THE EYE AS AN OPTICAL INSTRUMENT 247 



arises behind from the bony margin of the foramen through which 

 the optic nerve enters the orbit. In the figure, which represents 

 the orbits opened from above, the superior rectus of the right 

 side has been removed. The superior oblique or pulley (trochlear) 

 muscle, t, arises behind near the straight muscles and forms an- 

 teriorly a tendon, u, which passes through a fibrocartilaginous 

 ring, or pulley, placed at the notch in the frontal bone where it 



FIG. 80. The eyeballs and their muscles as seen when the roof of the orbit 

 has been removed and the fat in the cavity has been partly cleared away. On the 

 right side the superior rectus muscle has been cut away, a, external rectus; s, su- 

 perior rectus; i, internal rectus; t, superior oblique. 



bounds superiorly the front end of the orbit. The tendon then 

 turns back and is inserted into the eyeball between the upper and 

 outer recti muscles. The inferior oblique muscle does not arise, 

 like the rest, at the back of the orbit, but near its front at the 

 inner side, close to the lachrymal sac. It passes thence outwards 

 and backwards beneath the eyeball to be inserted into its outer 

 and posterior part. 



The inner, upper, and lower straight muscles, the inferior 

 oblique, and the elevator of the upper lid are supplied by branches 

 of the third cranial nerve. The sixth cranial nerve goes to the 

 outer rectus; and the fourth to the superior oblique. 



The eye may be moved from side to side ; up or down ; obliquely, 



