STRUCTUKB OF THE EYEBAIJj 455 



and upper surface of the eyeball respectively, and by their 

 action turn the eyeball upward or downward. The other 

 two muscles (the inferior and superior oblique) serve to 

 rotate the eyeball outward or inward. The inferior oblique 

 has its origin near the front of the orbit on the inner side 

 and passing under the eyeball is inserted on the outer side. 

 Its contraction rotates the eyeball outward. The superior 

 oblique has its origin at the back of the orbit like the rectus 

 muscles, passes forward just above the superior rectus and 

 its tendinous end, after first passing through a loop or pulley 

 formed by a fibro-cartilage on the upper edge of the orbit, 

 turns outward over the top of the eyeball, and is inserted 

 in the eyeball at a point on the inner side. By this arrange- 

 ment the contraction of a straight muscle rolls the eye 

 inward. 



The combined action of these six muscles is responsible 

 for all the movements of the eyeball. The muscles of the 

 two eyeballs contract and relax in pairs, the internal rec- 

 tus of one eye contracting at the same time as the external 

 rectus of the other, and similarly for all the others. In this 

 •way, both eyes are turned in the same direction at the same 

 time. 



As has been already mentioned, the eyelids are closed by 

 the orbicularis muscle. The upper lids, however, are pro- 

 vided with elevator muscles attached to the back of the 

 orbit, and these muscles pull the upper lids open farther 

 than would be possible by the extension of the orbicularis 

 alone. 



Structure of the eyeball. — The eyeball itself, when de- 

 tached from its supports, is a nearly spherical sac of about 

 an inch in diameter. It is covered, except at the points 

 where the optic nerve enters and in front, with an opaque 



