Eye is the organ of vision. The paired eyes work in coordination to present NERVE impulses the BRAIN interprets as dimensional (stereovisual) images. The function of sight requires close integration among the structures of the eye, the neurologic system, and the muscular system. Each eye is a fluidfilled, elongated globe of fibrous tissue, about 1⁄4 inch from front to back and 1 inch from top to bottom and side to side, contained within the protective cavity of the orbital socket in the skull. The OPTIC NERVE, the second cranial nerve, provides a direct pathway from the back of the eye to the brain. Six muscles move each eye up and down, from side to side, and in rotation. These muscles direct the eye toward objects within the VISUAL FIELD and hold the eyes steady.
The process of vision begins when lightwaves enter the eye through the CORNEA, a transparent portion of the eye’s tough outer layer, the sclera. The cornea’s convex front surface initially refracts the lightwaves for preliminary focusing. The cornea is soft and flexible but fixed; it does not adjust or move. The LENS, a transparent and flexible convex disk behind the cornea, further refracts the lightwaves. Tiny muscles at the edge of the lens, the ciliary muscles, cause the lens to thicken or flatten to adjust the degree of refraction for optimal focus. The resulting light pattern strikes the RETINA, activating the specialized cells that detect color (cones) and brightness (rods). These cells convert the light to nerve impulses that converge at the back of the retina at the optic disk, their portal to the optic nerve. The optic nerve conveys the signals to the brain, which interprets them as images.
|Common Conditions of the Eye|
|DRY EYE SYNDROME||EYE STRAIN|
|ISCHEMIC OPTIC NEUROPATHY||KERATITIS|
|RETINAL DETACHMENT||RETINITIS PIGMENTOSA|
For further discussion of the eye within the context of ophthalmologic structure and function please see the overview section “The Eyes.”
See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; CRANIAL NERVES.
Resource: Facts On File Encyclopedia Of Health And Medicine
Eye is the organ of vision. The paired eyes work in coordination to present NERVE impulses the BRAIN interprets as dimensional (stereovisual) images. The function of sight requires close integration among the structures of the eye, the neurologic system, and the muscular system. Each eye is a fluidfilled, elongated globe of fibrous tissue, about 1⁄4
Xanthelasma - deposits of fatty plaque that form blisterlike lesions on the eyelids, usually the upper eyelids near the corner of the NOSE. The lesions are yellowish in color and often indicate HYPERLIPIDEMIA (elevated BLOOD levels of cholesterol and triglycerides). Though harmless, the lesions can cause the eyelid to droop, obscuring vision when an upper
Vitreous Detachment - the separation of the vitreous humor, the gelatinous substance within the EYE, from the RETINA. Vitreous detachment commonly occurs with advancing age as the vitreous humor thins and takes on more of a liquid consistency. By itself vitreous detachment is harmless and has no effect on vision, though it typically produces FLOATERS
Retina - the innermost layer of the EYE. The retina receives light images and converts them to NERVE impulses the OPTIC NERVE conveys to the BRAIN. The retina is two tissue-thin layers that together are less than 1⁄2 millimeter in thickness. The outer pigment layer provides a completely light-absorbing, nonreflective lining. The inner sensory layer
Visual Acuity - the ability to see objects clearly and sharply. Visual acuity assesses central vision and represents the function of the CORNEA, iris (pupil size), LENS, and RETINA. The SNELLEN CHART, which presents lines of letters of diminishing size, is the standard measure of visual acuity. Environmental factors that influence visual acuity include
Visual Field - the total area or scope of vision. EYE care specialists map the visual field by measuring the boundaries of peripheral vision in degrees from the point of central vision. A normal field of vision is 135 degrees vertically (60 degrees up and 75 degrees down) and 160 degrees horizontally (100 degrees outward and 60 degrees inward). Everyone has
Vitrectomy - an OPERATION to remove the vitreous humor from within the EYE as treatment for RETINAL DETACHMENT, vitreous HEMORRHAGE (bleeding into the vitreous humor), RETINOPATHY, and foreign body penetration. In vitrectomy, the ophthalmologist makes three tiny incisions in the sclera (white portion) of the eye for the insertion of a cutting instrument, a
Vision Health - personal care for the eyes to protect the eyes and preserve vision. The two most important elements of vision health are EYE protection and regular ophthalmic examinations. Vision Health and Protective Eyewear Most injuries to the eyes are preventable by wearing appropriate protective eyewear, which ranges from sunglasses to protect the
Vision Impairment Definition Vision Impairment - the uncorrectable loss of vision. About 12 million Americans have vision impairments that prevent them from participating in occupations and activities that have requirements or legal standards for VISUAL ACUITY (the ability to see clearly) and VISUAL FIELD (the scope of peripheral vision). People who have
Uveitis - INFLAMMATION of the uveal structures of the EYE, which include the iris, ciliary body, and choroid. Uveitis most commonly affects the front of the eye (anterior uveitis) though may involve specific segments of the eye or the uveal tract throughout the eye (diffuse uveitis). Symptoms of Uveitis Symptoms can vary from mild to severe and may