HAIRS, FEATHERS, AND SCALES. 5 



(2) a fibrous substance, extending from the bulb to the 

 point of the hair. By soaking the hair in hot sulphuric 

 acid, this fibrous substance resolves itself into an immense 

 number of very long cells, pointed at each end, and 

 squeezed by mutual pressure into various angular forms. 

 " A human hair, of one-tenth of a line in thickness,* has 

 about 250 fibrils in its mere diameter, and about 50,000 

 in its entire calibre : so that these ultimate fibrils are 

 finer than those of almost any other known tissue, from 

 the great elongation and narrowing of their constituent 

 cells as they are drawn out into the shaft of the hair 

 during growth ; and hence the expanded bulb of the hair, 

 where the cells are yet spherical and soft."t (3) Running 

 through the very centre of the fibrous portion may be 

 sometimes discerned a dark slender line, which is a sort of 

 pith (medulla) composed of minute roundish cells, filled 

 with air, and arranged in two or three rows. J 



HOO'fl BRISTLES. 



The bristles of the Hog bear much resemblance to the 

 human hair. On this slide is one (a), which you perceive 



* This is nearly thrice as great as the diameter I have given above, 

 which is the result of several careful admeasurements of different 

 hairs, taken from childhood and adult age. 



t Grant, " Outl. Comp. Anat.," 647. 



J " The cortical (or bark-like) substance has different colour, accord- 

 ing to the colour of the hair ; generally, the colour is diffused through 

 its whole mass ; less frequently, the colour depends on granular pigment 

 scattered through its substance in small masses. The cortical substance 



