SIGHT AND, SWIFTNESS OF BIRDS. 193 



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 noc- 

 turnal birds of prey from being dazzled when, exposed to daylight. 



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, which perceives, while on rapid wing, the smallest 

 insect on which it feeds. According to Spallanzani, the Swift has 

 sight so piercing, that it can see only five lines in diameter at the 

 distance of five hundred feet. 



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

 greatest rapidity. The fleetest among the Mammifera cannot 

 run over five or six 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 body is more than a yard from wing to 

 wing. It is assumed, from these facts, that these birds traverse more 

 than fifteen hundred 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 two hundred and fifty 

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

 remarkable lightness which contributes so much to its velocity. 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 pectoral muscles are inserted which, besides being 

 largely developed, in birds of flight possess remarkable contractile 

 properties. 



The vocal apparatus in birds, represented in Figs. 70 and 71, 



