THE SENSE OF SIGHT. 299 



towel. By not observing this rule certain contagious dis- 

 eases of the eyes often spread rapidly. 



13. Consult a Reliable Oculist. If there is any con 

 tinuous trouble with the eyes, consult a reliable oculist. 

 Many headaches are due to eye-strain, the real cause being 

 unsuspected. If a child has frequent headaches, it is well 

 to have the eyes examined. Many persons injure their 

 eyes by not wearing suitable glasses. On the other hand, 

 do not buy glasses of peddlers nor of any but reliable 

 specialists. One may ruin the eyes by wearing glasses 

 when they are not needed. Sight is priceless. 



READING. Sight, Le Conte. 



Summary. i. Sight, like hearing, acts through space, outstripping 

 the "contact senses " of touch, taste, and smell. 



2. The eye is protected by its bony surroundings, lids, lashes, tears, 

 sensitiveness of the conjunctiva, etc. 



3. The eye is moved by muscles under nerve control. 



4. The eye has three coats sclerotic, choroid, and retina. 



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

 amount of light admitted. 



6. The refracting media of the eye are the cornea, aqueous humor, 

 lens, and vitreous humor. 



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. 



