state is for an animal its highest end, since the exercise of

sense-perception or of thought is the highest end for all beings to

which either of these appertains; inasmuch as these are best, and

the highest end is what is best: whence it follows that sleep

belongs of necessity to each animal. I use the term 'necessity' in its

conditional sense, meaning that if an animal is to exist and have

its own proper nature, it must have certain endowments; and, if

these are to belong to it, certain others likewise must belong to it

[as their condition.]

The next question to be discussed is that of the kind of movement or

action, taking place within their bodies, from which the affection

of waking or sleeping arises in animals. Now, we must assume that

the causes of this affection in all other animals are identical

with, or analogous to, those which operate in sanguineous animals; and

that the causes operating in sanguineous animals generally are

identical with those operating in man. Hence we must consider the

entire subject in the light of these instances [afforded by

sanguineous animals, especially man]. Now, it has been definitely

settled already in another work that sense-perception in animals

originates ill the same part of the organism in which movement

originates. This locus of origination is one of three determinate

loci, viz. that which lies midway between the head and the abdomen.

This is sanguineous animals is the region of the heart; for all

sanguineous animals have a heart; and from this it is that both motion

and the controlling sense-perception originate. Now, as regards

movement, it is obvious that that of breathing and of the cooling

process generally takes its rise there; and it is with a view to the

conservation of the [due amount of] heat in this part that nature

has formed as she has both the animals which respire, and those

which cool themselves by moisture. Of this [cooling process] per se we

shall treat hereafter. In bloodless animals, and insects, and such

as do not respire, the 'connatural spirit' is seen alternately