THE EYEBALL
This illustration shows the 
eyeball and the muscles that 
control its position. The cornea 
and the lens focus the light 
rays onto the back of the eye. 
The lens regulates the focusing 
for near and far objects by 
becoming more or less globular.

    The collective function of 
the nonretinal parts of the eye 
is to keep a focused, clear image 
of the outside world anchored 
on the two retinas. Each eye is 
positioned in its socket by the 
six small extraocular muscles 
mentioned in Chapter 2. That 
there are six for each eye is no 
accident; they consist of three 
pairs, with the muscles in each 
pair working in opposition, so 
as to take care of movements in 
one of three orthogonal 
(perpendicular) planes. For both 
eyes, the job of tracking an 
object has to be done with a 
precision of a few minutes of 
arc--or else we see double. (To 
see how distressing that can be, 
try looking at something and 
pressing on the side of one eye 
with your index finger.) Such 
precise movements require a 
collection of finely tuned 
reflexes, including those that 
control head position.