262 Physiology. 



3. The pupil is a hole in the iris, and varies in size to regulate the 

 amount of light admitted. 



4. The cornea, aqueous humor, lens, and vitreous humor form an 

 inverted image on the retina. The eye is a camera, darkened inside. 



5. The lens changes its thickness for seeing at different distances. 



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



7. The retina is an expansion of the optic nerve, and is exceedingly 

 complicated in its structure. 



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



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

 sensations of light. 



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

 the brain, but some do not cross. 



11. Defects in eyesight are much more common among civilized 

 men than with the uncivilized. 



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

 ful thought by all reading people. 



Questions. i . What is " cataract " ? 



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



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



4. Why does looking at a bright light often cause a person to 

 sneeze ? 



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



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



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

 eyesight ? 



8. Why is one blinded on entering a bright room from the dark ? 



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

 see at first, but gradually sees more distinctly ? 



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



11. Should the lights which illumine a pulpit or platform be so 

 placed that they can shine into the eyes of the congregation ? How 

 should they be arranged ? 



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

 the field of vision ? 



13. What advantage has a stereoscopic view over a single view ? 

 How are stereoscopic views made ? 



