GENERATION OF ANIMALS, V. iv.-v. 



during a period of infirmity just as the wJiole body 

 is afflicted by a deficiency of natural heat," so the 

 parts, including even the verj' small ones, share in 

 .this infirmity ; also, a great deal of residue is formed 

 in the body and in its parts : hence the lack of con- 

 coction in the flesh produces grey hairs. But when 

 health and strength is restored, people accomplish a 

 change, as it might be old men renewing their youth, 

 and, in consequence, the conditions also accomphsh 

 a corresponding change. In fact, we might justifi- 

 ably go so far as to describe disease as " adventitious 

 old age " and old age as " natural disease " ; at any 

 rate, some diseases produce the same effects as old 

 age does. 



The temples are the first part to go grey, and the 

 reason is this. The back of the head, since it contains 

 no brain,^ is empty of fluid. The bregma '" contains a 

 great deal ; but a large volume of fluid does not 

 easil}' putrefH". On the other hand, the hair on the 

 temples has neither a small enough amount of fluid 

 to secure concoction for it, nor a large enough amount 

 for it to avoid putrefaction, as this region of the head 

 is intermediate between the two extremes, and 

 therefore stands outside both of these two conditions. 



We have now given the reason for greyness so far 

 as man is concerned. 



The reason why this change does not noticeably V 

 occur on account of age in the other animals is the 

 same as the one already given in the case of baldness : 

 their brain is small and (less) fluid,** thus the heat 

 does not become completely unable to effect concoc- 



' See 744 a 25, n. 



'' The insertion of " less " is necessary to the sense : man's 

 brain is the most fluid of all (see 784 a 4). 



5^1 



