8 EYE OF BIRDS. 



The sight of birds is almost invariably remarkable for its development and its adapta- 

 tion for near or distant objects. The swallow, for example, when darting through the air 

 with that swiftness which has become proverbial, is capable of accommodating its sight to 

 the insect which it pursues even in the short space of time which is occupied by its swoop 

 at its victim. The same phenomenon may be noticed in the falcon, which is able to per- 

 ceive a little bird or animal on the ground, and though sweeping downwards with such 

 wonderful rapidity that it looks merely like a dark streak in the air, is able to calculate 

 its distance so exactly, that it just avoids dashing itself to pieces on the ground, and 

 snatches up its prey with the same lightning speed which characterizes its descent. 



It is very probable that a curious structure, named from its shape the "pecten," or 

 comb, which is found in the interior of the bird's eye, may contribute to this peculiarity of 

 vision. This comb is of a fan-like shape, and is situate upon the spot where the optic 

 nerve enters the eye, projecting obliquely upwards, and evidently playing some very con- 

 spicuous part in the economy of the eye. The teeth, or folds of which this fan or comb is 

 composed, are black in color and very variable in number, being only six or seven in the 

 owls, and twenty or thirty in the sparrow. There is a plentiful supply of blood-vessels in 

 the comb, but no muscular tissues, and it is supposed by several anatomists that its expan- 

 sion or contraction, caused by the greater or less amount of blood which fills the vessels, 

 may have some effect in the peculiarly delicate adjustment of the eye which has already 

 been mentioned. 



From the contapt of external substances, as well as for the purpose of excluding unneces- 

 sary light, the eye of the bird is furnished with two ordinary eyelids, and a third, or supple- 

 mentary eyelid, which plays within the others, and is technically called the nictitating mem- 

 brane. This membrane is elastic, and by its own contractility is kept within the angle of the 

 eye as long as its services are not needed. When, however, the bird wishes to cleanse its eyes 

 from dust or other annoyances, it draws the membrane rapidly over the eye, letting it return 

 to its place by its own powers of contraction. The eye of the bird is further remarkable for a 

 series of bony plates which surround the eye, and are supposed to have a great influence in 

 increasing or lessening the convexity of the eyeball. The number of these plates is nearly as 

 various as the teeth of the comb, but upon an average their number is thirteen or fourteen. 

 There are many other curious and interesting details in the anatomy and general structure of 

 the birds, but as this publication is not intended as a work on comparative anatomy, we must 

 proceed to the histories of the birds themselves. 



