50 ORGANS OF THE SENSES. 



prehension of the food, or the dividing of it into such 

 portions as can pass the gullet (which is generally 

 capable of considerable distension) into the stomach; 

 and there is no mastication, or chewing by the mouth, 

 of birds, and no apparatus for the performing of such 

 an office. 



The organs of smell, sight, and hearing, are placed 

 in the head ; the nostrils at the base of the bill, in 

 the substance of it, or even near its extremity, accord- 

 ing to the habit ; and they are variously defended by 

 feathers, hairs, scales, and valvular membranes. 



The eyes of birds are placed in the sides of the 

 head, generally speaking better adapted for seeing 

 under them or laterally than above them or directly 

 in front. But the position varies with the habit : birds 

 which prey only in clear light and under them have 

 the eyes far apart, and shaded ; while those which 

 prey in the twilight have them turned more to the 

 front. The eye is better fortified than in the mam- 

 malia ; the anterior part of the ball has a circle of 

 bone ; and there is a nictitating membrane, or third 

 eyelid, which, when not used, is protected from the 

 action of the weather by a lodgment in the inner 

 canthus of the eye. It is thus more soft and moist 

 than an exposed eyelid, and it works over the whole 

 eye in a direction crosswise to the usual opening. 

 The cornea of the eye is very convex, and beauti- 

 fully clear ; but the crystalline lens is rather flat. 

 The eyes of birds are in many instances much more 

 exposed to the action of the weather than the eyes of 

 most other vertebrated animals, and they appear to be 

 the organs of sense upon which birds have their chief 

 dependence ; and, true to the general law of nature, 

 that the supply is always proportionate to the want, they 

 are furnished and fortified in an extraordinary manner. 



