HAIK 



1031 



the surface of the latter a fluted appearance. The hair of the ornitho- 

 rhynchus is a very curious object ; for whilst the lower part of it 

 resembles the fine hair of the mouse or squirrel, this thins away and 

 then dilates again into a very thick fibre, having a central portion 

 composed of polygonal cells, inclosed in 

 a flattened sheath of a brown fibrous 

 substance. 



The structure of the human hair is 

 in certain respects peculiar. When its 

 outer surface is examined, it is seen to 

 be traversed by irregular lines (fig 1 . 764, 

 A), which are most strongly marked in 

 foetal hairs ; and these are the indications 

 of the imbricated arrangement of the 

 flattened cells or scales which form the cuticular layer. This 

 layer, as is shown by transverse sections (C, D), is a very thin 

 and transparent cylinder ; and it incloses the peculiar fibrous sub- 

 stance that constitutes the principal part of the shaft of the hair. 

 The constituent fibres of the substance, which are marked out by 

 the delicate striae that may be traced in longitudinal sections of the 

 hair (B), may be separated from each other by crushing the hair, 

 especially after it has been macerated for some time in sulphuric 

 acid ; and each of them, when completely isolated from its fellows, 

 is found to be a long spindle-shaped cell. In the axis of this fibrous 

 cylinder there is very commonly a band which is formed of spheroidal 



FIG. 763. Transverse section 

 of hair of peccary. 



FIG. 764. Structure of human hair : A, external surface of the shaft, show- 

 ing the transverse striae and jagged boundary caused by the imbrications of 

 the cuticular layer; B, longitudinal section of the shaft, showing the 

 fibrous character of the cortical substance, and the arrangement of the 

 pigmentary matter; C, transverse section, showing the distinction be- 

 tween the cuticular envelope, the cylinder of cortical substance, and the 

 medullary centre ; D, another transverse section, showing deficiency of 

 the central cellular substance. 



cells ; but this ' medullary ' substance is usually deficient in the fine 

 hairs scattered over the general surface of the body, and is not 

 always present in those of the head. The hue of the hair is due 

 partly to the presence of pigmentary granules, either collected into 

 patches or diffused through its substance, but partly also to the 

 existence of a multitude of minute air-spaces, which cause it to 



