HAIRS. 325 



the innermost of which lies immediately on the outer root 

 sheath, and is known as the hyaline layer (Glashaut). This 

 varies in thickness, and is sometimes hardly visible. Its inner 

 surface is distinctly grooved. Outside the hyaline layer we 

 find the circular sheath, in which bundles of connective-tissue 

 fibres run around the hair root. This extends from the bot- 

 tom of the follicle to the level of the sebaceous glands. Outside 

 the circular sheath there are longitudinal bundles of connective- 

 tissue fibres containing vessels and nerves. 



Development of Hairs. 



Although the hair with its root sheaths is a somewhat 

 complicated structure, all the various parts are found to have 

 a common origin in the stratum germinativum of the skin. 

 Toward the end of the third month of foetal life epithelial 

 thickenings appear in those places where hairs are to develop 

 (Fig. 247). In consequence of the farther increase of cells 



FIG. 247. 



Older hair Rudiment of 



Hair germ germ hair follicle Hair germ 



~ 



Epidermis [Q 



Corium \ 

 Blood-vessel' 



Corium 



Vertical section through the scalp of a human embryo of the fifth month. X 230. 



the epidermis dips down into the corium in the form of solid 

 epidermal columns, each of which forms a hair germ. This 

 increases in length and becomes thicker at its lower end. At 

 this time we notice that the corium is differentiated to form the 

 hair follicle ; and the connective-tissue papilla grows up from 

 the corium into the bulb of the hair. Various differentiations 

 now take place in the cells of the hair germ. The axial cells 



