AFFECTIONS OF THE MUSCLES 6l 



The action of the muscles are as follows: The external 

 and internal recti cause the eyeball to move outward 

 and inward respectively, and they balance, so to speak, 

 the horizontal movements. The superior rectus causes 

 an upward and shghtly inward movement, while the 

 inferior rectus causes a downward and inward movement. . 

 The obhque muscles cause the eyeball to rotate on its 

 anteroposterior axis. They oppose or balance the 

 movements of the superior and inferior recti. 



The retractor is the largest and most powerful muscle 

 of the eyeball. It has its origin in common with the 

 recti muscles, surrounding the optic foramen, passes 

 forward, completely encasing the optic sheath, expand- 

 ing in a funnel shape, and is inserted into the posterior 

 third of the sclerotic coat. Its function is to pull the eye- 

 ball backward. 



The insertion of the muscles are tendinous, and these 

 tendons are ensheathed in a fascia, which is in reality 

 a portion of Tenon's capsule. 



All the extrinsic muscles, except the superior ob- 

 hque and the external rectus, are supplied by the third 

 cranial nerve. The superior obhque is supphed by the 

 fourth cranial nerve, and the external rectus by the 

 sixth cranial nerve. 



Affections of the Muscles 

 In man we have many affections of the muscles, be- 

 cause the eyes are so placed to produce, in a normal case, 



