1738 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- 
rematkable. 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 eact. 
minute, or more than fifty miles in an hour. A falcon of Henn 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 aéréa/ 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. 
