THE HAIR. 



393 



Nerve- 



plexus of 

 Bonnet. 



would become its cuticle, while the layer of Huxley would form 

 the cuticle of the inner root-sheath (Mertsching). The other 

 theory assumes that the hair is derived from a matrix, consisting of 

 proliferating cells situated on the surface of the papilla. From 

 these germinal cells would be derived the medullary and cortical 

 substance of the hair, its cuticle, and the inner root-sheath (Unna). 



The shedding of hair is common to all mammalia, a phenomenon 

 occurring periodically in the majority of species. In man the pro- 

 cess is continuous. Microscopic examination shows that the hair 

 destined to be shed becomes loosened from its papilla by a cornifi- 

 cation of the cells of its bulb. At the same time the cortical por- 

 tion of the hair bulb breaks up into a brush-like mass. Such hairs 

 are called club hairs or bulb hairs, in contradistinction to papillary 

 hairs. In the region of the former 

 papilla there arises, by a prolifera- 

 tion of the external root-sheath, a 

 bud which grows downward, from 

 which a new hair with its sheaths 

 and connective-tissue papilla is 

 developed. The result is that the 

 developing new hair gradually 

 pushes the old hair outward until 

 the latter finally drops out. The 

 exact details of this process have 

 given rise to considerable discus- 

 sion (yid. Gotte and Stieda, 87). 



Adjacent to the hair follicles 

 are bundles of smooth muscle- 

 fibers, known as the arrectores pi- 

 lorum. They originate from the 

 papillary layer of the corium and 

 extend to the lower part of the 

 connective-tissue sheath of the hair 

 follicles. In their course they not 

 infrequently encircle the sebace- 

 ous glands of the follicle. Since 



the hair follicles have a direction oblique to the skin surface, forming 

 with it an acute and an obtuse angle, and since the muscle is situated 

 within the obtuse angle, its function may easily be conceived as 

 being that of an erector of the hair. The hair papillae are very 

 vascular. 



The nerve-fibers of the hair follicles have recently been studied by 

 a number of investigators, with both the Golgi and the methylene- 

 blue methods. It has been shown that the hair follicles receive 

 their nerve supply from the nerve-fibers which terminate in the 

 immediate skin area. Each follicle receives, as a rule, only one 

 nerve-fiber, which reaches the follicle a short distance below the 

 mouth of the sebaceous gland. The nerve-fiber, on reaching the 



Fig. 316 Longitudinal section 

 through hair and hair follicle of cat ; 



X 1 60. 



