THE SKIN AND ITS APPENDAGES. 269 



The medulla is composed of rows of irregular cuboidal or 

 spherical cells, 15-20 n in diameter, filled with dark granules, which 

 really are minute air- vesicles ; by reflected light the pith appears 

 silvery-white, while by transmitted light it is dark and opaque. The 

 air gains access to the medulla in consequence of the partial drying 

 out of the soft protoplasm of the cells. In many animals the medulla- 

 cells form a conspicuous and relatively large portion of the hair, and 

 present characteristic details sufficiently dis- 

 tinctive to determine the kind of animal from 

 which the specimen was obtained. 



The color of the hair depends upon the 

 presence of pigment-granules, diffuse pig- 

 ment, and air. The granular pigment 

 occurs as colored particles varying from light 

 brown to black ; in dark hair the pigment lies 

 within the elements of the cortical substance, as 

 well as often between the cells, the cortex in 

 addition sometimes containing diffuse soluble 

 coloring-matter in combination with the proto- 

 plasm of the cells. Diffuse pigment is en- 

 tirely wanting in white hair, is sparingly present 

 in light blond hair, and exists in abundance in 

 dark blond, red, and dark hair. 



The hair-follicles are tubular or flask-shaped 

 depressions within the integument (2-7 mm. in 

 length) which tightly embrace the hair-shafts ; 

 those of the finer hairs lie entirely within the 

 corium, while those of the large hairs frequently 

 extend deeply into the subcutaneous stratum. 



The hair-follicle serves the double purpose of 

 supplying the tissue from which the hair is formed 

 and of affording the necessary attachment and 

 support to the hair after its development. The 

 relation of the hair-follicle to the general integu- 

 ment is best appreciated by remembering that 

 the follicle develops by an ingrowth of the 

 epidermis into the subjacent connective tis- 

 sue ; the hair subsequently appears as the result 

 of the metamorphosis and the differentiation of 

 the cells occupying the most dependent part of 

 the epidermal plug. While in the follicles of the 

 finer hairs the epithelium forms almost the entire structure, in those 

 of the large hairs the surrounding connective tissue takes part to the 

 extent of supplying a strong protective sheath, the fibrous coat. 



Hair-follicle from human 

 scalp : a, hair ; b, inner 

 root-sheath ; c , outer root- 

 sheath ; d, glassy mem- 

 brane ; e, fibrous sheath ; 

 f t hair-bulb; h, hair-pa- 



