vi FORM AND FUNCTION 127 



focus. When they contract the choroid is drawn 

 forward, the strain upon the lens is reduced, and, con- 

 sequently, its surface becomes more rounded. This is 

 the process that takes place when the sight is adjusted 

 for near objects. At the same time the Iris contracts 

 and lessens the amount of light admitted. This 

 wonderful curtain adapts itself to all circumstances : 

 involuntarily, by a reflex action we reduce the size of 

 the pupil when a strong light falls upon the eye ; 

 voluntarily, though habit makes the action unconscious, 

 and by calling into play a different set of nerves, 

 we contract it, when we cast our eyes upon a near 

 object. 



It is now time to mention some of the peculiarities 

 of the bird's eye. The eyeball is not so globular as in 

 man ; in front it is much contracted, behind it opens 

 out like a decanter ; the cornea is highly curved. In 

 birds of prey, which see great distances, the front 

 surface of the lens is nearly flat ; in owls, on the con- 

 trary, it is much rounded, and at the same time the 

 pupil is very large to admit as much moonlight as 

 possible. 



At the back of the eye, springing from the entrance 

 of the nerve, is a peculiar fanlike object, the Pecten, 

 which projects into the eyeball (P). It is full of 

 blood vessels, and is deeply pigmented, like the 

 choroid to which it is akin in structure. It is 

 thought to nourish the vitreous humour ; certainly 

 it does not push the lens forward for focussing- 

 purposes as some writers have maintained. Any 

 one who examines it, not in a diagram, but in the 

 eye itself, will find that it is far too limp to produce 



