HISTOLOGY. 89 



cells overlapping each other. The cortical is the thick- 

 est and contains the coloring- matter of the hair. The 

 medulla is not always present, but when it is, it con- 

 tains air and gives an appearance according to tta 

 light used to examine the specimen. 



At the bottom of the hair follicle is the fibrous tissue 

 that contains the blood vessels of the hair. The sheaths 

 of the hair are named according to their location. The 

 arrectores muscles of the hair are composed of a few 

 fibers of smooth muscle-tissue arising in the fibrous 

 of the corium, and cause the hair to stand on ends in 

 case of fright. A longitudinal section of the hair pre- 

 sents for examination : shaft, composed of three lay- 

 ers of modified epithelial cells ; root, that part of the 

 hair within the skin; the bulb, a division of the root; 

 the papillae, an elevation of the true skin into the root 

 of the hair; the sheaths, surrounding the root and 

 bulb, called root sheaths (the opening and the sheaths 

 are called the hair follicle) ; sebaceous glands, com- 

 posed of a saccular variety of glands, located about 

 midway the root of the hair. 



The theories as to the growth of hair are two : first, 

 that the hair grows from an infolding of the epidermis 

 of the skin ; second, that the hair grows from the papil- 

 lae at the bottom of the root. The nerves of the hair 

 are few and extend only to the duct of the sebaceous 



