394 



HISTOLOGY 



Cortex. 



Medulla. 



layer which covers the papilla (Fig. 399) . An inner stratum of the hyaline 

 membrane is formed, according to Stohr, from the epithelial cells of the 

 root sheath. This inner stratum is provided with fine pores, and is 

 always clear and homogeneous. It may unite with the connective tissue 

 stratum so that both may appear as a single membrane. The connective 

 tissue sheath is found fully developed only around the lower half of the root. 

 The outer epithelial sheath is an inpocketing of the epidermis. The 

 stratum corneum extends to the sebaceous gland; the stratum granulosum 

 continues somewhat deeper, but only a thinned stratum germinativum 

 can be followed to the bulb. All of these are included in the outer epi- 

 thelial sheath (Figs. 401-405, I, II, and 5). 



The inner epithelial sheath extends from the sebaceous gland to the bulb. 

 It begins as a layer of cornified cells below the termination of the stratum 



granulosum, but it is not a continuation of 

 that layer. Toward the bulb the inner 

 sheath is divisible into two layers. The 

 outer or Henle's layer consists of one or two 

 rows of cells with occasional atrophic nuclei; 

 for the most part they are non-nucleated. 

 The inner or Huxley's layer is a row of 

 nucleated cells. The inner surface of Hux- 

 ley's layer is covered by a membrane, the 

 cuticula of the sheath, composed of non- 

 nucleated cornified scales. Traced down- 

 ward, the elements of the inner epithelial 

 sheath and its cuticula all become nucleated 

 cells, but the layers may be distinguished 

 almost to the neck of the papilla. There 

 they lose their sharp boundaries, but may 

 still be distinguished from the pigmented 

 cells of the bulb. Traced upward, it is 



found that kerato-hyalin granules appear in Henle's layer at the level of the 

 papilla, and in Huxley's layer somewhat higher (Fig. 399) ; still higher these 

 granules disappear and the cells of the inner sheath become cornified. 



The shaft of the hair is entirely epithelial; it consists of cuticula, 

 cortex and medulla (Fig. 400). The cuticula, which covers its surface, is a 

 thin layer formed of transparent scales directed from the center of the shaft 

 outward and upward, thus overlapping like inverted shingles. This 

 arrangement is readily seen in wool and the hairs of various mammals, but 

 is much less evident in human hair. The cuticula is composed of non- 

 nucleated cornified cells. 



The greater portion of the shaft is included in the cortex. Toward the 

 bulb, the cortex consists of soft round cells; distally these cells become corni- 



Cuticula. 



FIG. 400. PART OF A WHITE HUMAN 

 HAIR. X 240. 



