426 



THE SKIN. 



ment may be carried to the cells of the hair-bulb through their agency (Riehl). The 

 young hair continuing to grow, at last perforates the cuticle, either directly, or after 

 first slanting up for some way between the mucous and horny strata : it is often 

 bent like a whip, and then the doubled part protrudes. 



The first hairs prqduced constitute the lanugo ; their eruption takes place about 

 the fifth month of intra-uterine life, but some of them are shed before birth, and are 

 found floating in the liquor amnii. 



According to Kolliker's observations, the infantile hairs are entirely shed and 

 renewed within a few months after birth ; those of the general surface first, and 

 afterwards the hairs of the eyelashes and head, which he finds in process of change 

 in infants about a year old. A formation of new hairs, preceded or accompanied by 

 a shedding of the old ones, also goes on during the whole of life. The papilla of the 



Fig. 488. LONGITUDINAL SECTION THROUGH THE FOLLICLE OF A HAIR WHICH HAS CEASED GROWING. 



(Ranvier. ) 



m, epithelium at the bottom of the follicle (which contains no papilla) ; b, modified hair-bulb ; c, 

 neck of the hair-follicle somewhat opened in preparing the section ; s, sebaceous gland ; o, epithelial bud 

 at the level of the insertion of the arrector pili. 



old hair atrophies and the hair-bulb disappears (fig. 488). The t hair-root acquires a 

 closer attachment to the sides of the follicle within which it gradually becomes shifted 

 towards the surface of the skin, but all active growth ceases, and finally the hair 

 drops out of the follicle. The new hairs are generated in the follicles of the old 

 (fig. 487). An increased growth of cells takes place at one part, usually the bottom 

 of the follicle, and a new papilla is produced within the mass of cells thus formed. 

 These cells occupying the lower part of the follicle, and resting on the papilla, are 

 gradually converted into a new hair with its root-sheath, just as in the primitive 

 process of formation in the embryo. 



The sebaceous glands (figs. 481, 489) are small saccular glands which pour 

 out their secretion at the roots of the hairs, for, with very few exceptions (labia 



