202 VERTEBRATES. 



reversed in birds. Beasts grind their food with their teeth, and 

 then it passes into the stomach, where it is softened and digested. 

 On the contrary, birds of this sort first macerate and soften it in 

 the crop, and then it is ground and comminuted in the stomach 

 or gizzard. Birds are also careful to pick up sand, gravel, and ■ 

 other hard substances, not to grind their food, as has been sup- 

 posed, but to prevent the too violent action of the coats of the 

 stomach against each other. 



This structure, however, is not fully carried out in all birds. 

 It exists in its most complete state in those which are granivorous, 

 or which live upon fruit, seed, etc. ; but in the carnivorous birds, 

 or those which feed upon flesh or fish, the dilatation, constituting 

 the crop, is very small or altogether wanting ; and the gizzard is 

 a thin and weak organ, hardly to be distinguished from the second 

 or membranous stomach. This is a difi"erence corresponding to 

 that which has been described as existing in the Mammalia; 

 among which those feeding upon vegetable food are provided with 

 powerful and extensive organs of digestion, whilst in those livins; 

 upon animal food they are comparatively weak, and limited in 

 extent. 



The sight of birds is very perfect. They possess the power 

 of seeing objects distinctly, when very remote. Birds of prey are 

 particularly remarkable for the very great distance at which they 

 perceive their prey, and the accuracy with which they direct their 

 flight towards it. Besides the upper and under eyelids, birds 

 have a third, which is semi-transparent, and serves the purpose ol 

 protecting the eye from the contact of external bcdies, or from too 

 powerful light, whilst, at the same time, it does not prevent them 

 from distinguishing the objects around them. This membrane is 

 "situated at the inner angle of the eye, and is drawn over the globe 

 of it, like a curtain, at will. It is by means of this protection, 

 that the eagle is enabled to look steadily at the sun. 



The senses of hearing and smelling are also possessed in 

 considerable perfection by birds ; the former more particularly by 

 the nocturnal, and the latter by those feeding principally upon 

 carrion, the scent of which they are thus able to trace to an im- 



