The Pupil is located in the centre of
each eye in the human body.
It generally appears to be the dark "centre" of the eye, but
can be more accurately described as the circular aperture in the centre
of the iris
through which light passes into the eye.
The size of the pupil (and therefore the amount of light that is admitted
into the eye) is regulated by the pupillary reflex (also
known as the "light reflex"). That is, when bright light reaches
the retina, nerves of the parasympathetic nervous system are stimulated,
a ring of muscle around the margin of the iris contracts, the size of
the pupil is reduced, hence less light is able to enter the eye. Conversely,
in dim lighting conditions the pupil opens due to stimulation of the sympathetic
nervous system that contracts of radiating muscles, hence increases the
size of the pupil.
Note that although some animals' eyes
are basically structured in a similar way to human eyes, they may
appear to be very different.
E.g. Differently shaped pupils of cats compared with people.
The subject of visual (also known as "physiological") optics
is a key component of many courses within the fields of both biology and
physics. It is also an essential consideration in the design of displays
and control units used in many applications from televisions and mobile
telephones to advanced aircraft. In the context of engineering "visual
optics" is one of several medical and psychological topics in the
important area of "Human Factors".