312 



EXCRETION 



is particularly marked when the weather is cold and dry. The electricity 

 thus excited is negative. Sections of the shaft of the hairs show that 



they are oval, but their shape is va- 

 riable, straight hairs being nearly 

 round while curled hairs are quite 

 flat. Another peculiarity of the 

 hairs is that they are hygrometric. 

 They readily absorb moisture and 

 become sensibly elongated, a prop- 

 erty that has been made use of 

 by physicists in the construction of 

 hygrometers. 



Roots of the Hairs, and Hair- 

 follicles. The roots of the hairs 

 are embedded in follicular open- 

 ings in the skin, which differ in 

 the different varieties only in the 

 depth to which they penetrate the 

 cutaneous structure. In the downy 

 hairs, the roots pass into the super- 

 ficial layers only of the true skin ; 

 but in the thicker hairs, the roots 

 pass through the skin and penetrate 

 the subcutaneous cellulo-adipose 

 tissue. 



The root of the hair is softer, 

 rounder and a little larger than the 

 shaft. It becomes enlarged into a 

 rounded bulb at the bottom of the 

 follicle, and rests on a fungiform 



Fig. 67. Hair and hair-follicle (Sappey). papilla, Constricted at its base, to 



i, root of the hair; 2, bulb of the hair; 3, in- which the hair is closely attached, 

 ternal root-sheath; 4, external root-sheath; 5, mem- -pu hair follicles are tubular 



brane of the hair-follicle (the internal, amorphous 



membrane of the follicle is very delicate and is not inversions of the Structures that 

 represented in the figure) ; 6, external membrane .-, , ., 



of the follicle; 7, 7, muscular bands attached to the Compose the COHUm, and their 



follicle; 8, 8, extremities of these bands passing to wa lls present three membranes. 



the skin; 9, compound sebaceous gland, with its _. . . , . 1 -, r . , 



duct (10) opening into the upper third of the fol- Their length IS T % to \ Ot ail inch 



licle; n, simple sebaceous gland; 12, opening of ( 2> r to g . millimeters). The mem- 

 the hair-follicle. 



brane that forms the external coat 



of the follicles is composed of inelastic fibres usually arranged longi- 

 tudinally. It is provided with bloodvessels, a few nerves and some 

 connective-tissue elements, but no elastic tissue. This is the thickest 



