56 STRUCTURE OP BIRDS. 



perforated by many small openings, permits the air to enter 

 into them, and likewise to pass into several cavities situated 

 in the chest and belly. It even extends into the interior of 

 the bones, and by thus pervading various parts of the body, 

 not only exercises very extensively its peculiar influence on 

 the blood, but also renders the whole body lighter and better 

 adapted for flight. 



The organs of digestion also are somewhat varied. As 

 birds cannot chew their food, a provision is made to supply 

 this defect by means of the structure of their internal organs. 

 The food is carried first into the crop, which appears to be 

 merely an enlargement of the oasophagus or gullet, at the 

 bottom of the neck, where it is softened by a liquor poured 

 out from the internal surface of this cavity. It is then carried 

 into a membranous sack, called the ventriculus succcnturiatus , 

 where it is further macerated and soaked ; and from thence 

 into the gizzard, which is composed of two very strong and 

 firm muscles, united by radiated tendons, and lined on its 

 inside by a rough cartilaginous membrane. In this organ 

 the food is powerfully acted upon, and is triturated and ground 

 up into a substance resembling that prepared by the teeth and 

 stomach of the Mammalia. 



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

 It exists in its most complete state in those which are gra- 

 nivorous, or which live upon fruit, seed, &,c. ; but in the car- 

 nivorous 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 difference 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 living 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 sem transparent, 

 and serves the purpose of protecting the eye from the contact 

 of external bodies, 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 



