HAIRS 



197 



cells coating the papilla. These coalesced and cornified 

 cells being continually replaced by new growths from 

 below, which undergo the same metamorphosis, the shaft 

 of the hair is thrust out until it attains the full length 

 natural to it. Its base then ceases to grow, and the old 

 papilla and sac die away, but not before a new sac and 

 papilla have been formed by budding from the sides of 

 the old one. These give rise to a new hair. The shaft 

 of a hair of the head consists of a central pith, or 

 medullary matter, of a loose and open texture, which 

 sometimes contains air ; of a cortical substance sui'- 



b <" ^^^ 





A 



Fig. 61. — Part of the Sn ' 'N''~lo5ed withik its Root- 



Sheaths AND TREATED WITH CaUSTIC SoDA, WHICH HAS CAUSED THE 



Shaft to become distorted. 



a, medulla ; 6, cortical substance ; c, cuticle of the shaft ; from dtof. 

 the root-sheaths, in section. (Magnified about 200 diameters.) 



rounding this, made up of coalesced elongated homy cells ; 

 and of an outer cuticle composed of fiat horny plates, 

 arranged transversely round the shaft, so as to overlap 

 one another by their outer edges, like tiles on the roof of 

 a house. The superficial epidermic cells of the hair-sac 

 also coalesce by their edges, and become converted into 

 root-sheaths, which embrace the root of the hair, and 

 usually come away with it when it is plucked out. 



The sebaceous glands are small glands whose duct opens 

 into the follicle of a hair. They form a fatty secretion 

 which lubricates the hairs. 



