268 



NORMAL HISTOLOGY. 



FIG. 304. 



within the integument, the root ; at its lower extremity the root ter- 

 minates in a bulbous expansion, the hair-bulb, which at its lowest 

 point is indented to receive the connective-tissue papilla. The hair- 

 bulb lies embraced within a pocket of modified integument, the 

 hair-follicle, to which the corium and the epidermis contribute 

 respectively the fibrous and the epitKelial root-sheaths. 



The hair consists entirely of ^epithelial cells disposed as three dis- 

 tinctly defined strata, the cuticle, the cortical substance, and the 

 medulla, or pith. The hair cuticle is composed of a single layer 

 of thin, horny, imbricated scales, which envelop the entire sur- 

 face of the hair, both on the root and on the shaft ; in these situations 

 it forms a layer respectively 6-8 /JL and 2-4 p. in thickness. Owing 

 to the imbricated arrangement of the cells, as tiles upon a roof, only 

 their free projecting borders are visible, which produce in surface views 



the characteristic oblique transverse 

 markings so distinctive of hair; in pro- 

 file the edges of the cells appear as deli- 

 cate serrations. 



The cortical substance constitutes by 

 far the greater part of the hair, when the 

 medulla is wanting sometimes forming its 

 entire bulk. This portion of the hair- 

 shaft is composed of greatly elongated 

 horny epithelial cells, which possess 

 attenuated nuclei and are so intimately 

 united that the boundaries of the individ- 

 ual elements under ordinary circumstances 

 are not distinguishable. On the root the 

 cells are broader, less horny, and assume 

 more and more the character of the ele- 

 ments of the stratum mucosum as the prox- 

 imal end of the hair-bulb is approached ; 

 ^^^ immediately around the papilla the 



cells of the cortical substance become con- 

 tinuous with the extension of the stratum mucosum, the outer root- 

 sheath. 



The medulla, or pith, occupies the central tract of the hair-shaft, 

 and extends in favorable examples from near the hair-bulb almost to 

 the extremity of the hair. Many hairs possess no pith, this part 

 being usually wanting in the fine hairs of the general body-surface 

 and the colored hairs of the head, as well as in the hairs of chil- 

 dren under four or five years of age. In the thick short and in the 

 robust long hairs, likewise in most white scalp-hairs, the medulla is 

 present, and constitutes sometimes one-third of the diameter of the hair. 



mi 



A , human hair ; the upper half 

 of the figure represents the super- 

 ficial horny cells (A) constituting the 

 cuticle, the lower half, the fibrous 

 structure of the cortical substance 

 and the medulla ; B, isolated ele- 

 ments of the hair ; a, cuticular 

 scales ; b, thin fibre-cells of cortical 

 substance. 



