Further, if the
moisture be watery it dries up quickly and the hairs do not gain in
size, but if it be greasy the opposite happens, for the greasy is
not easily dried up. Therefore the thicker-skinned animals are as a
general rule thicker-haired for the causes mentioned; however, the
thickest-skinned are not more so than other thick-skinned ones, as
is shown by the class of swine compared to that of oxen and to the
elephant and many others. And for the same reason also the hairs of
the head in man are thickest, for this part of his skin is thickest
and lies over most moisture and besides is very porous.
The cause of the hairs being long or short depends on the
evaporating moisture not being easily dried. Of this there are two
causes, quantity and quality; if the liquid is much it does not dry up
easily nor if it is greasy. And for this reason the hairs of the
head are longest in man, for the brain, being fluid and cold, supplies
great abundance of moisture.
The hairs become straight or curly on account of the vapour
arising in them. If it be smoke-like, it is hot and dry and so makes
the hair curly, for it is twisted as being carried with a double
motion, the earthy part tending downwards and the hot upwards. Thus,
being easily bent, it is twisted owing to its weakness, and this is
what is meant by curliness in hair. It is possible then that this is
the cause, but it is also possible that, owing to its having but
little moisture and much earthy matter in it, it is dried by the
surrounding air and so coiled up together.
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