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.