STRUCTURE OF HAIR. 341 



skin, descending into the subcutaneous fat, generally to a 

 greater depth than the sudoriparous glands, and at its deepest 

 part enlarging in a bulbous form, and often curving from its 



FIG. 116. 



A, surface of a white hair, magnified 160 diameters. The wave lines mark the 

 upper or free edges of the cortical scales. , separated scales, magnified 350 diame- 

 ters (after Kolliker). 



previous rectilinear course. It is lined throughout by cells of 

 epithelium, continuous with those of the epidermis, and its 

 walls are formed of pellucid membrane, which commonly, in 

 the follicles of the largest hairs, has the structure of vascular 

 fibro-cellular tissue. 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 pa- 

 pilla that the hair is formed. The inner wall of the follicle is 

 lined by epidermal cells continuous with those covering the 

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

 formed by a simple thrusting in of the surface of the integu- 

 ment (Figs. 117, 118). 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 imbricated 

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

 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 flattened, and of more horny consist- 

 ence. That specially modified portion of the corium, or true 

 skin, by which the nail is secreted, is called the matrix. 



The back edge of the nail, or the root as it is termed, is 

 received into a shallow crescentic groove in the matrix, while 

 the front part is free, and projects beyond the extremity of the 

 digit. The intermediate portion of the nail rests by its broad 



29 



