412 SENSE-ORGANS AND INTEGUMENT. 



eyeball than the recti and are therefore partly covered by the 

 latter. They alternate with the recti and are inserted into 

 the eyeball at about its equator, except the inferior division, 

 which is inserted on a line with the recti. They are supplied 

 by the oculomotor nerve. 



M. obliquus inferior (a). The inferior oblique arises from 

 the maxillary bone just laterad of the lachrymal bone and 

 curves over the ventral side of the eyeball along the lines of 

 insertion of the recti (crossing the tendon of the rectus inferior, 

 b} to the ventral edge of the tendon of the lateral rectus (^r), 

 where it is inserted. It rotates the eyeball and is supplied by 

 the third nerve (/"). 



M. obliquus superior. The superior oblique takes origin 

 from the cranial border of the optic foramen, passes dorso- 

 craniad, and ends in a small rounded tendon which passes 

 through a fibrous pulley-ring. This ring is situated near the 

 rim of the orbit, one or two centimeters craniad of the zygomatic 

 process of the frontal bone, and is attached to the wall of the 

 orbit by two fibrous bands. After passing through the pulley- 

 ring the tendon turns and passes laterocaudad ; it becomes 

 much expanded, passes caudad of the superior rectus, and is 

 inserted into the eyeball along the caudal margin of the inser- 

 tion of the superior rectus tendon. The superior oblique is 

 supplied by the trochlear (fourth) nerve. 



M. levator palpebrae superioris. This is a small thin 

 muscle which takes origin on the wall of the optic foramen 

 close to the rectus superior, passes over the outer surface of the 

 rectus superior and beneath the lachrymal gland, and forms a 

 thin tendon which is inserted into the margin of the upper 

 eyelid. It is innervated by the oculomotor. 



5. The Eyeball. The eyeball is approximately spherical 

 in form, but the cranial surface has a slightly, greater curvature 

 than the caudal portion, so that the eye appears slightly pointed 

 at the cranial (free) surface. The large cylindrical optic nerve 

 (Fig. 167, a) enters the eyeball at nearly the centre of the 

 caudal half of the eye. On the outer surface is seen the trans- 

 parent cornea (r), covering the free surface of the eye, and the 

 opaque sclerotic (^), covering the remainder of the ball. 



