300 PHYSIOLOGY. 



7. These refracting media form an inverted image on the retina. 

 The eye is a camera, darkened on the inside. 



8. The ciliary muscle, acting on the elastic lens, adjusts the lens 

 for seeing at different distances. 



9. Suitable lenses overcome many of the defects in eyesight. 



10. The retina is an expansion of the optic nerve, and is exceed- 

 ingly complicated in its structure. 



11. The blind spot is the place where the optic nerve enters the eye. 



12. The optic nerve is insensitive to light, but injury to it causes 

 sensations of light. 



13. Most of the fibers of the optic nerve cross to the other half of 

 the brain, but some do not cross. 



14. Color is due to difference in the rapidity of vibration in the 

 waves of light. 



15. Eyes that do not distinguish these differences are color blind. 



1 6. Pain in the eyes comes through the fifth pair of nerves, not 

 through the optic nerves. 



17. Binocular vision makes objects "stand out" more distinctly as 

 solid bodies. 



18. Impressions of light linger, making after-images. 



19. Defects in eyesight are much more common among civilized 

 men than with uncivilized men or animals. 



20. The care of the eyes must be made a subject of study and care- 

 ful thought by all reading people. 



Questions. I . What is the position of the eyeballs during sleep ? 



2. What is "cataract" ? 



3. What is the cause of " double vision " ? 



4. Why does the well eye sympathize with the affected one ? 



5. Why does looking at a bright light often cause a person to sneeze ? 



6. Why is weeping associated with grief ? 



7. What is the condition of one who is "cross-eyed" ? 



8. Compare the pupils of a man, a cat, and a cow. 



9. Does the color of the eye have any relation to the strength of 

 eyesight ? 



10. Why is one going from a bright room into the dark unable to 

 see at first, but gradually sees more distinctly ? 



11. Why can one not see well when the eye "waters" ? 



12. If each eye has a blind spot, why are there not blank spaces in 

 the field of vision ? 



