GENERATION OF ANIMALS, II. vi. 



as they are there at all, and they grow more during 

 diseases, and when old age advances, and when the 

 body is wasting. This is because old age and diseases 

 mean that less (nourishment) is expended on the 

 supreme parts of the body and therefore more residue 

 is left over ; though when even this begins to fail 

 through age, the hair follows suit. With the bones, 

 the reverse occurs : thev waste away along with the 

 bodv and its parts. Hair actuallv continues to grow 

 after life is extinct, though it vriW not begin growing 

 where it does not already exist. 



Teeth may present a puzzle. They possess the Teeth. 

 same nature as the bones and are formed out of the 

 bones ; nails, hair, horns and the Uke, however, are 

 formed out of the skin, and that is why they change 

 their colour along -with the skin : they turn white and 

 black and all shades according to the colour of the 

 skin. The teeth do none of this, because they are 

 formed out of the bones (this applies of course only 

 to such animals as have both teeth and bones). They 

 are unique among bones in that they continue grow- 

 ing all through life, as is clear in the case of teeth 

 which take an oblique direction and fail to conie 

 into contact with each other." The reason for their 

 growth, the purposeyor the sake of which they grow, is 

 to discharge their special function : they would soon 

 be worn down unless the loss were made good in some 

 way,* since even as it is, in some aged animals which 

 eat a great deal but have small teeth, they are quite 

 worn away, because their growth is not proportionate 

 to their loss. And so here too Nature has produced 



* L. & S. translate " unless there were some means of 

 saving them " ; but Scot translates si non rrescerent con- 

 sumerentur cito nisi esset materia ex qua crescunt. 



235 



