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On The Generation Of Animals   

We have now spoken pretty much of all the other conditions of hair.


But as to their colour, it is the nature of the skin that is the
cause of this in other animals and also of their being uni-coloured or
vari-coloured); but in man it is not the cause, except of the hair
going grey through disease (not through old age), for in what is
called leprosy the hairs become white; on the contrary, if the hairs
are white the whiteness does not invade the skin. The reason is that
the hairs grow out of skin; if, then, the skin is diseased and white
the hair becomes diseased with it, and the disease of hair is
greyness. But the greyness of hair which is due to age results from
weakness and deficiency of heat. For as the body declines in vigour we
tend to cold at every time of life, and especially in old age, this
age being cold and dry. We must remember that the nutriment coming
to each part of the body is concocted by the heat appropriate to the
part; if the heat is inadequate the part loses its efficiency, and
destruction or disease results. (We shall speak more in detail of
causes in the treatise on growth and nutrition.) Whenever, then,
the hair in man has naturally little heat and too much moisture enters
it, its own proper heat is unable to concoct the moisture and so it is
decayed by the heat in the environing air. All decay is caused by
heat, not the innate heat but external heat, as has been stated
elsewhere. And as there is a decay of water, of earth, and all such
material bodies, so there is also of the earthy vapour, for instance
what is called mould (for mould is a decay of earthy vapour). Thus
also the liquid nutriment in the hair decays because it is not
concocted, and what is called greyness results. It is white because
mould also, practically alone among decayed things, is white. The
reason of this is that it has much air in it, all earthy vapour
being equivalent to thick air. For mould is, as it were, the
antithesis of hoar-frost; if the ascending vapour be frozen it becomes
hoar-frost, if it be decayed, mould. Hence both are on the surface
of things, for vapour is superficial. And so the comic poets make a
good metaphor in jest when they call grey hairs 'mould of old age' and
For the one is generically the same as greyness, the other
specifically; hoar-frost generically (for both are a vapour),
mould specifically (for both are a form of decay). A proof that this
is so is this: grey hairs have often grown on men in consequence of
disease, and later on dark hairs instead of them after restoration
to health. The reason is that in sickness the whole body is
deficient in natural heat and so the parts besides, even the very
small ones, participate in this weakness; and again, much residual
matter is formed in the body and all its parts in illness, wherefore
the incapacity in the flesh to concoct the nutriment causes the grey
hairs. But when men have recovered health and strength again they
change, becoming as it were young again instead of old; in consequence
the states change also. Indeed, we may rightly call disease an
acquired old age, old age a natural disease; at any rate, some
diseases produce the same effects as old age.

Men go grey on the temples first, because the back of the head is
empty of moisture owing to its containing no brain, and the 'bregma'
has a great deal of moisture, a large quantity not being liable to
decay; the hair on the temples however has neither so little that it
can concoct it nor so much that it cannot decay, for this region of
the head being between the two extremes is exempt from both states.
The cause of greyness in man has now been stated.


The reason why this change does not take place visibly on account of

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