Structure of the Eyes of Birds. 267 
as it forms the segment of a smaller circle, added to the larger 
formed by the sclerotica. The reason or advantage of this 
flatness is not very evident. It prevents them, perhaps, from 
projecting so far as to expose them to danger from the trees 
and grass, amongst which these animals live. 
As no description, however accurate, can give an exact idea 
of the structure of any part of the animal body, I have caused 
small sketches to be made, explaining all the different circum- 
stances that I have mentioned in this 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 I found involved in much difficulty, as 
the eyes of quadrupeds appeared 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 four 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 state 
by the elasticity of the sclerotic coat. 
From a knowledge of these circumstances, we may from 
