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Pupil

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

          The term pupil can also mean student. For the use in optics, see
          entrance pupil and exit pupil.

   The human eyeThe pupil is the central transparent area (showing as
   black). The greenish-brown area surrounding it is the iris. The white
   outer area is the sclera, the central transparent part of which is the
   cornea.
   Enlarge
   The human eye
   The pupil is the central transparent area (showing as black). The
   greenish-brown area surrounding it is the iris. The white outer area is
   the sclera, the central transparent part of which is the cornea.

   The pupil is the variable-sized, black circular opening in the centre
   of the iris that regulates the amount of light that enters the eye. It
   appears black because most of the light entering the pupil is absorbed
   by the tissues inside the eye.

Comparative anatomy

   In humans and many animals (but few fish), the size of the pupil is
   controlled by involuntary contraction and dilation of the iris, in
   order to regulate the intensity of light entering the eye. This is
   known as the pupillary reflex. In bright light, the human pupil has a
   diameter of about 1.5 millimeters, in dim light the diameter is
   enlarged to about 8 millimeters.

   The shape of the pupil varies between species. Common shapes are
   circular or slit-shaped, although more convoluted shapes can be found
   in aquatic species. The reasons for the variation in shapes are
   complex; the shape is closely related to the optical characteristics of
   the lens, the shape and sensitivity of the retina, and the visual
   requirements of the species.

   Slit-shaped pupils are found in species which are active in a wide
   range of light levels. In strong light, the pupil constricts and is
   small, but still allows light to be cast over a large part of the
   retina.

   The orientation of the slit may be related to the direction of motions
   the eye is required to notice most sensitively (so a vertical pupil
   would increase the sensitivity of the eyes of a small cat to the
   horizontal scurrying of mice). The narrower the pupil, the more
   accurate the depth perception of peripheral vision is, so narrowing it
   in one direction would increase depth perception in that plane. Animals
   like goats and sheep may have evolved horizontal pupils because better
   vision in the vertical plane may be benficial in mountainous
   environments.

   Ground dwelling snakes, such as boas, pythons and vipers, have vertical
   slit-shaped pupils to help them hunt prey on the ground while tree
   snakes have circular pupils. Small cats and foxes also have slit shaped
   pupils while lions and wolves have round pupils even though they are in
   the same family. Some hypothesize that this is because slit pupils are
   more beneficial for animals that hunt small prey than large prey.

   When an eye is photographed with a flash, the iris cannot close the
   pupil fast enough and the blood-rich retina is illuminated, resulting
   in the red-eye effect.

Constriction of the pupil

   The pupils of sheep and goats are horizontal almost rectangular ovals.
   Enlarge
   The pupils of sheep and goats are horizontal almost rectangular ovals.

   When bright light is shone on the eye, it will automatically constrict.
   This is the pupillary reflex, which protects the retina from bright
   light, and is an important test of brainstem function. Also, the pupil
   will constrict if a person sees an object or a person which arouses
   anger or loathing.

   The oculomotor nerve, specifically the parasympathetic part coming from
   the Edinger-Westphal nucleus, terminates on the circular iris sphincter
   muscle. When this muscle contracts, it reduces the size of the pupil.

   Certain drugs cause constriction of the pupils, such as heroin.

   Another term for the constriction of the pupil is miosis. Substances
   that cause miosis are described as miotic.
   Retrieved from " http://en.wikipedia.org/wiki/Pupil"
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