34:6 HANDBOOK OF PHYSIOLOGY. 



Beneath this is a much thicker layer of elongated horny cells, closely 

 packed together so as to resemble a fibrous structure. This, very com- 

 monly, in the human subject, occupies the whole of the inside of the 

 hair; but in some cases there is left a small central space filled by a sub- 

 stance called the medulla or pith, composed of small collections of irreg- 

 ularly shaped cells, containing sometimes pigment granules or fat, but 

 mostly air. 



The follicle, in which the root of each hair is contained (Fig. 238), 

 forms a tubular depression from the surface of the skin, descending into 

 the subcutaneous fat, generally to a greater depth than the sudoriferous 

 glands, and at its deepest part enlarging in a bulbous form, and often 

 curving from its 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 folli- 

 cles of the largest hairs, has the structure of vascular fibrous tissue. At 

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



FIG. 237. Surf ace of a white hair, magnified 160 diameters. The wave lines mark the upper or 

 free edges of the cortical scales. B, separated scales, magnified 350 diameters. (Kolliker.) 



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 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. 238). This 

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

 posed of two layers, the inner one of which is so moulded on the im- 

 bricated scaly cuticle of the hair, that its inner surface becomes imbri- 

 cated 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 consistence. 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 



