THE ORGAN OF SIGHT. 22$ 



Sometimes in disease, the parts of the brain which give 

 us feelings are excited without waiting for any message 

 brought in along a sensory nerve. Then the person be- 

 comes delirious or suffers from delusions. He sees and 

 hears and smells things which do not really exist; but 

 to his mind they are just as real as if they did actually 

 exist, and were acting on his sensory nerves so as to ex- 

 cite the parts of the brain which feel. 



3. Why the Eye is the Organ of Sight. In the eyes there 

 are thousands of nerve-fibres, each of which has a little 

 " tip" or end on it which is so made as to be very easily 

 acted on by light. Any light, as from the sun or a lamp 

 or candle, which comes direct, or is first reflected from 

 some object, and reaches one of these peculiar little ends, 

 excites it, and the end in turn excites the nerve-fibre 

 joined to it, and this fibre then carries up some message 

 to the part of the brain which gives us feelings or sensa- 

 tions of sight. If the light comes direct it excites the 

 nerves in such a way that we see the sun or lamp or 

 candle. If it comes bounding back from some other 

 object which it has struck on its way, we see the other 

 object. No other nerves than those of the eye have this 

 particular kind of tips on their ends, and so light does 

 not excite them, as it does the nerves of the eye. 



4. The Eyeball (Fig. 55) is nearly as round as a 

 marble, but is buried in the eye-socket and covered by 

 the eyelids, so that only a small part of its front side can 

 be seen. On this front part is a round transparent win- 

 dow, set in it, like a pane of glass, to allow light to get 



3. What is there on the ends of the nerve-fibres in the eye ? What 

 happens when light reaches them ? Why cannot we use other parts 

 of the body for seeing ? 



4. Describe the shape and position of the eyeball. What is there 



