Pupil
- The term pupil can also mean student.
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.
In the eye, the pupil is the hole in the middle of the iris. It appears black because most of the light entering it is absorbed by the tissues inside the eye. 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.
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).
In snakes, slit-shaped pupils are associated with venomous species, while non-venomous snakes have round pupils.
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.
Dilation of the pupil
The human pupil dilates if the person sees something (or someone) of interest or is aroused. Studies have shown that humans (especially females) are judged as more attractive if their pupils are wide open and more dilated than normal. The name Belladonna (beautiful lady) comes from the fabled use of the juices of the Nightshade plant by Italian women who would use eyedrops made from an extract of the plant in their eyes in order to enlarge their pupils and make their eyes appear more beautiful. In some cases gradual blindness has occurred through the overuse and abuse of belladonna to enhance the beauty of the eyes. Atropine, a chemical found in belladonna, is used by modern eye doctors to dilate the pupils so that they can examine the retina. Excessive dilation is called mydriasis.
Dilation occurs when signals from sympathetic fibers (these fibres travel from the sympathetic trunk up to the face) cause the iris dilator muscles (dilator pupilae ) to contract. Damage to the sympathetic tract to the face (cranial nerve VII) results in Horner's syndrome, a symptom of which is excessive pupillary constriction (miosis).
Hallucinogenic drugs, such as LSD and psilocybin, and some stimulants such as cocaine also cause dilation of the pupils.
Constriction of the pupil
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.
The oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus , terminates on a circular muscle (a sphincter) within the iris. When this muscle contracts, it reduces the size of the pupil.
Certain drugs cause constriction of the pupils, such as heroin.
See also
