46 ELEMENTARY PHYSIOLOGY. 



(Sense of Sight.) 



Between the cornea and the iris, in the 



front P art of the e y e is a water y fluid 



called the aqueous humor (18). Back 

 of the iris lies the crystalline lens (23). This is a 

 beautiful gem-like little body as clear as a crystal. 

 Back of the lens, and between it and the retina, the 

 eyeball is filled with another clear jelly-like sub- 

 stance called the vitreous humor (25). The effect 

 of these three humors which are contained in the 

 eye, and especially the effect of the lens, is to produce 

 the image of things on the retina, as has already 

 been mentioned. 



(Third Section of Eye Manikin.) 



These muscles, which are shown to be attached to 

 the outer sclerotic coat, produce some of the prin- 

 cipal movements of the eye upward, downward, in- 

 ward and outward. By the way, we also mention 

 that the action of the muscles of the iris in regulat- 

 ing the size of the pupil, is an interesting example of 

 what we have already learned to name reflex nervous 

 action. 



THE EAR. 



The nerve of special sensation which 

 The iferve goes from the brain to the ear is called 



of Hearing. 



the auditory nerve. As the nerve of 

 sight is sensitive only to light, this is sensitive only, 

 to sound. The ear is an instrument to collect sounds 

 and bring them to bear on the auditory nerve in 



