GENERATION OF ANIMALS, V. i. 



small amount of fluid, because on the one hand it is 

 not so small in volume that it gets disturbed and so 

 hampers the movement produced by the colours, nor 

 on the other hand is it so large in volume that its 

 movement is rendered difficult. 



These are not the only causes of dullness and keen- 

 ness of sight. In addition to them we must mention 

 the nature of the skin upon what is known as the 

 pupil. This skin should be transparent, a condition 

 which must of necessity be satisfied by skin that is 

 thin, and white, and even — thin, in order that the 

 movement that comes from without may take a 

 straight course ; even, so that its \\'rinkles shall not 

 produce a shadow (the reason why old people do not 

 have keen ^ision is that the skin in the eyes, like that 

 elsewhere, gets wrinkled and thicker in old age) ; 

 white, because that which is black is not transparent, 

 non-transparency being precisely what blackness is ; 

 and that too is why lanterns cannot give any hght if 

 they are made of black skin. 



In old age and disease, then, these are the causes 

 OMing to which the sight is not keen ; in children, 

 however, it is the small volume of fluid which makes 

 the eyes appear blue to begin with.*' And odd- 

 coloured eyes occur more often in human beings and 

 horses than other animals for the same cause that 

 human beings are the only animals that go grey and 

 the horse is the only one of the remainder whose hairs 

 noticeably M'hiten in old age : — Greyness is a weak- 

 ness, viz., a lack of concoction, of the fluid in the 

 brain ; so is blueness of the eves ; since unduly thin ^ 

 fluid and unduly thick fluid are the equivalent "^ re- 

 spectively of a small amount and a large amount of 



* For exft Bwaiuv, cf. 733 b 15 784 b 14, and Introd. § 26. 



501 



