sanguineous animals, the natural substance blood, or, in bloodless

animals, that which is analogous to this; and since the veins are

the place of the blood, while the origin of these is the heart-an

assertion which is proved by anatomy-it is manifest that, when the

external nutriment enters the parts fitted for its reception, the

evaporation arising from it enters into the veins, and there,

undergoing a change, is converted into blood, and makes its way to

their source [the heart]. We have treated of all this when

discussing the subject of nutrition, but must here recapitulate what

was there said, in order that we may obtain a scientific view of the

beginnings of the process, and come to know what exactly happens to

the primary organ of sense-perception to account for the occurrence of

waking and sleep. For sleep, as has been shown, is not any given

impotence of the perceptive faculty; for unconsciousness, a certain

form of asphyxia, and swooning, all produce such impotence. Moreover

it is an established fact that some persons in a profound trance

have still had the imaginative faculty in play. This last point,

indeed, gives rise to a difficulty; for if it is conceivable that

one who had swooned should in this state fall asleep, the phantasm

also which then presented itself to his mind might be regarded as a

dream. Persons, too, who have fallen into a deep trance, and have come

to be regarded as dead, say many things while in this condition. The

same view, however, is to be taken of all these cases, [i.e. that they

are not cases of sleeping or dreaming].

As we observed above, sleep is not co-extensive with any and every

impotence of the perceptive faculty, but this affection is one which

arises from the evaporation attendant upon the process of nutrition.

The matter evaporated must be driven onwards to a certain point,

then turn back, and change its current to and fro, like a tide-race in

a narrow strait. Now, in every animal the hot naturally tends to

move [and carry other things] upwards, but when it has reached the

parts above [becoming cool], it turns back again, and moves