DIGESTIVE ORGANS. 1977 
seem to prove that they are attracted rather by the sight than 
smell. 
The organ of sight is, indeed, more highly developed in birds 
than 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-vessgls, 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 cz 
