350 EXCRETION BY THE SKIN AND KIDNEYS. 



friction ; and this 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 very variable, straight hairs being 

 nearly round, while curled hairs are quite flat. Another peculiarity of the 

 hairs is that they are strongly hygrometric. They readily absorb moisture 

 and become sensibly elongated, a property which has been made use of by 

 physicists in the construction of delicate hygrometers. 



Roots of the Hairs, and Hair-follicles. The roots of the hairs are em- 

 bedded in follicular openings in the skin, which differ in the different vari- 

 eties only in the depth to which they penetrate the cutaneous structure. In 

 the downy hairs, the roots pass only into the superficial layers 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 

 upon a fungiform papilla, constricted at its base, to which the hair is closely 

 attached. 



The hair-follicles are tubular inversions of the structures that compose the 

 corium, and their walls present three membranes. Their length is ^ to ^ of 

 an inch (2'1 to 6'4 mm.). The membrane that forms the external coat of the 

 follicles is composed of inelastic fibres, generally arranged longitudinally. It 

 is provided with blood-vessels, a few nerves and some connective-tissue ele- 

 ments, but no elastic tissue. This is the thickest of the three membranes and 

 is closely connected with the corium. Next to this, is a fibrous membrane 

 composed of fusiform, nucleated fibres arranged transversely. These re- 

 semble non-striated muscular fibres. The internal membrane is structureless 

 and corresponds to the amorphous layer of the true skin. The papilla at the 

 bottom of the hair-sac varies in size with the size of the hairs and is con- 

 nected with the fibrous layers of the walls of the follicle. It is composed of 

 amorphous matter, with a few granules and nuclei, and it probably contains 

 blood-vessels and nerves, although these are not very distinct. 



Although the different membranes of the hair-follicles are sufficiently rec- 

 ognizable, it is evident that the hair-sac is nothing more than an inversion of 

 the corium, with certain modifications in the character and arrangement 

 of its anatomical elements. The fibrous membranes correspond to the deeper 

 layers of the true skin, without the elastic elements ; and they present a pecul- 

 iar arrangement of its inelastic fibres, the external fibres being longitudinal 

 and the internal fibres transverse. The structureless membrane corresponds 

 to the upper layers of the true, skin, which are composed chiefly of amorphous 

 matter. The hair-papilla corresponds to the papillae on the general surface 

 of the corium. 



The investment of the root of the hair presents two distinct layers 

 called the external and internal root-sheaths. The external root-sheath is 

 three or four times as thick as the inner membrane, and it corresponds ex- 

 actly with the Malpighian layer of the epidermis. This sheath is continu- 

 ous with the bulb of the hair. The internal root-sheath is a transparent 



