172 MYOLOGY, OR ANATOMY OF MUSCULAR SYSTEM 



at the apex of the orbit from a ligamentous ring which encircles 

 the optic foramen and crosses the sphenoidal fissure; most 

 of the fibers spring from two common tendons; the upper one 

 rises from the inferior root of the small wing of the sphenoid, 

 and is prolonged into the internal, superior, and external 

 recti; the lower (Zinn) rises from the body of the sphenoid 

 and divides into three slips for the internal, inferior, and external 

 recti. All the recti are inserted into the sclerotic 3 or 4 lines 

 from the cornea; the external has two heads, between which 

 pass the third, nasal branch of the fifth, the sixth nerve, and 

 ophthalmic vein. The external and inferior recti are the longest, 

 internal broadest, and superior smallest. 



The superior oblique, or trochlearis, is internal to the levator 

 palpebrse, rises just in front of the optic foramen, and passes 

 forward to a round tendon which plays through a fibrocartilagi- 

 nous ring attached to the trochlear fossa of the frontal; it is 

 there bent out, back, and down between the superior rectus 

 and eye, and is inserted beneath the outer edge of the superior 

 rectus midway between the cornea and optic nerve. The 

 pulley is lined with a synovial sheath. 



The inferior oblique rises from the orbital plate of the superior 

 maxilla close outside the orifice of the nasal duct; the muscle 

 passes out, back, and up between the inferior rectus and floor 

 of the orbit, and is inserted under cover of the external rectus 

 at the back part of the eyeball, nearer the optic nerve than 

 at the cornea. 



Nerves. External rectus by the sixth nerve, superior oblique 

 by the fourth, and the other five by the third nerve. 



Actions. Levator palpebrce is the elevator of the upper 

 lid and antagonist of the palpebral part of the orbicularis. 

 The eyeball seems to move on a central fixed point without 

 shifting its place as a whole within the orbit; four movements 

 are possible: (1) Lateral; (2) elevation and depression; (3) 

 oblique rrfovements of elevation and depression; (4) rotation 

 about a sagittal axis. The external and internal recti produce 

 only lateral movements; the superior and inferior recti have 

 their line of direction internal to the centre of motion, and 

 so produce not only elevation and depression, but also inward 

 direction and slight rotation; this is corrected by the oblique 

 muscles, the inferior oblique being associated with the superior 

 rectus, and superior oblique with the inferior rectus; the superior 



