MUSCLES OF THE ORBIT 57 



sixth nerve passes forwards through the pons and emerges at 

 its lower border, near the median plane. Its subsequent 

 course is similar to that taken by the third nerve. Having 

 crossed the cisterna basalis, it passes through the lateral wall 

 of the cavernous sinus and gains the orbit through the superior 

 orbital fissure. 



The Muscles of the Orbit. The levator palpcbra superioris 

 acts as an elevator of the upper eyelid and is therefore antag- 

 onistic to the orbicularis oculi (palpebraruni) (p. 82). When the 

 latter muscle is involved in facial paralysis, the increased 

 tonus of the unopposed levator keeps the eye constantly open. 

 During sleep, however, the levator relaxes and the eye may 

 become almost completely closed. The levator palpebrae 

 superioris is supplied by the third nerve and, when it is 

 paralysed, the condition of ptosis results (cf. pseudo-ptosis, 

 p. 210). 



The superior rectus passes forwards from the posterior part 

 of the orbit and is inserted into the sclerotic coat a little in 

 front of the equator. It lies above the eyeball and conse- 

 quently acts as an upward rotator. The pull of the superior 

 rectus, however, does not impart a pure upward movement 

 to the pupil, but it adds a slight medial deviation as well. 



The superior rectus acts in concert with the inferior oblique, 

 which arises from the antero-medial part of the floor of the 

 orbit and passes laterally and backwards to be inserted into 

 the sclerotic behind the equator. It rotates the eyeball so as 

 to make the pupil look upwards and laterally. 



When these two muscles act together, they produce a pure 

 upward movement, since the medial pull of the superior rectus 

 is counterbalanced by the lateral pull of the inferior oblique. 

 Both are supplied by the oculo-motor nerve. 



The attachments of the inferior rectus are simliar to those 

 of the superior rectus, but the former muscle is applied to 

 the inferior aspect of the eyeball so that it acts chiefly as a 

 downward rotator. As in the case of the superior rectus, the 



