580 BIRD’S-EYE VIEWS. 
image formed on the retina is conveyed to the brain, and 
transformed into a mental perception, capable of being 
thought about. This is inscrutable; it is here the part of 
wisdom to confess ignorance, and acknowledge bounds that 
. human reason cannot overstep. Nor have we. need here to 
go into the general optical laws applicable to vision; they 
are well known, and moreover relate no more to a bird’s eye 
than to that of any other animal. What we want is to find 
out the meaning of the structural peculiarities by which a 
bird’s eye differs from other eyes. What is the reason and 
the purpose of the three eyelids? of the shape of the ball? 
of the very abundant aqueous humor? of the movable lens? 
of the marsupium? the tapering nerve? What special rela- 
tion do these and other features bear to the sense of sight in 
birds? In other words, why must a bird have just this sort 
of an eye to be able to see perfectly? Some of these ques- 
tions can be satisfactorily answered ; others not. Some we 
have already replied to as they arose in our mind invol- 
untarily during our dissection. Thus the third lid gives a 
subdued light, without excluding light altogether ; and also 
protects the eye, which could not be otherwise protected 
without closure of its outer opaque lids, and loss of sight 
altogether. The very convex and highly refractive cornea 
doubtless has some relation to a bird’s ability to see straight 
ahead, though its eyes look directly sidewise. 
Perhaps no reason has been assigned for the singular 
course and termination of the optic nerve. - These have po 
sibly no special optical relations; the cause may lie simply 
in the relative situations of the brain and eye, which are 
such, that the nerve would have to change its course 
ruptly to pierce directly through the sclerotic. We cannot 
round a corner. Nerves are the railroads of thoug 
train of thought might run off the track if the curves &Y 
grades were not easy. I believe that we find comparatively 
few instances of abrupt angles in the course of nerves. 
