MOVEMENTS OE THE EYEBALLS 291 



ments of the eyeball are carried out by the extrinsic muscles, 

 Avhich are six in number. Of these, four (called the recti) originate 

 in a common tendon surrounding the optic foramen, and pass 

 forward to be inserted a short distance in front of the equator 

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

 (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 inferior oblique arises from 

 the inner anterior part of the orbital floor, and passes outwards 



INF OBLIQUE 

 ipiH)<, 



EXT. RECTUS I / ^mW>>. \ '^"^^ f^ECTUS 



tsnN.)< — SBIH ^ (™^) <: 



SUP. RECTUS 



-> CniN.) ^ 



(iyn)< x.^________^^-£ ^ (niN.) 



SUP. OBLIQUE I I INF. R.ECTUS 



Fig. 75. — Diagram showing the directions in wliich tlie different external muscles of the eye 



rotate the eyeball. 



and backwards to its insertion rather beyond the equator. It acts 

 on the same line as the superior oblique. 



All the movements of the eyeball are rotations round axes passing 

 practically through the centre of the sphere, but it can be proved 

 experimentally that rotation never occurs round the visual axis. 



The internal and external recti rotate the eye roimd 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 tending 

 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 simul- 

 taneously. Its force can likewise be resolved, one component 

 tending to rotation round the horizontal axis at right angles to the 

 visual axis, the second component tending to rotation round 

 the visual axis, clockwise. The two rotations round the visual 



