PARTS OF ANIMALS, II. vii. 



The brain is present in order to preserve the animal 

 organism as a whole. Some " maintain that the Soul 

 of an animal is Fire or some such substance. This is 

 a crude way of putting it ; and might be improved 

 upon by saying that the Soul subsists in some body 

 of a fiery nature. The reason for this is that the 

 hot substance is the most serviceable of all for the 

 activities of the Soul, since one of the activities of 

 the Soul is to nourish ; another is to cause motion ; 

 and these are most readily effected by means of 

 this substance (\iz. the hot). So to say that the 

 Soul is fire is like saying that the craftsman, or his 

 craft, is the saw or the auger which he uses, on 

 the ground that the activity is performed while the 

 two are near together. From what we have said this 

 at any rate is clear : animals must of necessity have in 

 them a certain amount of heat. Now, everything needs 

 something to counterbalance it, so that it may achieve 

 moderation and the mean ; for it is the mean, and 

 not either of the extremes apart, which has re- 

 ality and rationality.^ For this cause nature has 

 contrived the brain to counterbalance the region of 

 the heart and the heat in it ; and that is Avhy animals 

 have a brain, the composition of which is a combina- 

 tion of Water and Earth. Hence, although all 

 blooded animals have a brain, practically none of 

 the others has (unless it be just a counterpart, as 

 in the case of the Octopus), for since they lack blood 

 they have but little heat. 



The brain, then, makes the heat and the boiling in 

 the heart well blent and tempered ; yet in order that 

 the brain may still have a moderate heat, blood- 

 vessels run from the great Blood-vessel and what is 

 known as the Aorta, till they reach the membrane 



151' 



