132 THE ESSENTIALS OF HISTOLOGY. 



its free end being at first thin, but as it grows forward over the bed it 

 appears to receive additions on its under surface at least in the pos- 

 terior part of the bed so that after a time the distal end becomes 

 thicker. The epitrichial layer of the cuticle which originally covered 

 the developing nail becomes detached about the fifth month, and, after 

 birth, only remains as the narrow border of cuticle which overlies the 



The hairs are growths of the epidermis, developed in little pits 

 the hair-follicles which extend downwards into the deeper part of the 

 jcorium, 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 (fig. 156). 



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

 fibrous material (fig. 155, /), which can be separated by the action of 

 sulphuric acid into long tapering fibrillated 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, 

 and frequently have a dark appearance from the presence of minute 

 air-bubbles. The latter may also occur in interstices in the fibrous sub- 

 stance. 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. 156, 6); this is composed 

 mainly of soft, growing cells, and fits over a vascular papilla (p\ which 

 projects 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. 156, /; 

 157, e)', and of an inner, thinner, horny stratum next the hair the 

 inner root-sheath (figs. 156, g\ 157, /). 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 nucleated 

 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. 157, 

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

 root-sheath by a basement-membrane termed the hyaline layer of the 



