DEVELOPMENT OF THE HAIR. 257 



Where a thick, permanent hair, the papilla of which extends 

 as far as the subcutaneous connective tissue, replaces a fine 

 lanuginous hair, the papilla of which only reaches to the middle 

 layer of the corium, it is obvious that the hair follicle of the 

 lanuginous hair must become elongated. 



This occurs, as Kolliker has described, by the external root- 

 sheath sending off a process into the deeper layers of the 

 corium. In the same mode, as in the formation of the first 

 rudiment of the hair, a papilla develops at the base of the 

 epithelial column from the hair follicle, the cells of the root 

 accumulating around it. The hair and Huxley's sheath pro- 

 ceed from the cells of the hair root, and perforate the cells of 

 the epithelial process, and then, lying in immediate contact 

 with the lanuginous hair, penetrate into its follicle. 



The papilla of the lanuginous hair now appears to undergo 

 atrophy, the hair falling out, and the thick persistent hair then 

 occupying its follicle. This process of succession of hairs is not, 

 in all probability, limited to a single occurrence during the 

 first year of life, but is frequently repeated in childhood, as the 

 skin augments in thickness. 



The permanent hair increases to a certain length, differing 

 with the individual and the region of the body. When it has 

 reached its proper limits, the papilla can no longer support the 

 weight of the hair, which falls out, a new hair developing at 

 the same spot. This is the physiological succession of hair, 

 which in Man is continuous, but in animals occurs periodically. 



The fall of the hair results from the circumstance that no 

 new cells form around the papilla. The last-formed cells of the 

 hair root, receiving no impetus from behind, become converted 

 into hair substance, and form either the conical or club-shaped 

 inferior extremity of the hair shaft, which is composed of 

 fibrillated hair scales. 



In accordance with the statements of Henle, most anatomists 

 have described the club-shaped end of a shed hair as a second 

 form of hair root (hair bulb of Henle), though incorrectly, since 

 these bulbs are only the ends of hair shafts that have ceased to 

 grow. 



The sheaths of the hair follicle, that in consequence of their 

 muscular structure (?) exert a constant pressure upon the con- 



VOL. II. S 



