Chap. 4i ] f 'different Animals. 391 



cartilaginous edge, which may be made to cover fome 

 part of their eyej and this is greater or lefs in differ- 

 ent animals, as their eyes are more or lefs expofed to 

 dangers in fearching after their food. This membrana 

 niffitans, as it is called, is however not very large in all 

 thefe animals. Cows and horfes have it fo large as to 

 cover one half of the eye like a curtain, and at the 

 fame time it is tranfparent enough to allow abundance 

 of the rays of light to pals through it. Fifties have a 

 cuticle always over their eyes, as they are ever in danger 

 in that inconftant element, the water. In this there- 

 fore we may obfeYve a fort of gradation. 



' All quadrupeds have a feventh mufcle belonging 

 to the eye, called Jufpenforius. It furrounds almoft the 

 'whole optic nerve, and is fixed into the fclerotic coat 

 as the others are, Its ufe is to fuftain the weight of 

 the globe of the eye, and to prevent the optic nerve 

 from being too much ftretched, without obliging the 

 four ft rait mufcles to be in a continual contraction, 

 which would be inconvenient: at the fame time this 

 mufcle may be brought to affift any of the other four, 

 by caufing one particular portion of it to act at a time. 



c The next thing to be remarked is the figure of the 

 pupil, which is different in different animals, but al- 

 ways exactly accommodated to the creature's way of 

 life, as well as to the different fpecies of objects that 

 are viewed. Man has it circular, for obvious reafons : 

 an ox has it oval, with the longeft diameter placed 

 tranfverfely, to take in a larger view of its food : cats, 

 again, have theirs likewife oval, but the longeft dia- 

 meter placed perpendicularly ; they can either exclude 

 a bright light altogether, or admit only as much as is 

 neceflary^ The pupil of different animals varies in 

 \videnefs, according as the internal organs of vifion are 

 more or lefs acute : thus cats and owls, who feek their 

 C c 4 prey 



