INTEGUMENT 



367 



shaft and slides against the cells of the epithelium which lines the hair 

 tube. These latter form the several layers of the outer hair sheath which 

 is thus a modified form of stratified epithelium continuous with that of 

 the surface. At about two thirds of the distance to the surface the inner 

 sheath degenerates, and the hair is surrounded directly by the epithelial 

 layers of the outer sheath. 



This epithelium, at two places, about a third to a half of the distance 

 from the surface to the bulb, is thickened by the growth of its basal layer. 

 The upper of these is the developing sebaceous gland which is described 



m. 



^^^^^ 



FIG. 331. Section of a hair root, just under the skin, m., medulla; i.ep.s., inner epithelial 

 sheath, Henle's layer and Huxley's layer; o.ep.s., outer epithelial sheath; h.m., hyaline 

 membrane; /./., fibrous layer. 



under lubrication. The lower is a center for the renewal of the hair, 

 which falls out after its term of usefulness is over (Fig. 331). 



The layers of the outer hair sheath are interesting. The upper is a 

 continuation of the outer epidermis. The stratum corneum extends 

 down as far as the sebaceous gland, where its place is taken by the inner 

 sheath. The stratum granulosum extends farther, and only the basal 

 layer can be traced continuously to the papillae. 



In the birds an epithelial structure is produced, somewhat similar to 

 hair, and called the feather. Like the hair, the feather begins in the 

 embryo as a thickening of the very thin epithelium which covers the 

 integument (Fig. 332). The same layers are found here as were seen 

 where the hair developed in the mammal, with the small differences of 

 detail, that in the hair anlage a thicker layer of epithelium is to be 



