Everything about The Pupil totally explained
The
pupil is the opening that's located in the center of the
iris of the eye and that controls 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. In optical terms, the anatomical pupil is the eye's
aperture and the iris is the
aperture stop. The image of the pupil as seen from outside the eye is the
entrance pupil, which doesn't exactly correspond to the location and size of the physical pupil because it's magnified by the
cornea.
Comparative anatomy
In
humans and many
animals (but few
fish), the size of the pupil is controlled by involuntary constriction and dilation of the
iris in order to regulate the intensity of
light entering the eye. This is known as the
pupillary reflex. In normal room light, a healthy human pupil has a diameter of about 3–4 millimeters, in bright light, the pupil has a diameter of about 1.5 millimeters, and in dim light the diameter is enlarged to about 8 millimeters. The narrowing of the pupil results in a greater focal range. (see
aperture for a more detailed explanation)
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 beneficial in mountainous environments.
Many
snakes, such as
boas,
pythons and
vipers, have vertical, slit-shaped pupils that help them to hunt prey under a wide range of light conditions. Small
cats and
foxes also have slit shaped pupils while
lions and
wolves have round pupils even though they're in the same respective families. Some hypothesize that this is because slit pupils are more beneficial for animals that hunt small prey rather than large prey.
When an eye is
photographed with a
flash, the iris can't close the pupil fast enough and the blood-rich
retina is illuminated, resulting in the
red-eye effect.
Constriction of the pupil
When bright light is shone on the eye, it'll automatically constrict. This is the
pupillary reflex, which is an important test of
brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.
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.
The iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the iris to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
Certain drugs cause constriction of the pupils, such as
alcohol and
opiates. Other drugs, such as
atropine and
amphetamines cause pupil dilation.
Another term for the constriction of the pupil is
miosis. Substances that cause miosis are described as
miotic.
Further Information
Get more info on 'Pupil'.
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