EYE SOCKETS AND LIDS 415 



414. Sight. Light is the cause, or stimulus, outside 

 of us that produces the sensation of sight, or seeing 

 (cf. 167). The eye is the organ that is sensitive to the 

 light waves that come to us. But the eye is not merely a 

 "light spot," an organ to distinguish light from darkness. 

 It is a structure that enables us to perceive light that has 

 been changed by objects that reflect it, or refract, trans- 

 mit, or partly absorb it, so that we see the illuminated 

 objects in all their variety of light and shade, form and 

 color. The eye collects these modified light waves, bends 

 them so that they come to a focus, and out of them forms 

 an image on the retina (cf. 416). 



415. Eye Sockets and Lids. The eye is admirably 

 protected. It is set into sockets in bones of the skull, so 

 that it is shielded on all sides except in front. The 

 sockets are lined with fatty tissue covered by a double 

 membrane. The fat and membrane not only make the 

 socket smooth, but they also form a soft cushion for the 

 eye ball, in case it is forced inward by a blow. 



The eye is protected in front by the eyebrow, the eye- 

 lashes, and the eyelids. The bone forming the brow pro- 

 jects over the eye like a roof, protecting it against blows 

 from above. At the same time the hairs of the eyebrow 

 act like eaves troughs, and drain off perspiration, so that 

 it does not run down into the eye. The eyelashes shade 

 the eye from "high light," and keep out dust. The 

 eyelids move with incredible speed when the eye is in 

 danger (cf. 405), so that the "twinkling of an eye" is 

 really a very short space of time. Experimenters have 

 again and again owed the preservation of their eye- 



