THE STRUCTURE AND FUNCTIONS OF THE SKIN. 



34T 



and projects beyond the extremity of the digit. The intermediate por- 

 tion of the nail rests by its broad under surface on the front part of the 

 matrix, which is here called the led of the nail. This part of the matrix 

 is not uniformly smooth on the surface, but is raised in the form of longi- 

 tudinal and nearly parallel ridges or laminae, on which are moulded the 

 epidermal cells of which the nail is made up (Fig. 241). 



The growth of the nail, like that of the hair, or of the epidermis 

 generally, is effected by a constant production of cells from beneath and 

 behind, to take the place of those which are worn or cut away. Inas- 



-7 



FIG. 238. 



FIG. 239. 



FIG. 238. Medium-sized hair in its follicle, a, stem cut short; 6, root; c, knob; d, hair cuticle; 

 e, internal, and /, external root-sheath; g, h, dermic coat of follicle; i, papilla; fc, fc, ducts of sebace- 

 ous glands; 1. corium; m, mucous layer of epidermis; o, upper limit of internal root sheath. X 50. 

 (Kolliker.) 



FIG. 239. Longitudinal section of a hair follicle, a, Stratum of Malpighi, deep layer forming 

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

 of the hair; ft, second external sheath; c, internal root sheath; d, fibroid sheath of the hair: e, 

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

 sheath. (Cadiat.) 



much, however, as the posterior edge of the nail, from its being lodged 

 in a groove of the skin, cannot grow back wards, on additions being made 

 to it, so easily as it can pass in the opposite direction, any growth at its 

 hinder part pushes the whole forwards. At the same time fresh cells are 

 added to its under surface, and thus each portion of the nail becomes 



