MOVEMENT OF ANIMALS, x.-xi. 



animals the same process goes on because of nature, 

 and because each part of them, since they are so 

 constituted, is naturally suited to perform its own 

 function ; so that there is no need of soul in each 

 part, but since it is situated in a central origin of 

 authority over the body,** the other parts live by 

 their structural attachment to it and perform their 

 own functions in the course of nature. 



XI. We have now discussed the manner of the 

 voluntary movements of animals, and the cause of 

 them. Some of their parts, hoAvever, undergo certain 

 involuntary movements, though most of these are 

 really non- voluntary. By involuntary I mean such 

 movements as those of the heart and of the privy 

 member, which are often moved by the presentation 

 of some image and not at the bidding of reason. By 

 non- voluntary I mean sleeping and waking and respira- 

 tion and the like. For neither imagination nor desire 

 is strictly speaking responsible for any of these move- 

 ments ; but, since animals must necessarily undergo 

 physical alteration, and, when their parts undergo 

 alteration, some increase and others decrease, and so 

 their bodies immediately move and undergo the 

 natural sequence of changes (the causes of their 

 movements being the natural heatings and chillings, 

 both external and internal), the movements too of 

 the above-mentioned parts ^ which occur contrary to 

 reason are due to the occurrence of a change. For 

 thought and imagination, as has already been said," 

 induce the states which cause the affections ; for 

 they present the images of the things which cause 

 them. Now these parts act in this way much more 

 conspicuously than any others, because each is as it 

 were a separate vital organism[, the reason being that 



477 



