The cause of greyness in man has now been stated.
5
The reason why this change does not take place visibly on account of
age in other animals is the same as that already given in the case
of baldness; their brain is small and less fluid than in man, so
that the heat required for concoction does not altogether fail.
Among them it is most clear in horses of all animals that we know,
because the bone about the brain is thinner in them than in others
in proportion to their size. A sign of this is that a blow to this
spot is fatal to them, wherefore Homer also has said: 'where the first
hairs grow on the skull of horses, and a wound is most fatal.' As then
the moisture easily flows to these hairs because of the thinness of
the bone, whilst the heat fails on account of age, they go grey. The
reddish hairs go grey sooner than the black, redness also being a sort
of weakness of hair and all weak things ageing sooner. It is said,
however, that cranes become darker as they grow old. The reason of
this would be, if it should prove true, that their feathers are
naturally moister than others and as they grow old the moisture in the
feathers is too much to decay easily.
Greyness comes about by some sort of decay, and is not, as some
think, a withering. (1) A proof of the former statement is the fact
that hair protected by hats or other coverings goes grey sooner
(for the winds prevent decay and the protection keeps off the winds),
and the fact that it is aided by anointing with a mixture of oil and
water.
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