HAIR. 



[ 309 ] 



HAIR. 



ruptly with a jagged margin. Externally it 

 is connected with the outer root-sheath, in- 

 ternally with the outer layer of the cuticle 

 of the hair ; hence no interval exists natu- 

 rally between it and the hair. At first sight 

 it appears as a perfectly homogeneous mem- 

 brane, but on closer examination it is seen 

 to be distinctly cellular; it consists of two 

 or three layers of polygonal, longish, trans- 

 parent cells, with their long axis parallel to 

 that of the hair. The outermost (Henle's) 

 layer (figs. 301 /, 305 A] consists of long, 



Fig. 305. 



Magnified 350 diameters. 



Elements of the inner root-sheath. A, external layer : 

 1, isolated plates ; 2, the same in connexion, showing the 

 interspaces (a) between the cells (b). B, cells of the inner 

 non-perforated layer. C, nucleated cells of the lower 

 part of the inner sheath, which consists of a single layer 

 only. 



flattened, non-nucleated cells, from l-/00to 

 1-500'' in length, with fissures between them, 

 forming a fenestrated layer. The innermost 

 (Huxley's) layer (figs. 301 e, 305 B) consists 



of one or two layers of shorter and broader 

 polygonal cells, from 1-1200 to 1-600" in 

 length ; their nuclei, which exist in the lower 

 part only of the coat, are often broader at 

 the ends than in the middle, sometimes 

 curved and pointed. At the base of the 

 hair-follicle, the inner root-sheath consists 

 of a single layer only of beautiful, polygonal, 

 nucleated cells (fig. 305 c) ; these becoming 

 soft, delicate and rounded, gradually pass 

 into the outer layers of the round cells of 

 the bulb. 



In regard to development, the rudiments 

 of the hair appear as processes of the rete 

 mucosum descending into the substance of 

 the cutis. These are solid and consist of 

 cells, the internal of which become horny 

 and form first a small slender hair in the 

 axis of the process, next an inner sheath 

 surrounding the former, whilst the outer 

 cells remain soft, and form the outer sheath 

 and the cells of the bulb. 



After birth the foetal hair appears to be 

 completely shed, new hairs being formed in 

 the old follicles, which displace the first set, 

 as shown in figs. 306, 307. 



Fig. 306. 

 A. 



Fig. 307. 



Magnified 20 diameters. 



Eye-lashes of a child a year old. A exhibits a process 

 (TO) of the bulb or outer root-sheath, in which the central 

 cells are elongated, and form a cone distinct from the 

 outer cells. B, one more advanced, in which the inner 

 cone has become developed into a hair and an inner root- 

 sheath : a, outer, b, inner root-sheath of the young hair ; 

 c, pit for the pulp ; d, bulb ; e, shaft of the old hair ; 

 f, bulb, g, shaft, A, summit of the young hair ; f, sebaceous 

 follicles ; k, three sudoriparous ducts opening into the 

 upper part of the hair-follicle. 



