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On The Motion Of Animals 
ordered, and one thing follows another in its accustomed order. So
in animals there is the same orderliness- nature taking the place of
custom- and each part naturally doing his own work as nature has
composed them. There is no need then of a soul in each part, but she
resides in a kind of central governing place of the body, and the
remaining parts live by continuity of natural structure, and play
the parts Nature would have them play.
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So much then for the voluntary movements of animal bodies, and the
reasons for them. These bodies, however, display in certain members
involuntary movements too, but most often non-voluntary movements.
By involuntary I mean motions of the heart and of the privy member;
for often upon an image arising and without express mandate of the
reason these parts are moved. By non-voluntary I mean sleep and waking
and respiration, and other similar organic movements. For neither
imagination nor desire is properly mistress of any of these; but since
the animal body must undergo natural changes of quality, and when
the parts are so altered some must increase and other decrease, the
body must straightway be moved and change with the changes that nature
makes dependent upon one another. Now the causes of the movements
are natural changes of temperature, both those coming from outside the
body, and those taking place within it. So the involuntary movements
which occur in spite of reason in the aforesaid parts occur when a
change of quality supervenes. For conception and imagination, as we
said above, produce the conditions necessary to affections, since they
bring to bear the images or forms which tend to create these states.
And the two parts aforesaid display this motion more conspicuously
than the rest, because each is in a sense a separate vital organism,
the reason being that each contains vital moisture. In the case of the
heart the cause is plain, for the heart is the seat of the senses,
while an indication that the generative organ too is vital is that
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