Basic Eye Anatomy
Important Parts of the Eye
Cornea: The transparent anterior portion of an eye serving as the first and largest refracting medium (approximately 70% of the refractive power of the eye).
Iris: The function of the iris is to regulate the amount of light entering the eye by changing the size of the pupil formed by the iris, and thus acting as a diaphragm.
Lens: A transparent substance in the eye that allows rays of light to converge or to diverge onto the retina.
Macula: A uniquely pigmented area of the eye that allows fine details to be seen more clearly.
Optic Nerve: Composed of fibers from all the retinal ganglion cells (over 800,000) and responsible for carrying impulses from the retina to the geniculate body of the brain resulting in the sense of vision.
Pupil: The opening in the iris that allows light to reach the retina. The size of the pupillary aperture is controlled by the sphincter muscle of the iris. Under normal illumination a human pupil ranges from 2 to 5 millimeters in diameter.
Vitreous: A non-cellular, transparent, colorless mass of soft, gelatinous material filling the posterior 4/5 of the eyeball between the crystalline lens and the retina.
Retina: The light-sensitive layer of tissue that lines the back of the eyeball and sends visual impulses through the optic nerve to the brain.
When light enters the eye initially, it is inverted by the cornea and lens and projected onto the back of the eye. Contained within the retina are seven layers of alternating cells and processes that translate light from the pupil into a signal.
Types of Refractive Errors
Myopia: Myopia is classified as the condition of the eye when accommodation is relaxed and when parallel rays of light entering the eye focus in front of the retina. A minus-powered lens is required to correct this condition, which is commonly referred to as "near-sighted."
Hyperopia: Hyperopia is a refractive condition of the eye where the conjugate focus is behind the retina when accommodation is relaxed. It occurs when parallel rays of light entering the eye focus behind the retina in the absence of accommodation. Commonly referred to as "far sighted.”
Astigmatism: Astigmatism is a refractive error in which light rays entering the eye fail to come to a single focal point. It is caused by different degrees of refraction in various meridians of the cornea and/or lens, most often at right angles to each other.
Presbyopia: From the Greek words meaning “old eye.” Presbyopia is the condition where the crystalline lens is no longer able to accommodate sufficiently to provide clear near vision. The condition becomes noticeable in humans sometime after the age of 40 and may be corrected with bifocal, trifocal, or progressive spectacle lenses, as well as contact lenses.
Diseases of the eye
Cataracts: Cataract, from the Latin word meaning “waterfall”, is a condition where clouding develops in the crystalline lens of the eye or its envelope. It can vary in degree from slight to complete opacity. Visual acuity becomes affected and individuals have difficulty identifying color and shadows.
Glaucoma: Glaucoma involves loss of retinal ganglion cells affecting the optic nerve. Glaucoma can be degenerative over a long period of time, often occurring without the affected individual even realizing there is a problem. The condition is most common among elderly segments of a population.
Age-Related Macular Degeneration: Age-Related Macular Degeneration (AMD) occurs as the result of damage to the retina with the loss of vision in the center of the visual field. As its label indicates, the disease is most prevalent among the elderly. It is one of the leading causes of blindness in people over the age of 50.
Diabetic Retinopathy: This is the result of the onset of diabetes mellitus and can eventually lead to blindness. Put simply, damage to the eyes can occur as a result of improper diabetic maintenance. Small blood vessels located in the eye can be vulnerable to poor blood sugar control, and may lead to additional vision complications over time or even complete loss of vision.
Detached Retina: An eye disorder that occurs when the retina peels away from its underlying layer of support tissue. Even nominal detachment is considered a medical emergency and can lead to vision loss and blindness.
Strabismus: Strabismus is present when the eyes are improperly aligned with one another. The affected individual’s gaze lacks coordination due to weak or impaired extraocular muscles. It is a common condition that can be the result of either muscle dysfunction or incorrect brain coordination.