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 (comprime) 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 



* Araer. Ornith., iv., 56, 1st edit. 



