For, as has been observed elsewhere, sleep comes on when the
corporeal element [in the 'evaporation'] conveyed upwards by the
hot, along the veins, to the head. But when that which has been thus
carried up can no longer ascend, but is too great in quantity [to do
so], it forces the hot back again and flows downwards. Hence it is
that men sink down [as they do in sleep] when the heat which tends
to keep them erect (man alone, among animals, being naturally erect)
is withdrawn; and this, when it befalls them, causes
unconsciousness, and afterwards phantasy.
Or are the solutions thus proposed barely conceivable accounts of
the refrigeration which takes place, while, as a matter of fact, the
region of the brain is, as stated elsewhere, the main determinant of
the matter? For the brain, or in creatures without a brain that
which corresponds to it, is of all parts of the body the coolest.
Therefore, as moisture turned into vapour by the sun's heat is, when
it has ascended to the upper regions, cooled by the coldness of the
latter, and becoming condensed, is carried downwards, and turned
into water once more; just so the excrementitious evaporation, when
carried up by the heat to the region of the brain, is condensed into a
'phlegm' (which explains why catarrhs are seen to proceed from the
head); while that evaporation which is nutrient and not unwholesome,
becoming condensed, descends and cools the hot.
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