322 



MUSCLES AND FASCIA. 



and Superior Oblique, and from the fibrous sheath of the optic nerve ; and is 

 inserted, by a tendinous expansion, into the sclerotic coat, about three or four 

 lines from the margin of the cornea. 



Relations. By its upper surface, with the Levator Palpebrae. By its under 

 surface, with the optic nerve, the ophthalmic artery, the nasal nerve, and the 



Fig. 216. Muscles of the Right Orbit. 



Fig. 217. The relative position and 

 attachment of the Muscles of the 

 Left Eyeball. 



branch of the third nerve, which supplies it ; and, in front, with the tendon of 

 the Superior Oblique, and the globe of the eye. 



The Inferior and Internal Recti arise by a common tendon (the ligament of 

 Zinn), which is attached round the circumference of the optic foramen, except 



at its upper and outer part. The External 

 Eectus has two heads : the upper one arises 

 from the outer margin of the optic foramen, 

 immediately beneath the Superior Rectus ; the 

 lower head, partly from the ligament of Zinn, 

 and partly from a small pointed process of 

 bone on the lower margin of the sphenoidal 

 fissure. Each muscle passes forward in the 

 position implied by its name, to be inserted, by 

 a tendinous expansion (the tunica albuginea), 

 into the sclerotic coat, about three or four 

 lines from the margin of the cornea. Between 

 the two heads of the External Eectus is a 

 narrow interval, through which pass the third, 

 nasal branch of the fifth, and sixth nerves, 

 and the ophthalmic vein. Although nearly all these muscles present a com- 

 mon origin, and are inserted in a similar manner in the sclerotic coat, there 

 are certain differences to be observed in them, as regards their length and 

 breadth. The Internal Rectus is the broadest ; the External, the longest ; and 

 the Superior, the thinnest and narrowest. 



The Superior Oblique is a fusiform muscle, placed at the upper and inner side 

 of the orbit, internal to the Levator Palpebrge. It arises about a line above 

 the inner margin of the optic foramen, and, passing forwards to the inner 

 angle of the orbit, terminates in a rounded tendon, which plays in a ring or 

 pulley, formed by fibre-cartilaginous tissue attached to a depression beneath 

 the internal angular process of the frontal bone, the contiguous surfaces of the 

 tendon and ring being lined by a delicate synovial membrane, and inclosed in 

 a thin fibrous investment. The tendon is reflected backwards and outwards 



Ilcctus Inje-nor 



