PHYSIOLOGICAL ANATOMY OF THE HAIRS. 



349 



head. The average number of hairs upon a square inch of the scalp is about 

 1,000 (155 in a square centimetre) and the number upon the entire head, 

 about 120,000 (Wilson). 



When the hairs are in a perfectly normal condition, they are very elastic 

 and may be stretched to one-fifth or one-third more than their original 



FIG. 107. Hair and hair-follicle (Sappey). 

 1, root of the hair ; 2, bulb of the hair ; 3, internal root- 

 sheath ; 4, external root-sheath ; 5, membrane of 

 the hair-follicle (the internal, amorphous mem- 

 brane of the follicle is very delicate and is not rep- 

 resented in the figure) ; 6, external membrane of 

 the follicle ; 7, 7, muscular bands attached to the 

 follicle ; 8, 8, extremities of these bands passing to 

 the skin ; 9, compound sebaceous gland, with its 

 duct (10) opening into the upper third of the fol- 

 licle ; 11. simple sebaceous gland; 13, opening of 

 the hair-follicle. 



FIG. 108. Root of the hair (Sappey). 

 1, root of the hair ; 2, hair-bulb : 3, papilla of 

 the follicle ; 4, opening of the follicle ; 5, 

 5, internal root-sheath ; 6, external root- 

 sheath ; 7, 7, sebaceous glands : 8, 8, ex- 

 cretory ducts of the sebaceous glands. 



length. Their strength varies with their thickness, but an ordinary hair from 

 the head will bear a weight of six to seven ounces (170 to 200 grammes). A 

 well known property of the hair is that of becoming strongly electric by 



