HAIRS. 323 



thelium, which together form the sheaths of the hair root. 

 Outside these we find connective- tissue layers, which enclose 

 them in a saccular structure, which is known as the hair 

 follicle. A true hair consists of three parts (Figs. 245 and 

 246): 



1. The medullary substance ; 



2. The cortical substance ; 



3. The outer hair membrane (hair cuticle, cuticula pili). 

 The medullary substance lies in the axis of the hairs ; but is 



found usually only in the thicker hairs, and in the lower part 

 of the hair root. It is made up of cubical cells containing large 

 spherical nuclei. As a rule, the whole thickness of the medul- 

 lary substance is formed by one or two adjacent cells. 



The main part of the true hair is composed of the cortical 

 substance. This consists of spindle-shaped cells, which show a 

 distinct fibrillar structure and contain oval nuclei. Since these 

 cells lie with their long axes parallel to that of the hair, the 

 whole hair has the appearance of being longitudinally striated. 

 In this part of colored hairs there are pigment granules, inside 

 and between the cells. In the neighborhood of the papilla, 

 however, we find branched pigment cells. There is to be 

 found, likewise, coloring-matter in solution, which infiltrates 

 the cortical cells. The color of the hair is due to these two 

 kinds of pigment. We find also between both cortical arid 

 medullary cells small spaces filled with air. In the medulla 

 these may be very abundant, and, when pigment is scanty, 

 they cause the hair to be distinctly white. Hairs which have 

 entirely lost their pigment, but contain no air spaces in the 

 medullary substance, are gray, but never white. 



The outermost layer of the true hair is the hair cuticle. 

 This is made up of fine, transparent, structureless, and almost 

 rectangular scales, which are placed like tiles in such a way 

 that their lower borders lie on the cortex, while their free 

 edges project toward the outside and the end of the hair. In 

 longitudinal sections of a hair these are directed from the out- 

 side downward and inward. They overlie one another, so that 

 four to six cells form the thickness of the cuticle. In cross- 



