374 THE ANIMALS AND MAN 



focus by changing the thickness of the lens. That is, the 

 eye accommodates itself, by a self-regulating mechanism, 

 to near and far objects. 



The choroid coat exerts a constant pull upon the lens 

 through the suspensory ligaments (fig. 185, SL). This pull 

 slightly flattens the lens, that is, makes it less convex, so that 

 it will focus far objects upon the retina. But when the 

 ciliary muscles (fig. 185, Cm) contract they pull the choroid 

 coat forward so that it fails to exert a pull upon the lens. 

 The lens then, of its own elasticity, becomes more convex 

 and will then focus near objects upon the retina. The invol- 

 untary movements of these muscles therefore enable the eye 

 automatically to accommodate itself to near and far objects. 



Sensation of sight. — The sensation of sight is, however, 

 not located in the retina, but in a certain part of the cortex 

 of the cerebral hemispheres. The cells affected by the 

 light are in the retina. Nerve fibers from these cells pass 

 back, as shown in figure 182, through the optic nerve to the 

 nerve-centers of sight in the brain. 



The essential parts therefore, of the sight-apparatus 

 are the retina, the optic nerve and nerve centers in the 

 brain. All the other parts are accessory parts to render the 

 function of these more perfect. 



Hygiene or care of the eyes. — Because the eyes are 

 such extremely delicate structures they must be treated with 

 as much care as the most delicate piece of machinery. 



Anything that affects the general health affects the eyes 

 very quickly. Therefore what has been said about good 

 food, fresh air and exercise being necessary for other parts 

 of the body applies to the care of the eyes as well. 



Reading in a bright light, as with sunshine upon the 

 book, or in a dim light, as at dusk, weakens the muscles that 

 operate the lenses and the eyeballs. A flickering light is 

 exceedingly bad as it over-exerts the muscles of accommo- 

 dation. 



