344 NORMAL HISTOLOGY AND ORGAN OGRAPHY. 



medulla. In thin and light hairs the medulla is usu- 

 ally absent. 



The hair cuticle, or outer hair membrane, is made up 

 of structureless transparent epithelial scales that 

 overlap each other in the direction of the distal end 

 of the hair. A hair feels smooth, therefore, if pulled 

 tnrough the fingers from the root to the free end. 

 These scales overlap each other sometimes to such 

 an extent that the cuticle has the appearance of 

 being stratified. The scales are derived from epi- 

 thelial cells that have become cornified, and they 

 are thus closely related to the horny epithelial plates 

 of the epidermis. 



The cortical substance forms the main bulk of the 

 hair and lies just beneath the cuticle. The cortex 

 consists of spindle-shaped nucleated cells which 

 show a distinct fibrillar structure, giving the whole 

 hair the appearance of being longitudinally striated. 

 Pigment granules are deposited in these cells and 

 between them, to which the hair owes most of its 

 color. Numerous small spaces filled with air are 

 frequently formed between the cells of the cortical 

 layer, and these give a white color to hairs that have 

 a scanty supply of pigment. Hairs that have en- 

 tirely lost their pigment and have none of these air 

 spaces, are gray but not white. 



The medullary substance forms the axis of the 

 hair and may be absent, but is usually present in 

 thick hairs. It is made up of nucleated cubical 

 epithelial cells forming two or three rows in thick- 

 ness. Pigment is also present in the medullary 

 cells. 



