OCULAR MOVEMENTS AND OCULAR MUSCLES. 



869 



of the paper: Q E is the direction in which the external rectus acts, 

 Q x I that in which the internal rectus acts. The axis of rotation is 

 perpendicular to the plane of the paper at the center of rotation O, and 

 thus coincides with the vertical axis of the eyeball. 2. The axis of 

 rotation of the superior and inferior recti (the dotted line R. sup. 

 R. inf.) lies in the horizontal plane of division of the eye, but it forms 

 an angle of about 20 with the transverse axis (Q Q x ). The line of 

 action for both muscles is indicated by the line s i. It will be seen at 

 once that by the action of the superior rectus the cornea must move 

 upward and somewhat inward; or downward and inward by the action 



vt 



E i I 



FIG. 302. Lines of Traction and Axes of Rotation of the Ocular Muscles. 



of the inferior rectus. 3. The axis of rotation of the two oblique muscles 

 (the dotted line Obi. sup. Obi. inf.) lies also in the horizontal plane 

 of division of the eyeball, but it forms an angle of 60 with the transverse 

 axis. The line of action of the inferior oblique is shown by the line a b, 

 that of the superior oblique by the line c d. The action of these muscles 

 causes the cornea to move respectively outward and upward, or out- 

 ward and downward. These actions of the muscles are effective only 

 when the eye is in the primary position; in every other position the 

 axis of rotation of each muscle changes. 



When the eye is at rest, the muscles are in equilibrium. Because of 



