102 CLASS AVES. 



The bird, using the anterior extremities for flight, and not for 

 locomotion or prehension, is, hke man, a biped. This posture 

 elevates the head, and gives it a different air from that of 

 quadrupeds. 



The femur is always shorter than the tibia ; the peroneum is 

 very slight, and never descends so far as the tibia ; the single 

 bone which represents the tarsus and the metatarsus, varies 

 considerably in length, and on this depends the height of the 

 bird on its legs. The toes have been sufficiently noticed. 



The beak, already described, varies greatly in length and 

 form, and, with the web, or interdigital membrane, will be 

 found to form the groundwork of the most prevailing principles 

 of artificial separation. 



Sight is extremely perfect in birds, and they have the pecu- 

 liar faculty of seeing objects near or distant equally well. The 

 means by which this is effected are not satisfactorily explained, 

 though a power of changing the convexity of the eye is probably 

 the proximate cause. Like all other physical peculiarities, it 

 is admirably adapted to the mode of existence of the class ; a 

 quick and perfect sight of objects and perception of distances 

 is necessary to the rapidity of movements and the securing of 

 their prey to birds. All the genera, except the owls, see a 

 single object but with one eye. The situation of these organs, 

 however, enables them to take in a much larger field of view 

 than animals whose eyes look straight before them. 



Not to dwell with minuteness on some peculiarities which 

 distinguish the eyes of birds, we shall pass to an additional 

 word or two on the third eyelid, or nictitating membrane : this 

 is folded in the angle of the eye next the nose, and is brought 

 over the organ like a curtain, in a vertical direction, and not 

 horizontally, or up and down, like the ordinary eyelids. This 

 membrane is partially transparent, and one of its purposes seems 

 to be, to prevent the access of too much light into the eye, 

 when the bird is exposed to that inconvenience. With a few 

 exceptions, the upper eyelid of birds is fixed, the lower one 

 only moving. 



