THE EYES
The eyes conduct the function of vision. Practitioners who provide care for the eyes and vision may be ophthalmologists (medical doctors who specialize in ophthalmology, providing medical and surgical treatment for diseases of the EYE) or optometrists (doctors of optometry who specialize in diagnosing and correcting REFRACTIVE ERRORS of vision). This section, “The Eyes,” presents a discussion of the structures of the eye and how they function to provide the sense of sight, an overview of VISION HEALTH and disorders, and entries about the health conditions that can affect the eyes and vision.
Structures of the Eye
Functions of the Eye
Ancient philosophers viewed the eyes as the windows to the soul, based on the belief that the PINEAL GLAND, located deep within the BRAIN, held the soul. Their rudimentary understanding of anatomy and physiology led them to conclude that the optic nerves connected the pineal gland and the soul directly to the outside world through the eyes. Though modern knowledge of the body’s structure and function clarifies that no such physical pathway exists, ancient scientists were not entirely off track. The pineal gland does appear to receive direct information from the external environment, which influences its production of MELATONIN, a HORMONE related to the body’s circadian cycles (cycles of wakefulness and sleep). Researchers do not fully understand the mechanisms of this, and it is possible the OPTIC NERVE plays some role. However, the primary function of the optic NERVE is to provide a direct conduit from the EYE to the brain through which the brain receives about two thirds of the information it processes about the environment out- side the body.
The eye resides within the protective enclosure of the orbit, a socket of BONE in the skull. Thin pads of fat cover the orbital bones to cushion the eye. A small opening in the back of the orbit allows passage of the optic nerve and the blood vessels that supply the eye. The eyelids, upper and lower, blink—automatically open and close—15 to 20 times a minute to rinse the eye with tears. Reduced blink rate is a characteristic of neurologic disorders such as PARKINSON’S DISEASE; increased blink rate occurs with eye irritation such as CON- JUNCTIVITIS and diseases such as MENINGITIS. The tears then drain from the lacrimal sac at the inner corner of the eye into the upper NOSE. The eyelids also close to protect the eye from hazards such as foreign objects and very bright light, and to cover the eye during sleep to keep it moist. The eye- lashes, extending from the eyelids, also help keep foreign objects from striking the eye and the eye- brows channel sweat around the eyes.
Six muscles attach the eye to the orbit, functioning in pairs as well as in coordination with one another to move the eye. These muscles integrate into the sclera, the fibrous outer layer of the eye, and extend to the back of the orbit where they anchor to the bone. When one MUSCLE in a pair contracts, the other relaxes. Typically both eyes move in tandem, which allows the eyes to simultaneously focus on the same object. This
provides depth perception and accommodates each eye’s “blind spot.” Some people have the ability to intentionally move their eyes independent of each other, though unintentional disparate movement generally indicates a pathologic condition. Discordant movement may characterize neurologic disorders such as progressive supranuclear palsy (PSP) and TRAUMATIC BRAIN INJURY (TBI).
Abnormal eye movements also accompany vestibular disorders (disturbances of the balance mechanisms of the inner EAR).
MUSCLES THAT MOVE THE EYE
• Superior oblique and inferior oblique rotate the eye primarily in a circular motion.
• Superior rectus and inferior rectus move the eye primarily up and down.
• Lateral rectus and medial rectus move the eye primarily side to side.
How the eye “sees” The sclera gives the eye its shape and rigidity. The front part of the sclera forms the “white” of the eye, the coloration coming from the white pigmentation of the fiber cells. In its center, the sclera becomes transparent, forming the CORNEA. The middle layer of the eye’s wall is the choroid, a thin, dark membrane rich in BLOOD vessels. The choroid loosely attaches to and nourishes the sclera and the eye’s innermost layer, the RETINA, where sight becomes vision.
Specialized cells infuse the retina, which lines the back of the inner eye. These cells, rods and cones, convert lightwaves into electrical impulses. Rods are the most plentiful, numbering about 120 million on each retina, and detect light in perceptions of shades of gray. Cones detect color and detail; there are about 6 million of them on each retina. Cones are sensitive to red, green, or blue. Rods and cones contain photosensitive chemicals that react to different wavelengths of light. The reactions alter the electrical charges of the rods and cones, creating nerve signals. Each minute of wakefulness thousands of these impulses traverse the optic nerves, carrying messages the brain then interprets and assembles as visual images.
The optic nerve, which contains more than a million nerve fibers, carries these signals to the brain. The pigmented cells of the retina are rich in melanin, the same chemical that causes the SKIN to darken in response to sun exposure. In the retina, these cells form a “blackout screen” that eliminates reflection, allowing lightwaves to reach and activate the rods and cones without interference. The macula, a small circular area in the center of the retina, contains the most dense distribution of cones and handles fine detail vision. The “blind spot,” the point at which the optic nerve enters the retina, is the optic disk; it contains no rods or cones. RETINITIS PIGMENTOSA (hereditary degeneration of the retina) and RETINAL DETACHMENT (separation of the retina from the choroid) are among the conditions that can affect the retina, resulting in impaired vision and blindness.
The physics of vision Lightwaves pass through the cornea and the LENS to enter the eye through the pupil, the opening in the circular muscle that rings the lens, the iris. The iris is the colored part of the eye; the pupil in its center appears black because it reveals the dark interior of the eye. The iris dilates (increases the size of) the pupil to allow more light to enter the eye and constricts (decreases the size of) the pupil to reduce the light that enters the eye. The cornea and the lens each refract, or bend, the entering lightwaves. The ciliary muscles contract and relax to move the lens, which thickens or flattens, respectively, to improve focus. After about age 40 the lens gradually loses its FLEXIBILITY, accounting for age-related difficulty with near vision (PRESBYOPIA).
Refracted light forms a final focal point that, in the healthy eye, aligns in a pattern on the retina at the back of the eye. The mechanics of this refractory process are such that the image result- ing on the retina is upside down. When interpreting and assembling nerve signals from the eye, the brain automatically reverses the image to perceive it right-side up. Refractive ASTIGMATISM, HYPEROPIA, and MYOPIA when the final focal point falls short of or extends beyond the retina, resulting in images that are out of focus or distorted.
Helping keep the lightwaves from fragmenting during refraction are two chambers of fluid, the aqueous humor, which fills the space between the cornea and the lens (the anterior chamber), and the vitreous humor, which fills the interior of the eye. The ciliary processes, specialized folds of the eye’s choroid layer that extend into the posterior chamber at the corners of the lens behind the iris, produce aqueous humor. This watery fluid is about the consistency of saliva and serves also to lubricate and nourish the cornea. Aqueous humor circulates through the anterior chamber between the cornea and the lens, then drains from the eye via the drainage angle, a channel between the iris and the cornea. Dysfunction of the drainage angle is a hallmark characteristic of GLAUCOMA.
Vitreous humor forms when the eye completes its development during the final trimester of gestation. A substance similar to water in chemical composition and to gelatin in consistency, vitreous humor maintains the eye’s shape and helps keep the retina smooth and even against the back of the eye. The volume of vitreous humor increases as the eye grows though otherwise remains constant (unlike the aqueous humor, which the eye continuously produces). Around age 40 years the vitreous humor begins to liquefy as a normal process of aging, causing VITREOUS DETACHMENT, which usually has little effect on vision though can produce FLOATERS (fragments of tissue that become suspended in the vitreous humor).