THE ORGANS OF SENSE 307 



transparent cornea. Behind the cornea is a colored, cir- 

 cular partition, called the iris, which has a central, circular 

 aperture, the pupil, through which light is admitted 

 to the back part of the eye. Just behind the iris lies the 

 transparent crystalline lens. This separates the interior 

 of the eye into two chambers, (i) a small anterior one 

 filled with a transparent fluid called the aqueous humor, 

 and (2) a large posterior chamber filled with the trans- 

 parent vitreous humor. The back part of the eye is lined 

 internally by the sensitive retina which is really the 

 expanded end of the optic nerve. Between the retina and 

 the sclerotic is the black, pigmented choroid coat which 

 serves to absorb scattered light which enters the eyeball. 



The optical parts of the eye are so arranged as to throw 

 images of outer objects on the retina. The eye has often 

 been compared to a camera which, in a very similar way, 

 is constructed so as to throw images on the photographic 

 plate at the back. The Jens of the eye functions like that 

 of a camera in forming an image; the iris, which by con- 

 tracting or relaxing alters the size of the pupil, corre- 

 sponds to the shutter which regulates the amount of light 

 entering the camera; the choroid, like the black inside 

 of the camera, absorbs superfluous light; and the retina 

 on which images are thrown is analogous to the sensitive 

 photographic plate or film. As images formed by a lens 

 are clearly outlined only when the object is a certain 

 distance away, if the parts of the eye remained always 

 the same we could not see both distant and near objects 

 with equal clearness. We are enabled to do this because 

 the curvature of the lens can be increased by the con- 

 traction of a special muscle that surrounds it, while the 

 lens resumes its previous shape when the muscle is relaxed. 

 The eye thus has the power of focusing itself upon objects 

 at varying distances. 



