416 



THE NERVES AND THE SENSE ORGANS 



sight to the fact that reflex action closed their eyelids 

 before the pieces scattered by an explosion could travel 

 to the eye. 



The eyelids are lined with a very sensitive membrane, 

 the conjunctiva. The same membrane covers the front 

 of the eyeball. When we get a cinder into one of our 

 eyes, and it hurts badly, we ought to remember that 

 the pain is the conjunctiva's way of telling us that 

 the eye is in danger, and that we" must remove the tres- 

 passing cinder. 



416. Parts of the Eye. In many respects the eye is 

 like a camera (cf. 188). It contains a lens, called the 



crystalline lens 

 (Fig. 300), a 

 "shutter," called 

 the iris, for regu- 

 lating the amount 

 of light that can 

 get into the eye, 

 a sensitive mem- 

 brane, the retina, 

 for receiving the 

 image, and an ar- 

 rangement, called 

 accommodation, 

 which changes 

 the focusing as objects are near or distant. 



The crystalline lens divides the eye into two chambers 

 of unequal size. The smaller one is in front, and con- 

 tains a liquid called the aqueous (or "watery") humor. 



Sclerotic 

 Coat 



Fig. 300. 

 Parts' of the Eye. 



