VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 559 



the subsistence or safety of an animal entrusted to, or depending more particularly 

 on one sense than the rest, we are sure to fiml that sense proportionably perfect ; 

 as in quadrupeds the organ of smelling is remarkably perfect, and leads them to 

 their prey; so the eyes of birds are proportionably perfect, being the means not 

 only of their support, but from them they receive the first intimation of ap- 

 proaching danger. 



The eyes of birds like those of other animals, consist of 3 coats, the sclero- 

 tica, choroides, and retina. The human eye, as well as those of quadrupeds, is 

 nearly spherical ; in birds the sphere is more oblate, the sclerotica as it approaches 

 the cornea becoming suddenly flat. The cornea, though small when compared 

 with the size of the whole eye, is more convex as it forms the segment of a 

 smaller circle, added to the larger, formed by the sclerotica. The reason or ad- 

 vantage of this flatness is not very evident. It prevents them perhaps from pro- 

 jecting so far as to expose them to danger from the trees and grass, among which 

 these animals live. 



As no description, however accurate, can give an idea of the structure of any 

 part of the animal body, I have caused small sketches to be made explaining all 

 the different circumstances mentioned in the paper. 



After having examined the eyes of birds, and seeing this curious apparatus, I 

 was next led to the examination of the eyes of quadrupeds, that I might see in 

 what manner they resembled the eyes of birds, and if I could account for their 

 being able to accommodate their eyes to objects at different distances. 



This was a subject involved in much difficulty, as the eyes of quadrupeds ap- 

 peared on examination not to have these imbricated scales, which are so obvious 

 in birds; but all this difficulty vanished on taking hold of one of the 4 recti 

 muscles of the eye of a sheep; and by tearing and dissecting, I found that it 

 terminated in, and with the other parts composed the cornea; so that on the first 

 volition of the mind the recti muscles on contracting will have the power of 

 fixing the eye and keeping it steady, and at the same time by contracting more 

 or less, will adapt the focus of the eye to the distance of the object, but in a less 

 degree than in birds. On these muscles giving over acting, the eye will be 

 restored to its former stale by the elasticity of the sclerotic coat. 



From a knowledge of these circumstances, we may from rational principles 

 explain, why people by being long accustomed to view small objects, obtain in 

 time a sort of microscopic power, if it may be so called; that is, the muscles 

 which contract the cornea will by custom increase their power of action, and 

 grow stronger, like the other muscles of the body. Other phenomena of vision 

 on these principles may be explained. 



Fig. 1, pi. 7, represents the eye of a buzzard, blown up and dried, the lesser circle of the cornea 

 suddenly rising above the sclerotic coats. 



