AskDefine | Define pupil

Dictionary Definition

pupil

Noun

1 a learner who is enrolled in an educational institution [syn: student, educatee]
2 contractile aperture in the iris of the eye
3 a young person attending school (up through senior high school) [syn: schoolchild, school-age child]

User Contributed Dictionary

English

Pronunciation

Etymology 1

From pupille, from pupillus, variant of pupulus, from pupus.

Noun

  1. In the context of "law|obsolete": An orphan who is a minor and under the protection of the state.
  2. A student under the supervision of a teacher or professor.
Translations
student
  • Albanian: nxënës
  • Croatian: učenik, đak
  • Czech: žák
  • Faroese: næmingur
  • Finnish: oppilas
  • French: élève
  • German: Schüler, Schülerin
  • Greek: μαθητής, μαθήτρια
  • Hungarian: tanuló, diák
  • Icelandic: nemandi
  • Japanese: 児童 (じどう, jidō), 生徒 (せいと, seito), 学生 (がくせい, gakusei), 弟子 (でし, deshi)
  • Portuguese: pupilo , pupila
  • Romanian: elev, eleva
  • Russian: ученик (učeník), ученица (učeníca)
  • Serbian: ученик, ученица
  • Slovene: učenec , učenka
  • Swedish: elev
Related terms

Etymology 2

From pupille, from pupilla, named because of the small reflected image seen when looking into someone's eye.

Noun

  1. The hole in the middle of the iris of the eye, through which light passes to be focused on the retina.
Translations
the hole in the middle of the iris of the eye
  • Czech: zornice
  • Faroese: sjónop
  • Finnish: pupilli
  • French: pupille, prunelle
  • German: Pupille , Sehloch
  • Greek: κόρη
  • Hungarian: pupilla
  • Japanese: 瞳孔 (どうこう, dōkō)
  • Polish: źrenica
  • Portuguese: pupila
  • Romanian: pupilă
  • Russian: зрачок
  • Slovene: zenica
  • Spanish: pupila
  • Swedish: pupill
Derived terms

Extensive Definition

The pupil is the opening that is 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 does not exactly correspond to the location and size of the physical pupil because it is 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 are 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 cannot close the pupil fast enough and the blood-rich retina is illuminated, resulting in the red-eye effect.

Constriction of the pupil

contradict-other miosis
When bright light is shone on the eye, it will 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.

Additional images

References

1

External links

  • - "Sagittal Section Through the Eyeball"
  • - "Sagittal Section Through the Eyeball"
  • A pupil examination simulator, demonstrating the changes in pupil reactions for various nerve lesions.
pupil in Arabic: حدقة
pupil in Bengali: পিউপিল
pupil in Bulgarian: Зеница
pupil in Catalan: Pupil·la
pupil in Danish: Pupil
pupil in German: Pupille
pupil in Modern Greek (1453-): Κόρη (ανατομία)
pupil in Spanish: Pupila
pupil in Esperanto: Pupilo
pupil in Basque: Begi-nini
pupil in French: Pupille
pupil in Galician: Pupila
pupil in Italian: Pupilla (anatomia)
pupil in Latin: Pupilla
pupil in Lithuanian: Vyzdys
pupil in Dutch: Pupil (oog)
pupil in Japanese: 瞳孔
pupil in Norwegian: Pupill
pupil in Polish: Źrenica
pupil in Portuguese: Pupila
pupil in Romanian: Pupilă
pupil in Russian: Зрачок
pupil in Simple English: Pupil (eye)
pupil in Slovak: Zrenica
pupil in Slovenian: Dijak
pupil in Finnish: Pupilli
pupil in Swedish: Pupill
pupil in Yiddish: אויג אפעל
pupil in Chinese: 瞳孔

Synonyms, Antonyms and Related Words

apprentice, auditor, autodidact, baby blues, banjo eyes, beginner, bright eyes, catechumen, clear eyes, cornea, disciple, educatee, eye, eyeball, eyelid, goggle eyes, inquirer, iris, learner, lens, lid, monitor, naked eye, neophyte, nictitating membrane, novice, oculus, optic, optic nerve, orb, organ of vision, peeper, popeyes, praepostor, prefect, retina, saucer eyes, scholar, schoolboy, schoolgirl, sclera, self-taught person, starry orbs, student, studier, trainee, tyro, unaided eye, visual organ
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