HAIR. 267 



This inner layer is not of equal diameter throughout, but 

 tapers gradually from below upwards : it is well defined both 

 internally and externally, except where it comes in contact 

 with the bulb internally, with which its inner edge or surface 

 becomes incorporated ; above it terminates in a thin border 

 at a little distance below the level of the skin. (See Plate 

 XXVIII. Jiff. I.) 



The outer layer, although adherent to the inner at its 

 lower part, does not become incorporated with it : the latter, 

 except at the point indicated, preserves everywhere its inde- 

 pendence and individuality. The two layers, it will thus be 

 seen, might with much convenience have been described, as 

 two distinct sheaths, an inner and an outer : to do so, would 

 be, however, to lose sight in some measure of the similar 

 origin and nature of the two. 



In the fact, however, of the inner sheath exhibiting a 

 fibrous structure, and of its incorporation with the bulb, it 

 would appear to have more structural affinity with the fibrous 

 portion of the stem of the hair itself than with the outer layer 

 or cellular sheath. 



This inner layer might be appropriately termed the 

 " modelling sheath," since it doubtless regulates the form and 

 dimensions of the shaft of the hair, the substance of which 

 when first developed is soft and plastic. Henle describes 

 the inner layer as fenestrated : this structure I have never 

 seen exhibited. 



Shaft of the Hair. 



The stem or shaft of the hair is divisible, as already ob- 

 served, into cortical, medullary, and fibrous portions. 



Cortex. The cortical part of the hair consists of a layer of 

 scales, imbricated upon each other after the manner of tiles 

 upon the roof of a house. (See Plate XXIX.) These scales 

 are be'st seen in the larger hairs of the whiskers and pubis, and 

 in the small downy hairs ; they are smaller than the ordinary 

 cells of epidermis, and are rarely seen to be nucleated. 

 Maceration of the hair in sulphuric acid causes them to fall 



z 3 



