414 LIFE ON THE EARTH 



the lacking sense organ. These sensations, like all others, 

 are carried to the brain by the nerves and there interpreted 

 into the sensation of touch. 



Sight. The organ of sight, the eye, is an exceedingly 

 sensitive, automatically adjustable camera that records 

 through the nerves. The camera box is the hard, bony 

 socket in which the eye is placed, the eyelid is the shutter, 

 and the iris, the diaphragm. The iris is the membrane in 



the front of the eye which 

 opens or contracts to let 

 in more or less light. In 

 the center of it is a hole, 

 the pupil. 



Back of the diaphragm, 

 or iris, is a small adjust- 

 able lens and beyond this 

 the sensitive plate, the 

 CROSS SECTION OF THE HUMAN EYE retina. Between the iris 



The pupil is the opening surrounded and the front of the eye 

 by the iris. . 1M . -t 



is a wateryhke material, 



the aqueous humor, which keeps the front of the eye ex- 

 tended into its rounded form. Back of the lens is a thick, 

 transparent, jellylike material, the vitreous humor, which 

 holds the retina extended and keeps the eye from collapsing. 

 Instead of moving the retina back and forth to focus a 

 picture, as is done with the ground-glass plate in a camera, 

 the eye lens is capable of adjusting itself so as to focus objects 

 which are at different distances. Leading back to the brain 

 from the retina is the optic nerve, which carries the impres- 

 sions made on the retina to the brain, where they are inter- 

 preted into the sensation of sight. 



OPTlfc 

 NERVE 



