EYE OF BIEDS. 9 



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 

 perceive 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 characterises its 

 descent. 



It is very probable that a curious structure, named fl-om 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 conspicuous part in the economy of the eye. The teeth, or folds of which this fan 

 or comb is composed, are black in colour and very variable in number, being only six or 

 seven in the owls, and twenty or thirty in the sparrow. Tliere is a plentiful supply of 

 blood-vessels in the comb, but no muscular tissues, and it is supposed by several anato- 

 mists that its expansion 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. 



Erom the contact of external substances, as well as for the purpose of excluding 

 unnecessary light, the eye of the bird is furnished with two ordinary eyelids, and a third, 

 or supplementary eyelid, which plays within the others, and is technically called the 

 nictitating membrane. 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. 



