,416 THE HUMAN BODY. 



united to form fibres; and in the centre of the shaft there 

 is found, in many hairs, a medulla, made up of more or less 

 rounded cells. The color of hair is mainly dependent upon 

 pigment granules lying between the fibres of the cortex. 

 All hairs contain some air cavities, especially in the medulla. 

 They are very abundant in white hairs and cause the white- 

 ness by reflecting all the incident light, just as a liquid beaten 

 into fine foam looks white because of the light reflected 

 from the walls of all the little air cavities in it. In dark 

 hairs the air cavities are few. 



The hair follicle (Fig. 119) is a narrow pit of the dermis, 

 projecting down into the subcutaneous areolar tissue, and 

 lined by an involution of the epidermis. At the bottom of the 



follicle is a papilla and the 

 epidermis, turning up over 

 this, becomes continuous 

 with the hair. On the 

 papilla epidermic cells 

 multiply rapidly so long as 

 the hair is growing, and 

 the whole hair is there 

 made up of roundish cells. 



Fiot. 119. Parts of two hairs imbedded As these get pushed Up by 



in their follicles, a, the skin, which is seen f -, f -, u 



to dip down and line the follicle; 6, the iresll Ones lOrmed beneath 



subcutaneous ti ssue ; c, the muscles of the XT-.^ J.T... .-.,-, f.-.,, ^ O f "| O T- 



hair follicle, which by their contraction them, the Outermost la} 6P 



can erect the hair. become flattened and form 



the hair cuticle; several succeeding layers elongate and 

 form the cortex; while, in hairs' with a medulla, the middle 

 cells retain pretty much their original form and size. 

 Pulled apart by the elongating cortical cells, these central 

 ones then form the medulla with its air cavities. The 

 innermost layer of the epidermis, lining the follicle, has its 

 cells projecting, with overlapping edges turned down- 

 wards. Accordingly these interlock with the upward 

 directed edges of the cells of the hair cuticle; consequently 

 when a hair is pulled out the epidermic lining of the follicle 

 is usually brought with it. So long as the dermic papilla 

 is left intact a new hair will be formed, but not otherwise. 

 Slender bundles of unstriped muscle (c, Fig. 119) run from 



