CHAPTER XXV. 

 THE SENSE OF SIGHT. 



Protection of the Eye. The eye is set well back in its 

 socket and guarded by three bony projections, the brow, 

 cheek bone, and the bridge of the nose. It is further 

 protected by the eyelids and eyelashes. 



The LaJrymal Secretion. The lacrymal gland, or tear 

 gland, is just above the outer angle of the eye, and pours 

 its secretion over the eyeball. The lids serve as curtains 

 to admit or shut out light, and, by winking, wash the eye. 

 It is as though a man were kept all the time in front of a 

 plate-glass window, with water and rubber scraper, to keep 

 it clean and bright. The lacrymal secretion is ordinarily 

 carried off into the nasal cavity as fast as it is made. If 

 the ducts are stopped, or if the secretion is formed very 

 rapidly, the liquid overflows on the face as tears. 



The External Parts of the Eye. The " white of the 

 eye " is the sclerotic coat. It has blood tubes, but ordina- 

 rily they are not conspicuous. The front part of the eye- 

 ball is covered with the cornea. This is transparent, and 

 the color of the iris shows through the cornea. In the 

 center of the iris is the hole, or pupil, by which light enters 

 the interior of the eye. 



The Conjunctiva. The front of the eyeball is covered 

 by a thin, transparent, mucous membrane, the conjunctiva, 

 which turns back and lines the inside of the eyelids. It is 

 very sensitive. 



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