HAIRS, FEATHERS, AND SCALES 21 



are transparent and apparently empty; but their walls 

 appear opaque and almost black an optical illusion, de- 

 pendent on the absorption of the light by their surfaces 

 at certain angles with the eye of the beholder. The fibrous 

 portion is here almost displaced by the great development 

 of the medullary cells. 



In the larger hairs of the Mole, which we will now look 

 at, the bark is very thin; and though the surface is marked 

 with sinuous lines, these do not project into teeth. The 

 pith here again forms the greater portion of the hair, the 

 cells of which it is composed being placed in single series, 

 which, for the most part, extend all across the body of the 

 hair, though they are somewhat irregular both in size and 

 shape. They are rather flattened, and appear perfectly 

 black (that is, opaque) by transmitted light, their surfaces 

 absorbing all the rays of light. The small hairs of the 

 same animal, however, are very different in form: they are 

 flattened, so as to appear twice as broad in one aspect as 

 in another at right angles to it; and, what is curious, the 

 scales of the bark project into strongly-marked imbrica- 

 tions on one side, and are scarcely perceptible on the 

 other. Here, as in the larger hairs, there is a single row 

 of oval transverse cells, perfectly opaque. 



The hair of many of the smaller Mammalia shows con- 

 siderable diversity of form, according to the part which we 

 select for observation. Thus, if we take a long hair out 

 of this Sable tippet, and examine it near the base, we see 

 that it is very slender, transparent, and colorless, covered 

 with strongly-marked imbrications, which are not obtuse 

 teeth, but long, pointed, overlapping scales, about ten of 

 which complete a whorl. The fibrous portion is moder- 

 ately thick; enclosing a wide pith of roundish cells, set in 



