THE HAIRS. 



1393 



basement membrane separating corium from cuticle. Greatly attenuated, it is 

 prolonged over the hair-papilla, which, as a special vascularized thickening of the 

 connective tissue of the follicle, carries nutrition to the bulb of the growing hair. 



The outer root-sheath is the continuation of the stratum germinativum alone, 

 the other layers of the epidermis thinning out and disappearing before reaching the 

 neck of the follicle. Its cells present the characteristics of those of the germinating 

 layer, with exceptionally well marked fibrillae. On approaching the level of the 

 papilla, the outer root-sheath, which farther above consists of numerous layers, 

 rapidly diminishes in thickness until, on the sides of the papilla, it is reduced to a 

 single row of low columnar cells. 



The inner root-sheath, which is best developed over the middle third of the 

 hair-root and fades away on reaching the upper third, includes three layers. The 

 outer, known as Henle 1 s layer, consists of a single row of flat polygonal cells, often 

 partially separated by oval spaces. Their nuclei are very indistinct or invisible 



FIG. 1157. 



Theca folliculi 



. Middle layer 



.'j?'^ ^- Glassy membrane 



2 





Transverse section of hair-follicle, showing hair surrounded by internal and external root-sheaths. X 285. 



within the cornified cytoplasm. The middle or Hitxley 1 s layer, also horny in 

 nature, often comprises only one stratum of nucleated cuboidal cells, but in the 

 thicker hairs two or even three rows of irregularly interlocked cells may be present. 

 The third layer, known as the sheath cuticle, resembles the external coat of the hair, 

 against which it lies, in being extremely thin and composed of flat horny plates. 

 The latter, however, are always nucleated and so disposed that they are opposed to 

 the serrations of the thicker hair-cuticle. 



Traced towards the bottom of the follicle, the root-sheaths and the hair, which above are 

 sharply defined from one another, become more and more alike until, in the immediate vicinity 

 of the hair-papilla, they blend into a still imperfectly differentiated mass of cells. The deepest 

 elements of this complex, however, are cuboidal or low columnar and form an uninterrupted 

 tract over the papilla, continuous with the outermost cells of the outer root-sheath. It is from 

 the proliferation of these deepest cells that the formative material, or matrix, is provided 

 to meet the requirements of growth and replacement of the hairs. Without anticipating the 

 account of the detailed changes described in connection with the development of the hair 

 (page 1401), it may be here noted that of the three parts of the hair, the medulla is produced by 



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