MOVEMENTS OF EYEBALL 



223 



TRAN3VER5E 



and pass forward to be inserted a short distance in front of the 

 equator of the eyeball. One is placed above (R. superior), one 

 below (R. inferior), one on the outside (R. externus) and one on 

 the inside (R. internus). The remaining two muscles are the 

 superior and the inferior oblique. The former arises from the 

 same tendon as the recti, and follows a course similar to but above 

 the internal rectus. At the front of the orbit its tendon passes 

 through a fibrous loop or 

 pulley, and bends sharply 

 back to be inserted in the 

 eyeball about the equator. 

 Its line of action makes an 

 angle of about 60 with 

 the visual axis. The in- 

 ferior oblique arises from 

 the inner anterior part of 

 the orbital floor, and passes 

 outwards and backwards 

 to its insertion rather be- 

 yond the equator. It acts 

 on the same line as the 

 superior oblique. 



All the movements of 

 the eyeball are rotations 

 round axes passing prac- 

 tically through the centre 

 of the sphere, but it can 

 be proved experimentally 

 that rotation never occurs 

 round the visual axis. 



The internal and exter- 

 nal recti rotate the eye 

 round a vertical axis, and 

 their action is unaffected by the relative obliquity of the visual 

 and orbital axes. 



The rectus superior acts along the line of the orbital axis, 

 and its force can be resolved into two components, the one tend- 

 ing to rotation round a horizontal axis at right angles to the 

 visual axis, the other tending to rotation round the visual axis 

 itself in a counterclockwise direction (viewed from the front). 

 In order to overcome the latter tendency, the inferior oblique 

 acts simultaneously. Its force can likewise be resolved, one 



FIG. 51. Diagram of extrinsic muscles of eye. 



