eye definition science

Posted Aug 27, 2009 In addition to the muscles already described, other facial muscles often cooperate in the act of lid closure or opening. QUESTION What causes dry eyes? a disease of plants, characterized by elliptical lesions on the … Two ligaments, the medial and lateral palpebral ligaments, attached to the orbit and to the septum orbitale, stabilize the position of the lids in relation to the globe. “Eye.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/eye. You may experience dry eyes in certain situations, such as on an airplane, in an a… These nerves may carry nonvisual sensory messages—e.g., pain—or they may be motor nerves controlling the muscles of the eye. The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and lids but permits a considerable degree of rotation of the eyeball in the orbit. Is it true that we only use ten percent of our brain? Both pupils are usually of equal size. The eye is kept moist by secretions of the lacrimal glands (tear glands). Please tell us where you read or heard it (including the quote, if possible). Dissect these questions and learn more about human organs. The opening of the iris forms the pupil. The orbital portion of the orbicularis is not normally concerned with blinking, which may be carried out entirely by the palpebral portion; however, it is concerned with closing the eyes tightly. Our eye doctor will provide the latest in eye exams and eye care treatment, and thorough answers to your questions and concerns. The eye is protected from mechanical injury by being enclosed in a socket, or orbit, which is made up of portions of several of the bones of the skull to form a four-sided pyramid, the apex of which points back into the head. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Send us feedback. Plural irises irides (ī′rĭ-dēz′, ĭr′ĭ-) The colored, muscular ring around the pupil of the eye in vertebrate animals, located between the cornea and lens. Delivered to your inbox! The pupil may appear to open (dilate) and close (constrict), but it is really the iris that is the prime mover; the pupil is merely the absence of iris. It must be appreciated that the two portions can be activated independently; thus, the orbital portion may contract, causing a furrowing of the brows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and allows the eyes to remain open. Get a Britannica Premium subscription and gain access to exclusive content. The muscle of Riolan, lying close to the lid margins, contributes to keeping the lids in close apposition. To protect the eye the bones of the skull are shaped so that an orbital cavity protects the dorsal aspect of each eyeball. Updates? Learn a new word every day. In the embryo the eye develops as a direct extension of the brain, and thus is a very delicate organ. The eye is a region of mostly calm weather at the center of tropical cyclones.The eye of a storm is a roughly circular area, typically 30–65 kilometers (19–40 miles) in diameter. The orbicularis and levator are striated muscles under voluntary control. Opening of the eye is not just the result of passive relaxation of the orbicularis muscle but also is the effect of the contraction of the levator palpebrae superioris muscle of the upper lid. Eyespot, a heavily pigmented region in certain one-celled organisms that apparently functions in light reception. before the 12th century, in the meaning defined at sense 1a, 15th century, in the meaning defined at transitive sense 1a(1), Middle English, from Old English ēage; akin to Old High German ouga eye, Latin oculus, Greek ōps eye, face, Sanskrit aká¹£i eye. The lids are essentially folds of tissue covering the front of the orbit and, when the eye is open, leaving an almond-shaped aperture. The term is also applied to certain light-sensitive cells in the epidermis (skin) of some invertebrate animals (e.g., worms, starfishes). If you have dry eyes, your eyes may sting or burn. Articles from Britannica Encyclopedias for elementary and high school students. Is the retina the lens at the front of the eye? A concave lens is thinner in the middle and thicker near the edges, and it causes light rays that pass through it to spread apart. Eyes are animal organs that are specialized for sight. Omissions? The outermost layer of the lid is the skin, with features not greatly different from skin on the rest of the body, with the possible exception of large pigment cells, which, although found elsewhere, are much more numerous in the skin of the lids. Contraction and expansion of the iris controls the size of the pupil, thereby regulating the amount of light reaching the retina. Author of. The medial ligament is by far the stronger. Eyes are amazing and complex organs. Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free! The lids also contain smooth (involuntary) muscle fibres that are activated by the sympathetic division of the autonomic system and tend to widen the palpebral fissure (the eye opening) by elevation of the upper, and depression of the lower, lid. It is divided into orbital and palpebral portions, and it is essentially the palpebral portion, within the lid, that causes lid closure.

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