Light energy enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina ( photoreceptor cones and rods) are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. Further information: Arthropod eye An image of a house fly compound eye surface by using scanning electron microscope Anatomy of the compound eye of an insect Arthropods such as this bluebottle fly have compound eyes

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This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above). [16] Multiple lenses The average human eye can see around 100 different shades of color and has a resolution that equals 576 gigapixels. These remarkable features of our eye are enabled by the complex structure of the eyeball. The eyeball consists of three layers; fibrous, vascular and nervous ( retina). Functionally, the most important layer is the retina, which receives the external visual stimuli. The posterior pole of the eyeball is connected with the optic nerve (CN II), which conveys the information from the retina to the brain. After the processing in the cerebral cortex, the visual stimuli become visual information, i.e. the conscious perception of a human’s surroundings. Wagner, H.J.; Douglas, R.H.; Frank, T.M.; Roberts, N.W. & Partridge, J.C. (Jan 27, 2009). "A Novel Vertebrate Eye Using Both Refractive and Reflective Optics". Current Biology. 19 (2): 108–114. doi: 10.1016/j.cub.2008.11.061. PMID 19110427. S2CID 18680315. Rolfs, Martin (2009). "Microsaccades: Small steps on a long way". Vision Research. 49 (20): 2415–2441. doi: 10.1016/j.visres.2009.08.010. PMID 19683016.This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes. [9] Other The Retina of the Human Eye". hyperphysics.phy-astr.gsu.edu. Archived from the original on 2015-05-04 . Retrieved 2015-06-03. Each eye has seven extraocular muscles located in its orbit. [5] Six of these muscles control the eye movements, the seventh controls the movement of the upper eyelid. The six muscles are four recti muscles – the lateral rectus, the medial rectus, the inferior rectus, and the superior rectus, and two oblique muscles the inferior oblique, and the superior oblique. The seventh muscle is the levator palpebrae superioris muscle. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. [6] Thus, the eye can be considered as undergoing rotations about a single point in the centre of the eye. a b c d e f Land, M.F. (1989). "The eyes of hyperiid amphipods: relations of optical structure to depth". Journal of Comparative Physiology A. 164 (6): 751–762. doi: 10.1007/BF00616747. S2CID 23819801.

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a b c d e f Nilsson, E.; Arendt, D. (Dec 2008). "Eye Evolution: the Blurry Beginning". Current Biology. 18 (23): R1096–R1098. doi: 10.1016/j.cub.2008.10.025. PMID 19081043. S2CID 11554469.The size of the eye differs among adults by only one or 2 millimetres. The eyeball is generally less tall than it is wide. The sagittal vertical (height) of a human adult eye is approximately 23.7mm (0.93in), the transverse horizontal diameter (width) is 24.2mm (0.95in) and the axial anteroposterior size (depth) averages 22.0–24.8mm (0.87–0.98in) with no significant difference between sexes and age groups. [2] Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88). [2] The typical adult eye has an anterior to posterior diameter of 24mm (0.94in), and a volume of 6 cubic centimetres (0.37cuin). [3] white or pale skin – eye melanoma mostly affects white people and is more common in those with fair skin The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones. These three layers comprise the circular outline of the eyeball. The inside of the eye contains the two refractive structures of the eye called the lens and vitreous body. Together with the cornea and aqueous humor, the vitreous body and lens belong to the refractive media of the eyeball. The role of refractive structures to bend the direction of the light that falls onto the eye and focus it onto the retina.

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The eye is made up of three coats, or layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the cornea and sclera, which provide shape to the eye and support the deeper structures. The middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly). Nairne, James (2005). Psychology. Belmont: Wadsworth Publishing. ISBN 978-0-495-03150-5. OCLC 61361417. Archived from the original on 2023-01-17 . Retrieved 2020-10-19.There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp, Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye. If your GP or optician (optometrist) suspects you have a serious problem with your eyes, they'll refer you to a specialist eye doctor called an ophthalmologist for an assessment. The inner third of the sclera is pierced by the numerous fibers of optic nerve forming a sievelike structure known as the lamina cribrosa. Besides the optic nerve axons, lamina cribrosa allows the passage of the central retinal artery and vein. a b Jochen Zeil; Maha M. Al-Mutairi (1996). "Variations in the optical properties of the compound eyes of Uca lactea annulipes" (PDF). The Journal of Experimental Biology. 199 (7): 1569–1577. doi: 10.1242/jeb.199.7.1569. PMID 9319471. Archived (PDF) from the original on 2009-02-25 . Retrieved 2008-09-15. Tombs, Selina; Silverman, Irwin (2004-07-01). "Pupillometry: A sexual selection approach". Evolution and Human Behavior. 25 (4): 221–228. doi: 10.1016/j.evolhumbehav.2004.05.001. ISSN 1090-5138.

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Chu WK, Choi HL, Bhat AK, Jhanji V. Pterygium: new insights. Eye (Lond). 2020 Jun;34(6):1047-1050. doi:10.1038/s41433-020-0786-3. Cunningham, Emmett T.; Riordan-Eva, Paul (2011-05-17). Vaughan & Asbury's General Ophthalmology (18thed.). New York: McGraw-Hill Medical. ISBN 978-0-07-163420-5. Nilsson, Dan-E. (1989). "Vision optics and evolution". BioScience. 39 (5): 298–307. doi: 10.2307/1311112. JSTOR 1311112. a b Saladin, Kenneth S. (2011). Anatomy & physiology: the unity of form and function (6thed.). New York: McGraw-Hill. pp.620–622. ISBN 978-0-07-337825-1. Hikichi, T; Yoshida, A; Fukui, Y; Hamano, T; Ri, M; Araki, K; Horimoto, K; Takamura, E; Kitagawa, K; Oyama, M (September 1995). "Prevalence of dry eye in Japanese eye centres". Graefe's Archive for Clinical and Experimental Ophthalmology. 233 (9): 555–558. doi: 10.1007/BF00404705. PMID 8543205. S2CID 20759190.