THE SKIN 



111 



root-sheath by a basement-membrane termed the 

 follicle. This inner vascular layer corresponds to 

 of the cutis vera. Its fibres and cells have a 

 regular circular arrangement around the follicle, 

 the cells being flattened against the hyaline layer. 

 Externally the dermic coat of the follicle has 

 a more open texture, corresponding to the reti- 

 cular part of the cutis, and containing the larger 

 branches of the arteries and veins. In the large 

 tactile hairs of animals, the veins near the bottom 

 of the follicle are dilated into sinuses, so as to pro- 

 duce a kind of erectile structure. 



The hair grows from the bottom of the follicle 

 by multiplication of the soft cells which cover the 

 papilla, these cells becoming elongated to form the 

 fibres of the fibrous substance, and otherwise modi- 

 fied to produce the medulla and cuticle. 



When a hair is eradicated, a new hair is pro- 

 duced from these cells. It is not uncommon to find 

 hair-follicles in which the whole of the lower part 

 has degenerated in such a way that the vascular 

 papilla, and the soft, growing cells which cover it, 

 may have entirely disappeared. The hair then 

 ceases to grow, and eventually becomes lost, but 

 its place may be again supplied by a new hair, 

 which becomes formed in a downgrowth from 

 either the bottom or the side of the hair-follicle, a 

 new papilla first becoming formed at the extremity 

 of the downgrowth (fig. 135). If not previously 

 detached, the old hair 'may be pushed from out the 

 follicle by the one which replaces it. 



The hairs are originally developed in the embryo 

 in the form of small solid downgrowths from the 

 Malpighian layer of the epidermis (fig. 136, A). 

 The hair-rudiment, as it is called", is at first com- 

 posed entirely of soft, growing cells ; but presently 

 those in the centre become differentiated, so as to 

 produce a minute hair invested by inner root- 

 sheath, and its base resting upon a papilla which 

 has grown up into the extremity of the hair-rudi- 

 ment from the corium (fig. 136, B). As the 

 minute hair grows, it pushes its way through the 

 superficial layers of the epidermis, which it finally 

 perforates (C). The hair-rudiments commence at 

 the third or fourth month of fetal life ; their 

 growth is completed about the fifth or sixth 

 month, and they form a complete hairy covering 



hyaline layer of the 

 the superficial layer 



FIG. 135. COMMEN- 

 CING REPLACEMENT 

 OF OLD BY NEW 

 HAIR. (Toldt.) 



a, outer root-sheath ; b, 

 dermic coat of follicle ; 

 /, downgrowth of epi- 

 thelium to form new 

 hair-follicle ; p, papilla 

 of new hair commen- 

 cing ;.;', root of old hair ; 

 t, duct of sebaceous 

 gland. 



