Eye (human) - definition and function
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.