EXCRETION AND THE EXCRETORY ORGANS 533 



The color of hair is mainly dependent upon pigment-granules 

 lying between the fibers of the cortex. All hairs contain some air 

 cavities, especially in the medulla. They are very abundant in 

 white hairs and cause the whiteness 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. 144) is a narrow pit of the dermis, pro- 

 jecting 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 con^ 

 tinuous 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. As these are pushed up by fresh ones 

 formed beneath them, the outermost layer 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, C 

 with overlapping edges turned 



downwards. 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 O 



FIG. 144. Parts of two hairs em- 

 Will be formed, but not Otherwise, bedded in their follicles, a, the skin, 



Slender bundles of smooth muscle 

 (c, Fig. 144) run from the dermis 



to the side of the hair-follicles, o, sebaceous gland. 

 The latter are in most regions obliquely implanted in the skin so 

 that the hairs lie down on the surface of the Body, and the muscles 

 are so fixed that when they shorten, they erect the hair and cause 

 it to bristle, as may be seen in an angry cat, or sometimes in a 

 greatly terrified man. Opening into each hair-follicle are usually a 



