These, however, are matters which have been
separately considered in the treatises on Sensation and on Sleep.
That the brain is a compound of earth and water is shown by what
occurs when it is boiled. For, when so treated, it turns hard and
solid, inasmuch as the water is evaporated by the heat, and leaves the
earthy part behind. Just the same occurs when pulse and other fruits
are boiled. For these also are hardened by the process, because the
water which enters into their composition is driven off and leaves the
earth, which is their main constituent, behind.
Of all animals, man has the largest brain in proportion to his size;
and it is larger in men than in women. This is because the region of
the heart and of the lung is hotter and richer in blood in man than in
any other animal; and in men than in women. This again explains why
man, alone of animals, stands erect. For the heat, overcoming any
opposite inclination, makes growth take its own line of direction,
which is from the centre of the body upwards. It is then as a
counterpoise to his excessive heat that in man's brain there is this
superabundant fluidity and coldness; and it is again owing to this
superabundance that the cranial bone, which some call the Bregma, is
the last to become solidified; so long does evaporation continue to
occur through it under the influence of heat.
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