THE SKIN 109 



generally. The corium of the nail-bed is beset with longitudinal 

 ridges instead of the papillae which are present over the rest of the 

 skin ; these, like the rest of the superficial part of the corium, are 

 extremely vascular. The nails are developed in the foetus at about 

 the third month, the groove being formed at this time in the corium, 

 and the nail-rudiment appearing in it as a thickening of the stratum 

 lucidum, which extends forward over the bed. It becomes free in the 

 sixth month, its free end being at first thin, but as it grows forward 

 over the bed it appears to receive additions on its under surface, so 

 that after a time the distal part becomes the thicker. The superficial 

 layers of the cuticle which originally covered the developing nail become 

 detached, and, after birth, only remain as the narrow border of cuticle 

 which overlies the lunula. 



The hairs are growths of the epidermis, which are developed in 

 little pits the hair-follicles which extend downwards into the deeper 

 part of the corium, or even into the subcutaneous tissue. The hair 

 grows from the bottom of the follicle, the part which thus lies within 

 the follicle being known as the root. 



The substance of a hair is mainly composed of a pigmented, horny, 

 fibrous material (fig. 132, /), which can be separated by the action 

 of sulphuric acid into long tapering cells, the nuclei of which are 

 still visible. This fibrous substance of the hair is covered by a layer 

 of delicate imbricated scales termed the hair-cuticle (c). In many hairs, 

 but not in all, the centre is occupied by a dark-looking axial substance 

 (medulla, m), formed of angular cells which contain granules of eleidin, 

 particles of dark pigment, and frequently minute air-bubbles. The 

 latter may also occur in interstices in the fibrous substance. When 

 they are present, the hair looks white by reflected light. The root has 

 the same structure as the body of the hair, except at its extremity, 

 which is enlarged into a knob (fig. 133, b) ; this is composed mainly 

 of soft, growing cells, and fits over a vascular papilla (p), which pro- 

 jects up into the bottom of the follicle. The follicle, like the skin 

 itself, of which it is a recess, is composed of two parts : one epithelial, 

 and the other connective tissue. The epithelial or epidermic part of 

 the follicle closely invests the hair-root, and is often in great part 

 dragged out with it ; hence it is known as the root-sheath. It consists 

 of an outer layer of soft columnar and polyhedral cells, like the Mal- 

 pighian layer of the epidermis the outer root-sheath (figs. 133, /; 

 134, e) ; and of an inner, thinner, horny stratum next the hair the 

 inner root-sheath (figs. 133, g ; 134, /). The inner root-sheath itself 

 consists of three layers, the outermost being composed of oblong cells 

 without nuclei (Henle's layer], the next of flattened polyhedral nu- 

 cleated cells (Huxley's layer], and the third the cuticle of the root- 

 sheath being a thin layer of downwardly imbricated scales, which fit 

 over the upwardly imbricated scales of the hair itself. 



The connective tissue or dermic part of the hair-follicle (fig. 134, 

 -a, c, d) is composed internally of a vascular layer, separated from the 



