398 



THE ORGANS 



' 1 



J 



reduced to a single layer of cuboidal cells. In the alveoli themselves 

 the cells are spheroidal or polyhedral, and usually fill the entire al- 

 veolus. These cells, like those lining the duct, are derivatives of 



the outer root sheath. The secretion 

 of the gland — an oily substance called 

 sebum — appears to be the direct prod- 

 uct of disintegration of the alveolar 

 ^^ cells, which can usually be seen in all 

 stages of the process of transformation 

 of the cell into the secretion of the 

 \l 1 5 gland. The most peripheral cells show 

 the least secretory changes, containing 

 a few small fat droplets. The central 

 cells and those in the lumen of the 

 duct show the most marked changes, 

 their protoplasm being almost wholly 

 converted into fat, their nuclei 

 shrunken or disintegrated or lost. In 

 the middle zone are cells showing in- 

 termediary stages in the process. 



Shedding of hair occurs in most mam- 

 malia at regularly recurring periods. In 

 man there is a constant death and replace- 

 ment of hair. In a hair about to be shed, 

 the bulb becomes corniiied and splits up into 

 a number of fibres. The hair next becomes 

 detached from the papilla and from the root 

 sheath and is cast off, the empty root sheaths 

 collapsing and forming a cord of cells between 

 the papilla and lower end of the shedding 

 hair. If the dead hair is to be replaced by 

 a new one, there sooner or later occurs a 

 proliferation of the cells of the outer root 

 sheath in the region of the old papilla. 



Fig. 282. — From Longitudinal 

 Section through Hair and Hair 

 Follicle. Enlarged to 800 diameters. 

 (Schafer.) A, Hair, a, Cortex of 

 hair; b, cuticle of hair. B, Inner 

 sheath, c, Cuticle of root sheath; 

 d, Huxley's layer; e, Henle's layer; 

 /, outer root sheath; g, vitreous 

 membrane; i, connective-tissue fol- 

 licle; m, fat cells. 



From this "hair germ" the new hair is 

 formed in a manner similar to embryonal hair formation, the new hair growing 

 upward, under, or to one side of the dead hair, which it finally replaces. 



As to the manner in which growth of hair takes place, two views are held. 

 According to one of these, the hair, cuticle, and inner root sheath are replenished 

 by proHferation of the epithelial cells surrounding the papiUa. These parts 

 thus grow from below toward the surface. The oldest cells of I he outer root- 

 sheath, on the .other hand, lie against the vitreous membrane, so that growth of 

 this sheath takes place from without inward. According to the second view. 



