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 
