322 



HISTOLOGY. 



The shaft of the hair is entirely epithelial. Its surface is covered by 

 a thin cutkula which is formed of transparent scales directed from the cen- 

 ter of the shaft outward and upward, and overlapping like shingles. These 

 are non-nucleated cornified cells. The greater portion of the shaft is 

 included in the cortex. Toward the bulb the cortex consists of soft cells, 

 but distally they become cornified, elongated and compact; their nuclei 

 are then linear. Except in white hairs pigment occurs both between and 

 in these cells. Very small intercellular air spaces are found in the cortex 

 of fully developed hairs. The medulla when present, occupies the center 



Cortical substance. - 



Medullary substance. 



Cuticle. _ 



FIG. 373. ELEMENTS OF A HUMAN HAIR AND ITS SHEATH. X 240. 



I, White hair; 2, scales of the cuticle; 3, cells of the cortical substance of the shaft; 4, cells of Huxley's 

 layer; 5, cells of Henle's layer, having the appearance of a fenestrated membrane; 6, cells of the 

 cortical substance of the root. 



of the shaft. It is generally a double row of cells containing kerato- 

 hyalin granules and degenerate nuclei. A medulla is found only in 

 large hairs and it terminates before reaching their tips. 



The shedding of hairs. Shortly before and after birth there is a gen- 

 eral shedding of hair. In the adult the loss and renewal of hairs is not 

 periodic but constant. The life of a hair in the scalp may last 1600 days. 

 The process of removal begins with a thickening of the hyaline membrane 

 and circular fiber sheath. The matrix ceases to produce the inner sheath 

 and consequently the cuticula and hair. The bulbus becomes cornified, 

 forming a solid frayed end of the shaft as seen in Figs. 375 and 376.. The 



