DIGESTIVE ORGANS. 



177 



seem to prove that they are attracted rather^ by the sight than 

 smell. 



The organ of sight is, indeed, more highly developed in birds 

 ihan in any other class of animals. The volume of the eye itself is 







Fig. 68. Internal Anatomy of Birds. 



large compared with the head. It includes an addition which seems 

 to be confined to Birds. This is a black membrane, with many folds, 

 very rich in blood-vessels, and situated at the bottom of the ocular 

 globe, and advancing towards the crystalline. Anatomy has failed 

 to explain the use of this, but it is supposed that by enlarging or 



