VEKTEBRATED ANIMALS. 



265 



FlG - 452 - 



cal substance forms a tube, which we see crossed at intervals by partitions 



that are sometimes complete,, sometimes only partial; these are the walls 



of the single or double line of cells, of which the medullary substance is 



made-up. The hairs of the Bat tribe are commonly distinguished by the 



projections on their surface, which are formed by extensions of the com- 



ponent scales of the cortical substance: these are particularly well seen in 



the hairs of one of the Indian species, which has a set of whorls of long 



narrow leaflets (so to speak) arranged at regular intervals on its stem (c). 



In the hair of thePecn'(Fig. 452), the cortical 



envelope sends inwards a set of radial prolonga- 



tions, the interspaces of which are occupied by 



the polygonal cells of the medullary substance; 



and this, on a larger scale, is the structure of 



the 'quills' of the Porcupine; the radiating 



partitions of which, when seen through the 



more transparent parts of the cortical sheath, 



ffive to the surface of the latter a fluted appear- 



ance. The hair of the OrnitkorkynchuSiu 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 dilales again into a very thick fibre, having a central portion com- 



posed of polygonal cells, inclosed in a flattened sheath of a brown fibrous 



substance. 



662. The structure of the human Hair is in certain respects peculiar. 



***"<*** 



FIG. 453. 



Structure of Human Hair: A, external surface of the shaft, showing 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 pig- 

 mentary matter; c, transverse section, showing the distinction between the cuticular envelope, the 

 cylinder of cortical substance, and the medullary centre; D, another transverse section, showing 

 deficiency of the central cellular substance. 



When its outer surface i ; examined, it is seen to be traversed by irregular 

 lines (Fig. 453, A), which are most strongly marked in fcetal 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 substance that constitutes the principal part 

 of the shaft of the hair. The constituent fibres of this 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 



