Pupil

Light must pass through the pupil before it can reach
the lens and be focused onto the retina. Parasympathetic
nerve fibres from the third (oculomotor) cranial nerve
innervate the muscle of the iris that causes constriction
of the pupil, whereas sympathetic nerve fibres control
dilation. The pupillary aperture also narrows when
focusing on close objects and dilates for more distant
viewing. At its maximum contraction, the adult pupil
may be less than 1 mm (0.04 inch) in diameter, and
it may increase up to 10 times to its maximum diameter.
The size of the human pupil may also vary as a result of
age, disease, trauma, or other abnormalities within the
visual system, including dysfunction of the pathways
controlling pupillary movement. Thus, careful evaluation
of the pupils is an important part of both eye and
neurologic exams.