On Sleep And Sleeplessness
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. The tenuity or
narrowness of the veins about the brain itself contributes to its
being kept cool, and to its not readily admitting the evaporation.
This, then, is a sufficient explanation of the cooling which takes
place, despite the fact that the evaporation is exceedingly hot.
A person awakes from sleep when digestion is completed: when the
heat, which had been previously forced together in large quantity
within a small compass from out the surrounding part, has once more
prevailed, and when a separation has been effected between the more
corporeal and the purer blood. The finest and purest blood is that
contained in the head, while the thickest and most turbid is that in
the lower parts. The source of all the blood is, as has been stated
both here and elsewhere, the heart. Now of the chambers in the heart
the central communicates with each of the two others. Each of the
latter again acts as receiver from each, respectively, of the two
vessels, called the 'great' and the 'aorta'. It is in the central
chamber that the [above-mentioned] separation takes place. To go
into these matters in detail would, however, be more properly the
business of a different treatise from the present. Owing to the fact
that the blood formed after the assimilation of food is especially
in need of separation, sleep [then especially] occurs [and lasts]
until the purest part of this blood has been separated off into the
upper parts of the body, and the most turbid into the lower parts.