Looking into a mirror, you will easily be able to see your pupils.
They are the round dark centers of the eye or the gap within the
iris, the coloured part of the eye. Health care professionals
assessing a patient’s pupils can provide insight into the health of
not only the eye but also the rest of the body.
The size and shape of the pupil is controlled by nerve impulses
from the brain. There are actually two different classes of nerves.
The first class is responsible for making the pupil larger (dilated),
while the other makes the pupil smaller (constricted).The dilating
group of nerve fibres originates from the lower portion of the
brain. They travel down to the thoracic region of the spinal cord
then back up to the head along the internal carotid artery and
eventually through the back of the eye. The constricting group of
nerve fibres originates from the lower portion of the brain, but
travels into the head and enters through the back of the eye. The
balance between these two types of nerve fibres determines the
size of your pupil.
The amount of light entering the eye can change the size of the
pupil. This is called the light reflex. Increasing the amount of light
will cause the pupil to constrict. In dim light conditions, the pupil
will dilate. When focusing on an object that is close to your face,
the pupils will constrict. This is called the near or accommodative
When assessing a patient’s pupils, we look at the pupil size, shape,
reaction to light, and any asymmetry between the pupils.
The normal size of pupils ranges from 3mm to 8mm in diameter.
Normally pupils are round however they can take an oval or
distorted shape due to previous eye inflammation, trauma,
intraocular eye surgery or congenital defects.

Abnormalities in pupils to the reaction of light can be caused by
optic nerve disease due to inflammation or reduce blood flow.
Significant arterial occlusions in the retinal blood vessels can also
cause pupil reaction changes as well.
Usually the pupils have the same diameter between left and right.
On occasion there can be a difference in the size between the eyes.
This is referred to anisocoria. For some people this can be normal.
In other instances this can be caused by certain eye drops, trauma,
intraocular surgery, tumours or blood vessel damage near the
pathways of the nerve fibres responsible for controlling the size of
the pupil.
The next time you look in the mirror, pay particular attention to
both your pupils. If anything should appear out of the ordinary
regarding your pupil size or shape, do not hesitate to consult the
care of your local optometrist or ophthalmologist.