The human eye
The pupil is the central transparent area (showing as black). The greenish-brown area surrounding it is the iris. The iris consists of Pigmented Fibrovascular tissue known as a stroma. The white outer area is the sclera, the central transparent part of which is the cornea. The sclera, called the white or white of the eye, is the opaque (usually white though certain animals such as horses and lizards can have black sclera fibrous The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber.

Schematic diagram of the human eye.

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. The iris consists of Pigmented Fibrovascular tissue known as a stroma. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain ^[1] It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. Tissue is a cellular organizational level intermediate between cells and a complete organism 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. In an optical system the entrance pupil is a virtual aperture that defines the area at the entrance of the system that can accept light The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber.

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. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two The iris consists of Pigmented Fibrovascular tissue known as a stroma. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 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. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. Common shapes are circular or slit-shaped, although more convoluted shapes can be found in aquatic species. An aquatic animal is an Animal which lives in water for most or all of the time 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. The vertebrate retina is a light sensitive part inside the inner layer of the Eye.

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. Peripheral vision is a part of vision that occurs outside the very center of gaze ^[2] Animals like goats and sheep may have evolved horizontal pupils because better vision in the vertical plane may be beneficial in mountainous environments. ^[3]

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. A snake is an elongate Reptile of the suborder Serpentes Like all reptiles snakes are covered in scales. Boa Kwon (born November 5, 1986) have contributed to her commercial success in South Korea and Japan and her popularity throughout Asia Common names pythons The Pythonidae are a family of non- Venomous Snakes found in Africa, Asia The Viperidae are a family of Venomous snakes commonly referred to as vipers, although the term viperids is more specific and distinguishes them from 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. WikipediaManual of Style (spelling, articles should conform to one overall spelling style of English typically the one most linked to the article topic (if it is geographic A fox is an Animal belonging to any one of about 27 Species (of which only 12 actually belong to the Vulpes genus or 'true foxes' of small The lion ( Panthera leo) is a member of the family Felidae and one of four Big cats in the Genus Panthera. The grey wolf or gray wolf ( Canis lupus) also known as the timber wolf or simply wolf, is a Mammal of the order Carnivora Some hypothesize that this is because slit pupils are more beneficial for animals that hunt small prey rather than large prey. ^[4]

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. A photograph (often shortened to photo) is an Image created by Light falling on a light-sensitive surface usually Photographic film or an electronic A flash is a device used in Photography that produces an instantaneous flash of artificial Light The vertebrate retina is a light sensitive part inside the inner layer of the Eye. The red-eye effect in Photography is the common appearance of Red Pupils in color Photographs of Eyes It occurs when using a photographic

Constriction of the pupil

The pupils of sheep and goats are horizontal, almost rectangular, ovals. The domestic goat ( Capra aegagrus hircus) is a subspecies of goat Domesticated from the Wild goat of Southwest Asia and Eastern Europe

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. The brain stem (or brainstem) is the lower part of the Brain, adjoining and structurally continuous with the Spinal cord. 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. The oculomotor nerve is the third of twelve paired Cranial nerves. The parasympathetic Nervous system ( PSNS) is a division of the Autonomic nervous system (ANS along with the Sympathetic nervous system The Edinger-Westphal nucleus (also known as the accessory oculomotor nucleus) is the accessory parasympathetic Cranial nerve nucleus of the Oculomotor The iris sphincter muscle ( pupillary sphincter, circular muscle of iris, circular fibers) is a Muscle in the part of the Eye 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. In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon For other uses see Opiate (disambiguation, or for the class of drugs see Opioid. Other drugs, such as atropine and amphetamines cause pupil dilation. Atropine is a Tropane Alkaloid extracted from Deadly nightshade ( Atropa belladonna) Jimsonweed (Datura stramonium and other plants Amphetamine, and related drugs such as Methamphetamine are a group of drugs that act by increasing levels of Norepinephrine, Serotonin, and Dopamine

Another term for the constriction of the pupil is miosis. Miosis is Constriction of the Pupil of the Eye. This is a normal response to an increase in light but can also be associated with certain Pathological Substances that cause miosis are described as miotic. Miosis is Constriction of the Pupil of the Eye. This is a normal response to an increase in light but can also be associated with certain Pathological

See also

• Pupil function (Eye examination)
• Dilated fundus examination
• Eye contact
• Horner's syndrome
• Mydriasis
• Synechia
• Anisocoria
• Adie's pupil
• Argyll Robertson pupil
• Marcus Gunn pupil
• Light Near dissociation

Additional images

Iris, front view. An eye examination is a battery of tests performed by an Optometrist or Ophthalmologist assessing vision and ability to focus on and discern Dilated fundus examination (DFE is diagnostic procedure that employs the use of mydriatic Eye drops to dilate or enlarge the Pupil Eye contact is an event in which two people look at each other's Eyes at the same time Horner's syndrome is a clinical Syndrome caused by damage to the Sympathetic nervous system. "Dilated pupil" redirects here An Eye examination sometimes requires the dilation of the pupil In medicine synechia refers to an adhesion, usually involving the iris (see article below but also within the uterus as in Asherman's syndrome. Anisocoria is a condition characterized by an unequal size of the Pupils Causes In the absence of any deformities of the iris or eyeball proper anisocoria is Adie syndrome, sometimes known as Holmes-Adie's syndrome or Adie's Tonic Pupil, is a Neurological disorder which affects the Pupil of the eye Argyll Robertson pupils (“AR pupils” are bilateral small Pupils that constrict when the patient focuses on a near object (they “ accommodate ” but do Marcus Gunn pupil is a Medical sign observed during the Swinging-flashlight test whereupon the patient's Pupils constrict less (therefore appearing to dilate

References

1

External links

• Atlas of anatomy at UMich eye_1 - "Sagittal Section Through the Eyeball"
• Atlas of anatomy at UMich eye_2 - "Sagittal Section Through the Eyeball"
• A pupil examination simulator, demonstrating the changes in pupil reactions for various nerve lesions. The University of Michigan Ann Arbor ( U of M, U-M, UM or simply Michigan) is a top-ranked Coeducational public research The University of Michigan Ann Arbor ( U of M, U-M, UM or simply Michigan) is a top-ranked Coeducational public research