THE ORG^N OF SIGHT. 



4ir 



4. Muscles of the Orbit (Fig. 166). — Within the orbit are 

 eleven muscles, ten connected with the eyeball, and one, 

 M. levator palpebrjB superioris, with the upper eyelid. 



Of the ten muscles of the eyeball, eight are straight 

 muscles, passing from an origin about the inner end of the 

 orbit directly distad ; the other two have an oblique direction. 

 Four of the straight muscles are larger and are known as the 

 Mm. recti (/^ c, d); these are distinguished according to their 

 position as lateral {c), medial, superior {d), and inferior (/;). 

 The four smaller straight muscles constitute together M. re- 

 tractor oculi {c). The two oblique muscles are known as 

 Mm. obliqui and are distinguished according to their position 

 as inferior {a) and superior. 



Fig. 166. — Muscles of the Eyeball, Lateral View. 



a, M. obliquus inferior; b, M. rectus inferior; c, M. rectus lateralis; d, M. rectus 

 superior; e, parts of M. retractor oculi; /, the oculomotor nerve. 



Mm. recti {b, c, d). — The four recti muscles arise from the 

 bone about the optic foramen and pass toward the eyeball. 

 They are inserted by thin, flat tendons along a line which 

 separates the darker caudal part of the sclerotic from the white 

 zone of the sclerotic which borders the cornea, — the line of 

 insertion of the four tendons forming thus a circle about the 

 eyeball. The rectus superior {d) is partly covered by M. 

 levator palpebrae superioris. The tendon of the rectus inferior 

 {b) is crossed by M. obliquus inferior (a). The recti muscles 

 are all innervated by the oculomotor (third) nerve (/"), except 

 the lateral (t), which is supplied by the abducens (sixth) nerve. 



M. retractor oculi {e). — The retractor oculi arises about the 

 optic foramen and divides into four heads which lie nearer the 



