262 SCIENCE OF COMMON THINGS. 



Structure of the eye. How the eye is moved. 



CHAPTER II. 



STRUCTURE OF THE EYE AND THE PHENOMENA OF VISION. 

 1*703 What is the structure of the human eye ? 



In man the organ of vision consists of two hollow 

 spheres, each about an inch in diameter, filled with 

 certain transparent liquids, and deposited in cavities of 

 suitable magnitude and form in the upper part of the 

 front of the head on each side the nose. 



7O3 How is it that we are enabled to move the eye in various directions t 



By means of muscles attached to different points of 

 its surface. 



These are shown in Fig. 76. where the external bones of the temple 

 are supposed to be removed in order to render visible the muscular 

 arrangements. The muscle, 1, raises the eye-lid, and is constantly in 

 action while we are awake. During sleep, the muscle being in repose 

 and relaxed, the eye-lid falls and protects the eye from the action of light. 

 The muscle, 4, turns the eye upwards ; 5, downwards ; 6, outwards ; and 

 a corresponding one on the inside, not seen in the figure, turns it inwards. 

 No. 2 and 10 turn the eye round its axis. No. 11 is tue great optic 



