HAIR. 269 



separated from each other and loose will frequently be de- 

 tected. (See Plate XXIX.) 



The whole of the fibres of the stem, however, do not ex- 

 tend its entire length, the majority terminating long before 

 the extremity is attained ; and it is to this fact that the at- 

 tenuated form of the hair is attributable. In some hairs the 

 fibres are seen to terminate at regular distances, their points 

 describing transverse lines on the stem of the hair. 



The splitting, of such common occurrence in hairs which 

 are allowed to attain an excessive length, is due to a separa- 

 tion of the fibres from each other. 



The fibrous portion in young and healthy hairs is coloured. 



Medullary Canal and Medulla. - In most hairs which are 

 not of too deep a colour a medullary canal may be detected 

 running up the centre. This commences in the upper por- 

 tion of the bulb, runs along the shaft, but ceases as it ap- 

 proaches the apex : its diameter varies with that of the 

 hair itself, but usually bears the proportion of a third of its 

 thickness. (See Plates XXVIII. and XXIX.) 



Henle is uncertain whether this canal is lined by a distinct 

 membrane or not, but inclines to the opinion that it is so. 



The medulla or contents of this canal exhibit a granular 

 appearance, and are made up of pigment granules, a few 

 perfect pigmentary cells, and particles of coloured oil : it is 

 therefore in the medulla that the greatest amount of colouring 

 matter of the hair is situated. (See Plate XXVIII. fig.\.) 



At the very commencement in the bulb of the hair the 

 medulla has distinctly a cellular origin. 



In young and healthy hairs it is also continuous through- 

 out the entire extent of the medullary canal ; in old and dis- 

 coloured hairs, on the contrary, its continuity is frequently 

 interrupted by distinct intervals, and it only partially fills the 

 cavity of the canal even where it is present. 



The medullary canal and medulla is best seen in the larger 

 hairs, which are not of too deep a colour, and in grey hairs : 

 in the fine and downy hairs of the general surface of the 

 body, the canal and its contents, as a necessary consequence, 

 are almost entirely absent. 



Z 4 



