238 FACULTIES OF BIRDS. 
“the conformation of the eye forms a curious and 
interesting subject to the young anatomist. The 
globe of the eye is immoveably fixed in its socket 
by a strong, elastic, hard cartilaginous case, in 
form of a truncated cone: this case, being closely 
covered with a skin, appears at first to be of one 
continued piece, but on removing the exterior mem- 
brane it is found to be formed of fifteen pieces, 
placed like the staves of a cask, overlapping a little 
at the base or narrow end, and seem as if capable 
of being enlarged or contracted, perhaps by the 
muscular membrane in which they are encased.” 
** In five other different species of owls,” adds Wil- 
son, ‘‘ which I have since examined, I found nearly 
the same conformation of this organ, and exactly 
the same number of staves. The eye being thus 
fixed, these birds, as they view different objects, 
are always obliged to turn the head, and nature has 
so excellently adapted their neck to this purpose, 
that they can with ease turn it round, without mo- 
ving the body, in almost a complete circle.”* 
In nocturnal birds, M. de Blainville remarks, that 
the eye, besides being comparatively very large, is 
flat (comprimé) both before and behind, while the 
transparent cornea is placed at the end of a sort of 
tube formed by the bony portion of the sclerotic. 
The retina is, consequently, comparatively very 
large and extended, and the iris also, while the mem- 
branes, being probably more soft and delicate, are 
more susceptible of impressions from a small quan- 
tity of light. The nictitating membrane is also very 
large, and the upper eyelid, unlike other birds, is 
moveable. 
It may be worth mentioning, that animals born 
with perfect eyes can use them the instant they 
enter the world. Sir James Hall, when making 
experiments on hatching, observed a chicken in the 
* Amer. Ornith., iv., 56, lst edit, 
