192 



INTEODUCTOEY CHAPTEE. 



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

 is 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. 



Fig. 69. 



Anatomy has failed to explain the use of this, but it is sup- 

 posed that by advancing or withdrawing it, it gives to birds 

 additional power of vision. Other parts of the eye, such as 

 the choroids, the thin membrane which covers the posterior part 

 of the eye, the iris, the retina, present nothing remarkable. 

 The white of the eye is surrounded by an osseous or cartilagi- 

 nous matter, evidently placed there for protection of this delicate 

 and useful organ. 



