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digestion。 Moreover； some kinds of illness have this same effect；
those arising from moist and hot secretions； as happens with
fever…patients and in cases of lethargy。 Extreme youth also has this
effect； infants； for example； sleep a great deal； because of the
food being all borne upwards…a mark whereof appears in the
disproportionately large size of the upper parts compared with the
lower during infancy； which is due to the fact that growth
predominates in the direction of the former。 Hence also they are
subject to epileptic seizures； for sleep is like epilepsy； and； in a
sense； actually is a seizure of this sort。 Accordingly； the
beginning of this malady takes place with many during sleep； and their
subsequent habitual seizures occur in sleep； not in waking hours。
For when the spirit ＇evaporation＇ moves upwards in a volume； on its
return downwards it distends the veins； and forcibly compresses the
passage through which respiration is effected。 This explains why wines
are not good for infants or for wet nurses （for it makes no
difference； doubtless； whether the infants themselves； or their
nurses； drink them）； but such persons should drink them ＇if at all＇
diluted with water and in small quantity。 For wine is spirituous；
and of all wines the dark more so than any other。 The upper parts；
in infants； are so filled with nutriment that within five months
＇after birth＇ they do not even turn the neck ＇sc。 to raise the
head＇； for in them； as in persons deeply intoxicated； there is ever
a large quantity of moisture ascending。 It is reasonable； too； to
think that this affection is the cause of the embryo's remaining at
rest in the womb at first。 Also； as a general rule； persons whose
veins are inconspicuous； as well as those who are dwarf…like； or
have abnormally large heads； are addicted to sleep。 For in the
former the veins are narrow； so that it is not easy for the moisture
to flow down through them； while in the case of dwarfs and those whose
heads are abnormally large； the impetus of the evaporation upwards
is excessive。 Those ＇on the contrary＇ whose veins are large are；
thanks to the easy flow through the veins； not addicted to sleep；
unless； indeed； they labour under some other affection which
counteracts ＇this easy flow＇。 Nor are the 'atrabilious' addicted to
sleep； for in them the inward region is cooled so that the quantity of
evaporation in their case is not great。 For this reason they have
large appetites； though spare and lean； for their bodily condition
is as if they derived no benefit from what they eat。 The dark bile；
too； being itself naturally cold； cools also the nutrient tract； and
the other parts wheresoever such secretion is potentially present
＇i。e。 tends to be formed＇。
Hence it is plain from what has been said that sleep is a sort of
concentration； or natural recoil； of the hot matter inwards ＇towards
its centre＇； due to the cause above mentioned。 Hence restless movement
is a marked feature in the case of a person when drowsy。 But where
it ＇the heat in the upper and outer parts＇ begins to fail； he grows
cool； and owing to this cooling process his eye…lids droop。
Accordingly ＇in sleep＇ the upper and outward parts are cool； but the
inward and lower； i。e。 the parts at the feet and in the interior of
the body； are hot。
Yet one might found a difficulty on the facts that sleep is most
oppressive in its onset after meals； and that wine； and other such
things； though they possess heating properties； are productive of
sleep； for it is not probable that sleep should be a process of
cooling while the things that cause sleeping are themselves hot。 Is
the explanation of this； then； to be found in the fact that； as the
stomach when empty is hot； while replenishment cools it by the
movement it occasions； so the passages and tracts in the head are
cooled as the 'evaporation' ascends thither？ Or； as those who have hot
water poured on them feel a sudden shiver of cold； just so in the case
before us； may it be that； when the hot substance ascends； the cold
rallying to meet it cools ＇the aforesaid parts＇ deprives their
native heat of all its power； and compels it to retire？ Moreover； when
much food is taken； which ＇i。e。 the nutrient evaporation from which＇
the hot substance carries upwards； this latter； like a fire when fresh
logs are laid upon it； is itself cooled； until the food has been
digested。
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。 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