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. These, however, are matters which may be
suitably considered in the Principles of Diseases, so far as natural
philosophy has anything to say to them.
It is the brain again-or, in animals that have no brain, the part
analogous to it-which is the cause of sleep. For either by chilling
the blood that streams upwards after food, or by some other similar
influences, it produces heaviness in the region in which it lies
(which is the reason why drowsy persons hang the head), and causes the
heat to escape downwards in company with the blood. It is the
accumulation of this in excess in the lower region that produces
complete sleep, taking away the power of standing upright from those
animals to whom that posture is natural, and from the rest the power
of holding up the head. 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
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