The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.[Cassin, B. and Solomon, S. (1990) Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company.] It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil. The size of the pupil is controlled by the iris, and varies depending on many factors, the most significant being the amount of light in the environment. The term "pupil" was coined by Gerard of Cremona.
In humans, the pupil is circular, but its shape varies between species; some Cat eyes, , and have vertical slit pupils, goats and sheep have horizontally oriented pupils, and some catfish have annular types.
Function
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. This is known as the pupillary light reflex.
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 pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye's fibers. This causes the effect in one eye to carry over to the other.
Effect of light
The pupil gets wider in the dark and narrower in light. When narrow, the diameter be 1.5 to 4 millimeters.
In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8 mm.
However, in any human age group there is considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals. After 25 years of age, the average pupil size decreases, though not at a steady rate.
At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as
hippus. The constriction of the pupil and near vision are closely tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark, this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.
["Sensory Reception: Human Vision: Structure and Function of the Eye" Encyclopædia Brtiannicam Chicago, 1987]
When bright light is shone on the eye, light-sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, will send signals to the oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil. This is the pupillary light reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.
Clinical significance
Effect of drugs
If the drug pilocarpine is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibers, conversely, atropine will cause paralysis of accommodation (cycloplegia) and dilation of the pupil.
Certain drugs cause constriction of the pupils, such as opioids.
The sphincter muscle has a parasympathetic innervation, and the dilator has a sympathetic innervation. In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited. The reverse is true, so control of pupil size is controlled by differences in contraction intensity of each muscle.
Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic. Dilation of the pupil is mydriasis. Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide.
Diseases
A condition called
bene dilitatism occurs when the optic nerves are partially damaged. This condition is typified by chronically widened pupils due to the decreased ability of the optic nerves to respond to light. In normal lighting, people affected by this condition normally have dilated pupils, and bright lighting can cause pain. At the other end of the spectrum, people with this condition have trouble seeing in darkness. It is necessary for these people to be especially careful when driving at night due to their inability to see objects in their full perspective. This condition is not otherwise dangerous.
Size
The size of the pupil (often measured as
diameter) can vary between 8 and 1.5 mm.
Pupil size can be a
symptom of an underlying disease. Dilation of the pupil is known as
mydriasis and contraction as
miosis.
Not all variations in size are indicative of disease however. In addition to dilation and contraction caused by light and darkness, it has been shown that solving simple multiplication problems affects the size of the pupil.
The simple act of recollection can dilate the size of the pupil,
however when the brain is required to process at a rate above its maximum capacity, the pupils contract.
There is also evidence that pupil size is related to the extent of positive or negative emotional arousal experienced by a person.
Near-sightedness individuals have Mydriasis resting and dark dilated pupils than Far-sightedness and Emmetropia individuals, likely due to requiring less accommodation (which results in pupil constriction).
Some humans are able to exert direct control over their iris muscles, giving them the ability to manipulate the size of their pupils (i.e. dilating and constricting them) on command, without any changes in lighting condition or eye accommodation state.
Animals
Not all
have circular pupils. Some have slits or ovals which may be oriented vertically, as in
,
Viperidae,
Felidae and
, or horizontally as in some
Batoidea,
,
and
such as
elk,
red deer,
reindeer and
hippopotamus, as well as the domestic
horse.
,
sheep,
and
octopus pupils tend to be horizontal and rectangular with rounded corners. Some skates and rays have crescent shaped pupils,
pupils range from circular, to a slit, to a series of pinholes,
and the
cuttlefish pupil is a smoothly curving W shape. Although human pupils are normally circular, abnormalities like
can result in unusual pupil shapes, such as teardrop, keyhole or oval pupil shapes.
There may be differences in pupil shape even between closely related animals. In felids, there are differences between small- and large eyed species. The domestic cat (Felis sylvestris domesticus) has vertical slit pupils, its large relative the Siberian tiger (Panthera tigris altaica) has circular pupils and the Eurasian lynx (Lynx lynx) is intermediate between those of the domestic cat and the Siberian tiger. A similar difference between small and large species may be present in canines. The small red fox (Vulpes vulpes) has vertical slit pupils whereas their large relatives, the gray wolf (Canis lupus lupus) and domestic (Canis lupus familiaris) have round pupils.
Evolution and adaptation
One explanation for the
evolution of slit pupils is that they can exclude light more effectively than a circular pupil. This would explain why slit pupils tend to be found in the eyes of animals with a
crepuscular or
nocturnal lifestyle that need to protect their eyes during daylight. Constriction of a circular pupil (by a ring-shaped muscle) is less complete than closure of a slit pupil, which uses two additional muscles that laterally compress the pupil.
For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans.
However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5 mm in the
tarsier) and the rectangular pupils of many
which do not close to a narrow slit in bright light.
An alternative explanation is that a partially constricted circular pupil shades the peripheral zones of the lens which would lead to poorly focused images at relevant wavelengths. The vertical slit pupil allows for use of all wavelengths across the full diameter of the lens, even in bright light.
It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, breaking up the circular outline of the eye.
Activity pattern and behavior
In a study of
, pupil shapes correlated both with
diel activity times and with foraging behavior. Most snake species with vertical pupils were
nocturnal and also ambush foragers, and most snakes with circular pupils were
diurnality and active foragers. Overall, foraging behaviour predicted pupil shape accurately in more cases than did diel time of activity, because many active-foraging snakes with circular pupils were not diurnal. It has been suggested that there may be a similar link between foraging behaviour and pupil shape amongst the felidae and canidae discussed above.
A 2015 study
File:Horizontal Rectangular Pupil of a Goat.jpg|A goat with horizontal rectangular pupils
File:Taeniura_grabata_eye.jpg|A stingray with crescent pupils
File:Crocodylus siamensis closeup.jpg|A crocodile with thin vertical slit pupils
File:Cuttlefish_eye.jpg|A cuttlefish with W-shaped pupils
File:Gecko-oeil.jpg|A gecko with 'thin string of pearls' pupils
File:Catpupil03042006.jpg|A cat with thick vertical slit pupils
Society and culture
The pupil plays a role in
Oculesics and nonverbal communication. The voluntary or involuntary enlargement or dilation of the pupils indicates cognitive
arousal, interest in the subject of attention, and/or
sexual arousal. On the other hand, when the pupil is voluntarily or involuntarily
Miosis, it could indicate the opposite - disinterest or disgust. Exceptionally large or
Mydriasis are also perceived to be an attractive feature in body language.
In a surprising number of unrelated languages, the etymology of the term for pupil is "little person".[" pupil, n.2.", Oxford English Dictionary Online, 3rd. edn (Oxford University Press, 2007).] This may be because the reflection of one's image in the pupil is a minuscule version of one's self.
The English phrase apple of my eye arises from an Old English usage, in which the word apple meant not only the fruit but also the pupil or eyeball.[ apple, n.", Oxford English Dictionary Online, 3rd ed. (Oxford University Press, 2008), § 6 B.]
See also
-
Pupillary response
-
Pupil function
-
Dilated fundus examination
-
Eye contact
-
Horner's syndrome
-
Mydriasis
-
Synechia (eye)
-
Anisocoria
-
Adie's pupil
-
Argyll Robertson pupil
-
Light-near dissociation
-
Marcus Gunn Pupil
External links
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— "Sagittal Section Through the Eyeball"
-
— "Sagittal Section Through the Eyeball"
-
A pupil examination simulator, demonstrating the changes in pupil reactions for various nerve lesions.
