THE HAIR 53 



materials necessary for the growth of the hair are supplied. Being 

 a development of the epidermis, the hair is composed of epithelial 

 cells, and these are arranged in three layers. From within outward 

 these are medulla, cortex and cuticle (Fig. 24). 



Bailey gives the following description of these layers : 

 (1) " The medulla, occupies the central axis of the hair. It is 

 absent in small hairs, and in large hairs does not extend throughout 

 their entire length. It is from 16 to 20 m. 5 in diameter, and con- 

 sists of -from two to four layers of polygonal or cuboidal cells with 



FIG. 24. Longitudinal section of a human hair. (From "Structure of the Wool Fi- 

 bre," Bowman. Courtesy of Macmillan Co.) 



finely granular, usually pigmented protoplasm and rudimentary 

 nuclei. 



(2) " The cortex makes up the main bulk of the hair and con- 

 sists of several layers of long spindle-shaped cells, the protoplasm 

 of which shows distinct longitudinal striations, while the nuclei 

 appear atrophied. As these striations give the hair the appearance 

 of being composed of fibrillae the term ( cortical fibers ' has been 

 applied to them. In colored hair pigment granules and pigment 

 in solution are found in and between the cells of this layer. This 

 pigment determines the color of the hair. 



(3) "The cuticle has a thicknevss of about 1 m. and consists 

 of clear, scale-like, non-nucleated epithelial cells. These overlap 

 one another like shingles on a roof, giving to the surface of the hair 

 a serrated appearance." 



G M. refers to a micron which is 1/1000 of a millimeter in length, and 

 the millimeter is 0.03937 part of an inch. 



