THE SENSE OF SIGHT. 



291 



move the retina back and forth, so we change the shape of 

 the lens. When we look at a near object the lens becomes 

 thicker, and when we look at a distant object the lens be- 

 comes less thick. This adjustment is called accommodation. 



CILIARY MUSCLE 



FAR NEAR CILIARY PROCESS 



Fig. 88. A Diagram to illustrate Accommodation. 



Action of the Ciliary Muscle. In looking at a near object, the 

 ciliary muscle pulls on the hyaloid membrane, and draws it forward 

 (since the muscle is fastened at the point where the iris joins the 

 cornea). When the hyaloid membrane is pulled forward, the lens is 

 released from pressure that was given it by the lens capsule. Now the 

 lens becomes thicker because it is elastic, and when it is not subject to 

 pressure it tends to become relatively thick. When we look at a dis- 

 tant object the muscle relaxes, and the capsule presses on the front of 

 the lens and flattens it, thus adjusting for far sight. It should be 

 noted that adjustment for near sight is brought about by muscular 

 effort, hence is fatiguing ; whereas adjustment for far sight is accom- 

 plished mechanically, without effort. 



(2) Near-sighted Eye. (I) Normal Eye. (3) Far-sighted Eye. 



Fig. 89. Defects in Eyesight. 



Defects of Eyesight. In old age the lens usually be- 

 comes less elastic, -and cannot adjust for near sight. Since 



