THE EYE 



381 



capsule. This enables the lens to thicken on account of its own elastic 

 force. To flatten the lens, the ciliary muscle relaxes, the elastic force 

 of the eyeball resumes its tension on the suspensory ligament, and the 

 membranous capsule resumes its pressure on the sides of the lens. 

 This pressure, overcoming the elastic force of the lens, flattens it. 



Movements of the Eyeballs. In order that the light 

 may enter the eyeballs to the best advantage, they must 

 be moved in various directions. These movements are 

 brought about through the action of six small muscles 

 attached to each eyeball. Four of these, named, from 



Superior 



ffx ternaf. 

 rec-fus 



Superior / 

 recCus 



ternal 

 rectua 



Optic nerve 



FIG. 162. Exterior muscles of eyeball. 



their positions, the superior, inferior, internal, and external 

 recti muscles, are attached at one end to the sides of the 

 eyeball and at the other end to the back of the orbit 

 (Fig. 162). These, in the order named, turn the eyes up- 

 ward, downward, inward, and outward. The other two, 

 the superior and inferior oblique muscles, aid in certain 

 movements of the recti muscles and, in addition, serve to 

 rotate the eyes slightly. The movements of the eyeballs 

 are similar to those of ball and socket joints. 



Binocular Vision. In addition to directing the eyeballs so that 

 light may enter them to the best advantage from different objects, the 



