair, magnified 160 diameters. The wave lines mark the upper or free 

 edges of the cortical scales. B, separated scales, magnified 360 diameters. (Kolliker.) 



the general surface of the skin; as if indeed the follicle had been formed 

 by a simple thrusting in of the surface of the integument (fig. 315). 

 This epidermal lining of the hair- follicle, or root-sheath of the hair, is 

 composed of two layers, the inner one of which is so moulded on the 

 imbricated scaly cuticle of the hair, that its inner surface becomes im- 

 bricated also, but of course in the opposite direction. When a hair is 



Fig. 815. Longitudinal section of a hair follicle, a, Stratum of Malpighi, deep layer forming 

 the external root-sneath, and continued to the surface of the papilla to form the medullary sheath 

 of the hair; 6, second external sheath; c, internal root-sheath; d, fibroid sheath of the hair; e, 

 medullary sheath or medulla; /, hair papilla; o, blood-vessels of the hair-papilla; h, fibro- vascular 

 sheath. (Cadiat.) 



pulled out, the inner layer of the root-sheath and part of the outer 

 layer also are commonly pulled out with it. 



Nails. A nail, like a hair, is a peculiar arrangement of epidermal 

 cells, the undermost of which, like those of the general surface of the 

 integument, are rounded or elongated, while the superficial are flat- 

 tened, and of more horny consistence. That specially modified portion 



