392 



THE SKIN AND ITS APPENDAGES. 



as well as toward the hair shaft. Below, in the region of the thick- 

 ened hair bulb, the root-sheaths begin to lessen in thickness, their 

 layers becoming more and more indistinct toward the base of the 

 hair papilla. Finally, all differentiation is lost in the region where 

 they encircle the neck of the papilla. Toward the shaft of the hair, 

 the root-sheath also undergoes changes. In the region into which 

 the sebaceous glands empty, the inner root-sheath disappears, while 

 the outer becomes continuous with the stratum germinativum of 

 the epidermis ; the outer layers of the latter the stratum granu- 

 losum, stratum lucidum, and stratum corneum push downward 

 between the outer root-sheath and the hair to the openings of the 

 sebaceous glands. 



Regarding the growth of the hair, two theories are prevalent. 



Glassy 

 layer. 



Fibrous-tis- 

 sue sheath. 



Fig- 3 J 5 Cross-section of human hair with its follicle-; X about 300. 



The one theory assumes that the elements destined to form the 

 epithelial root-sheaths are derived from the epidermis by a constant 

 process of invagination. The component parts of the hair would 

 thus be continuous with the layers of the root-sheaths, and conse- 

 quently with those of the epidermis. Thus the basal cells of the 

 external root-sheath would extend over the papilla, and be continu- 

 ous with the cells of the medulla of the hair (these relations are 

 especially well denned in the rabbit), and the stratum spinosum 

 (middle layer of stratum Malpighii) of the outer root-sheath would 

 be continuous with the cortical substance of the hair. According 

 to this theory also, the layer of Henle would correspond to the 

 stratum lucidum of the epidermis, and at the base of the hair 



