For when the nutriment steams upwards
through the blood-vessels, its refuse portion is chilled by the
influence of this region, and forms defluxions of phlegm and serum. We
must suppose, to compare small things with great, that the like
happens here as occurs in the production of showers. For when vapour
steams up from the earth and is carried by the heat into the upper
regions, so soon as it reaches the cold air that is above the earth,
it condenses again into water owing to the refrigeration, and falls
back to the earth as rain. 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.
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