422 



THE SKIN. 



satisfactorily explained : it appears generally to be due to the development of air between 

 the elongated cells composing the hair. 



The rate of growth of hair is about half-an-inch a month. 



Hairs are found on all parts of the skin except the palms of the hands and soles of the feet, the 

 dorsal surface of the third phalanges of the fingers and toes, the glans, and the inner surface of 

 the prepuce. On the scalp they are set in groups, on the rest of the skin for the most part singly. 

 Except those of the eyelashes, which are implanted perpendicularly to the surface, they have 

 usually a slanting direction, which is constant in the same parts. In the negro the hair-fol- 

 licles have been found to be curved, so that the papilla may look in a direction parallel to or 

 even away from the surface of the skin (Stewart). 



With the exception of the bones and teeth, no tissue of the body withstands decay after 

 death so long as the hair, and hence it is often found preserved in sepulchres, when nothing 

 else remains but the skeleton. 



Structure of the hair-follicle. The follicle, which receives near its mouth the 

 opening ducts of one or more sebaceous glands (fig. 481, t), consists of an outer coat 

 continuous with the corium, and an epidermic lining continuous with the cuticle. 



The outer or dermic coat of the follicle is thin but firm, and consists of three 

 layers. The most external (fig. 482, a) is formed of connective tissue in longitudinal 

 bundles, without elastic fibres, but with numerous corpuscles. It is highly vascular, 

 and provided with nerves. It is continuous above with the corium, and deter- 

 mines the form of the follicle. The most internal layer (hyaline layer, Kolliker) 

 (fig. 482, d) is a transparent homogeneous membrane, marked transversely on its 

 inner surface with some raised lines, and not reaching so high as the mouth of the 

 follicle ; it corresponds with the membrana propria or basement membrane of allied 

 structures. Between the two is a layer extending from the bottom of the follicle as 

 high as the entrance of the sebaceous glands, composed of an indistinctly fibrous 

 matrix, which tears transversely, and of transversely disposed connective tissue 

 corpuscles, with oblong nuclei (fig. 482, c). This layer is continuous with the 

 papillary part of the cutis vera, and its blood-vessels are continuous above with 

 those of that layer. 



The epidermic coat of the follicle adheres closely to the root of the hair, and 

 commonly separates, in great part, from the follicle and abides by the hair when the 



Fig. 482. SECTION OF HAIR-FOLLICLE. (Biesiadecki. ) 



1, Dermic coat of follicle ; 2, epidermic coat or root-sheath : 

 a, outer layer of dermic coat, with blood-vessels, b, I, cut across ; 

 c, middle layer ; d, inner or hyaline layer ; e, outer root-sheath ; 

 /, ff, inner root-sheath ; h, cuticle of root-sheath ; i, hair. 



latter is pulled out; hence it is sometimes named the " root- 

 sheath." It consists of an outer, softer, and more opaque 

 stratum (fig. 428, e}, next to the dermic coat of the follicle, 

 and an internal more transparent layer (fig. 482, /, a) 

 next to the hair. The former, named also the outer root- 

 > 2 sheath, and by much the thicker of the two, corresponds 

 with the Malpighian layer of the epidermis in general, 

 and contains soft polygonal cells, including pigment- 

 granules in the coloured races, which at the lower part 

 form a much thinner stratum and pass continuously 

 into those of the hair-knob ; the internal layer or inner 

 root-sheath, probably represents the stratum lucidum of 



the epidermis, but is not continuous with that part of the skin, ceasing abruptly a 

 little below the orifices of the sebaceous ducts. Lining the root-sheath internally 

 is a layer of imbricated, downwardly-projecting scales, the cuticle of the root-sheath 

 (fig. 482, h), which is applied to the cuticle of the hair proper, to whose upwardly 

 directed scales it fits like a mould. Its scales, as well as those of the hair-cuticle, 



