No animal has so much hair on the head as man. This, in the first
place, is the necessary result of the fluid character of his brain,
and of the presence of so many sutures in his skull. For wherever
there is the most fluid and the most heat, there also must necessarily
occur the greatest outgrowth. But, secondly, the thickness of the hair
in this part has a final cause, being intended to protect the head, by
preserving it from excess of either heat or cold. And as the brain
of man is larger and more fluid than that of any other animal, it
requires a proportionately greater amount of protection. For the
more fluid a substance is, the more readily does it get excessively
heated or excessively chilled, while substances of an opposite
character are less liable to such injurious affections.
These, however, are matters which by their close connexion with
eyelashes have led us to digress from our real topic, namely the cause
to which these lashes owe their existence. We must therefore defer any
further remarks we may have to make on these matters till the proper
occasion arises and then return to their consideration.
15
Both eyebrows and eyelashes exist for the protection of the eyes;
the former that they may shelter them, like the eaves of a house, from
any fluids that trickle down from the head; the latter to act like the
palisades which are sometimes placed in front of enclosures, and
keep out any objects which might otherwise get in.
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