APPENDICES OF THE SKIN. 91 
and that the colour changes with’age, and, in some animals, 
with the season, without any apparent change in the horny 
covering. ‘These arguments we consider as conclusive in 
favour of the opiion which supposes the colour to de- 
pend chiefly on the fluids of the contained vessels, and 
would Jead us to infer, that the structure of the central 
parts of slender hairs is different from that of the stronger 
kinds, termed Spines. Indeed it has been ascertained, 
that the pith of the bristles of the wild boar, form two ca- 
nals; and, in the whiskers of the seal, one canal may be 
distinctly perceived. 
In general, there is a close connection between the colour 
of the hair, and that of the mucous web. This is displayed 
in those animals which are spotted ; in which the colour of 
the skin is generally variegated like that of the hair. 
Hairs differ remarkably in form. In general they are 
round. Frequently on the body, they are thickest in the 
middle. Sometimes they are flat, or two-edged ; and, in 
the whiskers of seals, they are waved on the margins. In 
many animals, they are long and straight ; while, im others, 
they are crisped, and are then termed Wool. When stiff, 
they are termed Bristles; and, when inflexible, Spines. 
They obtain particular names, according to this situation, as 
beard, eye-lashes, and whiskers. In general, the motions 
of the hair depend on the movements of the muscular web. 
In some cases, however, small muscular fibres may be traced 
to the bulb of the root, by means of which, particular 
movements may be executed. 
Hairs grow by the roots. In some species, they are re- 
newed annually ; and, in all, they are readily reproduced. 
They are the most truly electrical of animal substances. 
They are probably nourished by capillary attraction, and 
the supply regulated by the moisture or dryness of the at- 
mosphere. 
