8 EYE OF BIRDS. 



The sight of birds is almost invariably remarkable for its develoi:)ment and its adapta- 

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

 with that swiftness wldch 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 thougii 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 u]) its prey with the same lightning speed which characterizes its descent. 



It is very jn-olialile that a curious structure, named from its shape the "i^ecten," or 

 comb, which is found in tlie interior of the bird's eye, may contribute to this i)eculiarity of 

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

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

 spicuous j)art in the economy of the eye. The teeth, or folds of ^vhich 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 musculai- 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 whicli has already 

 been mentioned. 



Prom the contact 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 j)lays 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 aiv not needed. When, however, the biid mshes to cleanse its eyes 

 from dust or othei' 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 furthei' 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 cuiious and interesting details in the anatomy and general structure of 

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

 proceed to the histories of the birds themselves. 



