108 PHYSIOLOGY 



Animals which are either subject to be pursued, or which fight with 

 their hind feet, have their eyes placed on the side of the head, and pro- 

 jecting, so as to throw the eye hackwards. A hare, rahbit, many 

 squirrels, &c. are of the first kind ; the horse, deer, &c. are of the 

 second. 



Of the Choroid Coat. 



That part of the choroid coat of the eyes of animals that is covered 

 with a pigmentum album, is much thicker and stronger than any other 

 part of the same coat. 



Of the Motion of the Iris. 



The eyes of animals have the motion of the iris increased in pro- 

 portion to the size of the eye ; but those that see in the dark have 

 a greater motion of the iris than those that do not, in proportion 

 to the size of the eye ; and, perhaps, the oblong shape of the pupil is 

 to allow a greater opening when it is brought to a round form than 

 possibly could have been done if it had been always round. 



It would appear from the disease of the eye called ' gutta serena,' 

 that most probably the dilatation of the pupil was owing to elasticity ; 

 for although the iris cannot be stimulated by the retina being affected 

 by light, so as to contract by muscular action, yet we can hardly sup- 

 pose that the dilators [if there are such] can or will act so constantly 

 as we find by the pupil being kept dilated for so long a time without 

 tiring. (D. Anderson's case, p. .) [Where ? asks Wm. Clift.] 



I conceive the iris is endowed with a sphinctoric property, or power 

 of contraction. 



It is very common for the Angola cats to have the iris of the two 

 eyes of different colours. 



Of the Flatness of the Bottom of Eyes. 



The flatness of the bottom of the eyes of some animals is, perhaps, 

 to have distinct vision on more parts than one, not as in the human 

 subject ; for the flatness keeps a greater surface of the eye at an equal 

 distance from the crystalline lens, therefore a greater [surface] at the 

 focal point of distance ; so that they can see lateral objects nearly as 

 perfectly as direct ones ; and we may observe that they do see lateral 

 objects sooner or quicker than we do, and can throw the eye in dif- 

 ferent directions on the same object. 



Of the Size of Eyes in different Animals. 



There is a great difference in the size of eyes in proportion to the 

 size of body in birds, and such difference is also observable in other 



