I78 REPTILES AND BIRDS. 



contracting it, birds obtain additional powers of vision. Other parts 

 of the eye, such as the choroids, the thin membrane which covers 

 the posterior part of the eye, the iris, the retina, present nothing re- 

 remarkable. The white of the eye is surrounded by an osseous or 

 cartilaginous matter, evidently placed there for protection of this 

 delicate and useful organ. 



Besides the ordinary upper and lower pupils, Birds possess a 

 third. This consists of an extensive transparent membrane, dis- 

 posed vertically, which covers the eye like a piece of network, 

 protecting it from the effects of a blaze of light. It is this pupil, 

 or nictating membrane, placed at the internal angle of the eye, 

 between the orb and the external pupil, which the animal uses at 

 will, which permits the eagle to gaze at the sun, and prevents the 

 nocturnal birds of prey from being dazzled when exposed to day- 

 light. 



The perfection of 'the sight of Birds seems to be proved from 

 the Vulture, so distant from his prey as to appear a mere speck 

 in the heavens, without deviation flying directly to it ; or the 

 Swallow, while on rapid wing, perceiving the smallest insect on 

 which it feeds. According to Spallanzani, the Swift has sight so 

 piercing, that it can see an object only five lines in diameter at the 

 distance of 500 feet. 



Birds, of all animal creation, can traverse distances with the 

 greatest rapidity. The fleetest among the Mammifera cannot ex- 

 ceed seven or eight leagues in an hour. Certain Birds easily traverse 

 their twenty leagues in the same interval of time. In less than three 

 minutes we lose sight of a large bird, such as a kite or an eagle, 

 whose extent from wing to wing is almost two yards. It is assumed, 

 from these facts, that these birds traverse more than 1,500 yards each 

 minute, or more than fifty miles in an hour. A falcon of Henri II. 

 strayed from Fontainebleau in pursuit of a bustard ; it was taken 

 the next day at Malta. Another falcon, sent from the Canaries to 

 the Duke de Lermes, in Spain, returned from Andalusia to the Peak 

 of Teneriffe in six hours ,the flight representing a distance of 250 

 leagues. In short, the whole organisation gives to a bird that 

 remarkable lightness which contributes so much to its velecity. Not 

 to speak of the feathers with which it is covered, its bones are hollow 

 and form large cells, called aerial sacs, which it is able to fill with 

 air at will, and its sternum is furnished with a bony frame or breast- 

 bone, formed somewhat like the keel of a ship, into which the pec- 

 toral muscles are inserted — which, besides being largely developed 

 in birds of flight, possess remarkable contractile properties. 



