388 



EXCRETION. 



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 resemble the non-striated muscular fibres. The internal membrane is structure- 

 less 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 connected 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 recognizable, it 

 is evident that the hair-sac is nothing more than an inversion of the corium, with some 

 slight 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 peculiar 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 papillaB 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 exactly with the Malpighian layer of the epider- 

 mis. This sheath is continuous with the bulb of the hair. The internal root -sheath is a 

 transparent membrane, composed of flattened cells, mostly without nuclei. This extends 

 from the bottom of the hair- follicle and covers the lower two-thirds of the root. 



Structure of the Hairs. The different varieties of hairs present certain peculiarities 

 in their anatomy, but all of them are composed of a fibrous structure forming the great- 

 er part of their substance, covered by a thin layer of imbricated cells. In the short, 

 stiff hairs, and in the long, white hairs, there is a distinct medullary substance ; but this 

 is wanting in the downy hairs and is indistinct in many of the long, dark hairs. 



FIG. 109. Human hair from the head of a white 

 cliild ; magnified 870 diameters. (From a photo- 

 graph taken at the United States Array Medical 

 Museum.) This figure shows the imbricated ar- 

 rangement of the epidermis of the hair. 



FIG. 110. Transverse section of a human hair from 

 the beard of a white adult ; magnified 370 diam- 

 eterK. (From a photograph taken at the United 

 States Army Medical Museum.) 



The fibrous substance is composed of hard, elongated, longitudinal fibres, which can- 

 not be isolated without the aid of reagents. They may be separated, however, by macer- 

 ation in warm sulphuric acid, when they present themselves in the form of dark, irregu- 

 lar, spindle-shaped plates, from ^ to ^ of an inch long, and from -^Vo to ^ Vo f 

 an inch wide. These contain pigmentary matter of various shades, occasional cavities 



