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HANDBOOK OF PHYSIOLOGY. 



either the surface of the skin, close to a hair, or, which is more usual, 

 directly into the follicle of the hair. In the latter case, there are gener- 

 ally two or more glands to each hair (fig. 312). 



Hair. A hair is produced by a peculiar growth and modification of 

 the epidermis. Externally it is covered by a layer of fine scales closely 

 imbricated, or overlapping like the tiles of a house, but with the free 

 edges turned upward (fig. 314, A). It is called the cuticle of the hair. 

 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 inside of the hair; but 



Fig. 3ia Transverse section of a hair and hair-follicle made below the opening of the sebaceous 

 erlaua. , medulla or pith of the hair; b, fibrous layer or cortex; o, cuticle; d, Huxley's layer: e, 

 Henle's layer of internal root-sheath: /and q. layers of external root-sheath, outside of g is a light 

 layer, or " glassy membrane, 1 ' which is equivalent to the basement membrane; h, fibrous coat of 

 hair sac; /, vessels. (Cadiat.) 



in some cases there is left a small central space filled by a substance 

 called the medulla or pith, composed of small collections of irregularly 

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

 air. 



The follicle, in which the root of each hair is contained (fi<r. 315), 

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

 into the subcutaneous fat, generally to a greater depth than the sudor- 

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

 often curving from its previous rectilinear course. It is lined through- 

 out 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 fibrous tissue. 



