What are eyes?

Structure and Functions

The eye captures pictures from the environment and transforms them into neural impulses that are processed by the brain into visual images. The retina, with its light-sensitive cells, acts as a camera to “put the picture on film,” while neural processing in the brain “develops the film” and forms a visual image that is meaningful and informative for the individual.

The human eye originates during development, that is, while the individual is being formed as an embryo in the uterus. Eye formation begins during the end of the third week of development when outgrowths of brain neural tissue, called the optic vesicles, form at the sides of the forebrain region. The optic vesicle induces overlying embryonic tissue to thicken in one region, forming a primitive lens structure called the lens placode. The lens placode, in turn, induces the optic vesicles to form a cuplike structure, the optic cup, while the brain’s connection of the vesicles narrows into a slender stalk that forms the optic nerve. The inner part of the optic cup forms the neural or sensory retina, with its photoreceptors, while the outer part of the optic cup develops into the layers of tissues, or tunics, that make up the wall of the eyeball. The lens placode further condenses and solidifies by forming lens fibers that become transparent. The function of the lens will eventually
be to focus light onto the retina. The major structures of the eye—the retina, lens, and eyeball coats—are initially formed by the fifth month of fetal development. During the remainder of the prenatal period, eye structures continue to enlarge, mature, and form increasingly complex neural networks with the visual processing regions of the brain.

At birth, an infant’s eyes are about two-thirds the size of adult eyes. Until after their first month of life, most newborns lack complete retinal development, especially in the area that is responsible for visual acuity. As a result, infants cannot focus their eyes properly and typically have a vacant stare during their first weeks of life. Most of the subsequent eye growth occurs rapidly during the remainder of the first year of life. From the second year of life until puberty, the rate of eye growth progressively slows. After puberty, eye growth is negligible.

The adult human eye weighs approximately 7.5 grams and measures approximately 24.5 millimeters in its anterior-to-posterior diameter. All movement of the eyeball, or globe, is accomplished by six voluntary muscles attached anteriorly by ligaments to the outer coat of the globe and posteriorly to a tendinous ring located behind the globe. One voluntary muscle elevates the upper lid.

Three concentric tunics form the globe itself. The outermost fibrous tunic consists of two portions. In the small, anterior portion, the tunic fibrils are arranged in a regular pattern, forming the transparent cornea. Posteriorly, the tunic fibrils are irregularly spaced, forming the opaque, white sclera. The innermost tunic, or nervous tunic, consists of two parts: the pars optica, or retina, containing photoreceptor cells, and the pars ceca lining the iris and ciliary body. Tucked between the outer and inner tunics lies the vascular tunic, consisting of the pigmented iris, which gives the eye its distinctive color; the ciliary body, which forms the aqueous humor to provide nourishment for the anterior structures of the globe; and the highly vascular choroid, which provides nourishment for the retina and also acts as a cooling system by regulating blood flow to the chemically active retina. In the center of the circular, pigmented iris lies the pupil, which is a small opening into the posterior parts of the eyeball.

The cavity that contains the globe, circumscribed by the concentric tunics, is filled with a clear, jellylike substance called the vitreous body. This substance is anteriorly bounded in the vitreous cavity by the transparent crystalline lens that lies just posterior to the pupil. The crystalline lens is elastic in structure, allowing for variations in thickness that change the focusing power of the eye.

The eye can refract, or bend, light rays because of the curved surfaces of two transparent structures, the
cornea and the crystalline lens, through which light rays must pass to reach the retina. Any curved surface, or lens, will refract light rays to a greater or lesser degree depending on the steepness or flatness of the surface curve. The steeper the curve, the greater the refracting power. If a curved surface refracts light rays to an intersection point one meter away from the refracting lens, this lens is defined as having one diopter of power. The human eye has approximately fifty-nine diopters of power in its constituent parts, including the cornea and crystalline lens.

Light rays emitted from a distant point of light enter the eye in a basically parallel pattern and are bent to intersect perfectly at the retina, forming an image of the distant point of light. If the point of light is near the eye, the rays that are emitted are divergent in pattern. These divergent rays must also be refracted to meet at a point on the retina, but these rays require more bending—hence, a steeper curved surface is needed. By a process called accommodation, the human eye automatically adjusts the thickness of the crystalline lens, forming a steeper curve on its surface and thereby creating a perfect image on the retina. Variations from the normal in either the length of an eyeball or the curves of the cornea and crystalline lens will result in a refractive error or blurred image on the retina.

The major task of the eye is to focus environmental light rays on the photoreceptor cells, the
rods and
cones of the retina. These photoreceptors absorb the light energy, transforming it into electrical signals that are carried to the visual center of the brain. Cones are specialized for color or daylight vision and have greater visual discrimination or acuity than the rods, which are specialized for black-and-white or nighttime vision.

The fovea is a pin-sized depression in the center of the retina that contains only cone cells in high concentrations. This makes the fovea the point of the most distinct vision, or greatest visual acuity. When the eye focuses on an object, the object’s image falls on the retina in the area of the fovea. Immediately surrounding the fovea is a larger area called the macula lutea that contains a relatively high concentration of cones. Macula lutea acuity, while not as great as in the fovea, is much greater than in the retina’s periphery, which contains fewer cones. The concentration of cones is greatest in the fovea and declines toward the periphery of the retina. Conversely, the concentration of the rods is greater at the more peripheral areas of the retina than in the macula luteal area. The retina of each eye contains about 100 million rod cells and about 300 million cone cells.

The
optic nerve carries impulses from the photoreceptors to the brain. This nerve exits the retina in a central location called the
blind spot. No image can be detected in this area because it contains neither rods nor cones. Normally, an individual is not aware of the retinal blind spot because the brain’s neural processing compensates for the missing information when some portion of a peripheral image falls across this part of the retina.

On a cellular level, rod and cone photoreceptors consist of three parts: an outer segment that detects the light stimulus, an inner segment that provides the metabolic energy for the cell, and a synaptic terminal that transmits the visual signal to the next nerve cell in the visual pathway leading to the brain. The outer segment is rod-shaped in the rods and cone-shaped in the cones (hence their names). This segment is made of a stack of flattened membranes containing photopigment molecules that undergo chemical changes when activated by light.

The rod photopigment, called rhodopsin, cannot discriminate between various colors of light. Thus rods provide vision only in shades of gray by detecting different intensities of light. Rhodopsin is a purple pigment (a combination of blue and red colors), and it transmits light in the blue and red regions of the visual spectrum while absorbing energy from the green region of the spectrum. The light that is absorbed best by a photopigment is called its absorption maximum. Thus at night, when rods are used for vision, a green car is seen far more easily than a red car, because red light is poorly absorbed by rhodopsin. Only absorbed light produces the photochemical reaction that results in vision.

When rhodopsin absorbs light, the photopigment dissociates or separates into two parts: retinene, which is derived from vitamin A, and opsin, a protein. This separation of retinene from opsin, called the bleaching reaction, causes the production of nerve impulses in the photoreceptors. In the presence of bright light, practically all the rhodopsin undergoes the bleaching reaction and the person is in a light-adapted state. When a light-adapted person initially enters a darkened room, vision is poor since the light sensitivity of the rod photoreceptors is very low. After some time in the dark, however, a gradual increase in light sensitivity, called dark adaptation, occurs as increased amounts of retinene and opsin are recombined by the rods to form rhodopsin. The increased level of rhodopsin occurs after a few minutes in the dark and reaches a maximum sensitivity in about twenty minutes.

Each kind of cone—red, green, and blue—is distinguished by its unique photopigment, which responds to a particular wavelength or color of light. Combinations of cone colors provide the basis for color vision. While each type of cone is most sensitive to the particular wavelength of light indicated by its color—red, green, or blue—cones can respond to other colors with varying degrees. One’s perception of color rests on the differential response of each cone type to a particular wavelength of light. The extent that each cone type is activated is coded and sent in separate parallel pathways to the brain. A color vision center in the brain combines and processes these parallel inputs to create the perception of color. Color is thus a concept in the mind of the viewer.

The intricacies of the human visual system require various methods to assess eye structure and function. Visual acuity is a measure of central cone function. Clinically, the most common method for testing
visual acuity is by the use of a Snellen chart, consisting of a white background with black letters. All symbols on the chart create, or subtend, a visual angle at the approximate center of the eye. The smaller the symbol, the smaller the angle and the more difficult cone recognition becomes. At the standard distance of twenty feet, the smallest letters on the Snellen chart subtend an angle of five minutes of arc at the eye’s center. The larger letters on the chart are calibrated such that each consecutively larger letter subtends a multiple unit of five minutes of arc. If the eye can detect the smallest letters on the chart, the patient is said to have normal (20/20) vision. The numerator of the clinical fraction designates the test distance of twenty feet. The denominator varies with the patient’s visual function, identifying the distance at which the smallest letter recognized by the patient subtends an angle of five minutes of arc. For example, if the smallest letter recognized is fifty minutes of arc
in size, the fraction used to record this visual acuity is 20/200 because the letter with fifty minutes of arc is ten times as large as the smallest letters on the chart. Therefore, the distance needed for this letter to create five minutes of arc at the eye is ten times as far as the normal twenty feet. In this example, the patient is said to have a refractive error.

Disorders and Diseases

Commonly existing refractive errors are astigmatism, myopia, hyperopia, and presbyopia. Presbyopia is an anomaly that occurs with aging when the crystalline lens loses its ability to accommodate. Causes include thickening of the lens and changes in the attachment fibers that anchor the lens. Because of these alterations, the lens is not able to change its shape and the eye remains focused at a specific distance. To compensate for this problem, bifocal lenses are normally prescribed, with the upper region of the lens focused for distant vision and the lower lens focused for near vision. Hyperopia, also called farsightedness, results when an eyeball is too short. Because light rays are not bent sufficiently by the lens system, the image is focused not on the retina but behind the retina. To compensate for this problem, convex lenses are prescribed, which bring the focus point back on the retina. Conversely, myopia, or nearsightedness, results from an abnormally long eyeball. In this case, the lens system focuses in front of the retina. This abnormal vision can be corrected by concave lenses. Astigmatism results from a refractive error of the lens system, usually caused by an irregular shape in the cornea or less frequently by an irregular shape in the lens. The consequence of this anomaly is that some light rays are focused in front of the retina and some behind the retina, creating a blurred image. To correct the focusing error, a special irregular pair of glasses (or the use of a contact lens) must be made to correct the abnormal irregularity of the eye’s lens system.

An examiner can assess the amount of refractive error based on a patient’s verbal choice as to which of a given series of lenses sharpens the retinal image of the letters on the Snellen chart. Refractive error can also be determined when a patient is not capable of response. A retinoscope is often used to shine a light through the pupil onto the retina. An image of the light is reflected back out to the examiner who, in turn, can assess refractive error by the movement and shape of the image.

Visual field testing is a measure of the integrity of the neural pathways to the vision center in the brain. To test visual fields clinically, the patient focuses on a central target. While continuing to focus centrally, test targets are serially brought into the patient’s peripheral vision, or visual field. The smaller and dimmer the test target, the more sensitive the test. The simplest visual field test technique is by confrontation. The patient and examiner sit facing each other one meter apart. If both patient and examiner cover their right eyes, the patient’s left visual field is being tested. Since the patient’s left visual field is congruent to the examiner’s left visual field, the examiner can detect visual field defects when the patient is not responsive to a test target brought into view from the side. Lesions to some portion of the visual pathway to the brain will result in a scotoma, or blind area, in the corresponding visual field.

A biomicroscope, or slitlamp microscope, is commonly used to assess external eye structures, including the eyelids, lashes, conjunctiva, cornea, sclera, and one internal structure, the crystalline lens. The white part of the eye, or sclera, is covered with a thin, transparent covering called the conjunctiva. Infections and tumors often invade this external structure. Though the normally transparent crystalline lens is essentially free of infections and tumors, it can become cloudy or opaque and develop a cataract. Causes for cataract formation are multiple, the most common being the aging process; less frequently, trauma to the lens or a secondary symptom of systemic disease can result in cataracts. When the cataract is so dense that it obstructs vision, the crystalline lens is surgically removed and replaced with an artificial, plastic lens.

To view internal eye structures, the pupil is dilated to allow more light to be introduced into the interior and posterior regions of the eyeball. Two commonly used instruments are the handheld ophthalmoscope and the head-mounted indirect ophthalmoscope. Diseases of the retina include retinal tears, detachments, artery or vein occlusions, degenerations, and retinopathies secondary to systemic disease.

Glaucoma is an eye disease characterized by raised pressure inside the eye. Normal eye pressure is stabilized by the balance between the production and removal of the aqueous humor, the solution that bathes the internal, anterior structures of the eye. Abnormal pressures are often associated with defects in the visual, or optic, nerve and in the visual field. Approximately 300 people per 100,000 are affected by glaucoma. Clinically, intraocular pressure is assessed by numerous methods in a process called tonometry.

Abnormalities of the eye muscles constitute a significant portion of visual problems. Binocular vision and good depth perception are present when both eyes are aligned properly toward an object. A weakness in any of the six rotatory eye muscles will result in a tendency for that eye to deviate away from the object, resulting in an obvious or latent eye turn called strabismus. Associated signs are eye fatigue, abnormal head postures, and double vision. To alleviate objectionable double vision, a patient often suppresses the retinal image at the brain level, resulting in functional amblyopia (often called lazy eye), in which visual acuity is deficient.

Color blindness, a trait that occurs more frequently in men than in women, is caused by a hereditary lack of one or more types of cones. For example, if the green-sensitive cones are not functioning, the colors in the visual spectral range from green to red can stimulate only red-sensitive cones. This person can perceive only one color in this range, since the ratio of stimulation of the green-red cones is constant for the colors in this range. Thus this individual is considered to be green-red color-blind and will have difficulty distinguishing green from red.

Perspective and Prospects

Early physicians recognized the importance of good eyesight, but because of limited understanding they had minimal means to treat major eye disorders. During the Middle Ages, surgeons performed eye operations, including ones for cataracts in which the lens was pushed down and out of the way with a needle inserted into the eyeball. In the eighteenth century, this operation was improved when cataract lenses were extracted from the eye. In the early seventeenth century, Johannes Kepler described how light was focused by the lens of the eye on the retina, thus providing insight into why spectacles are valuable in cases of poor eyesight. In 1801, Thomas Young published a foundational text entitled On the Mechanics of the Eye. Hermann von Helmholtz in the nineteenth century invented the first ophthalmoscope, which allowed inspection of the interior structures of the eye. Young and Helmholtz also developed theories to explain the phenomenon of color vision. From the invention of the ophthalmoscope, the range of clinical observation was extended to the inside of the eyeball, allowing the diagnosis of eye disorders. The modern understanding of eyesight and vision is increasing with contributions from ongoing research.

Ophthalmology is the study of the structure, function, and diseases of the eye. An ophthalmologist is a physician who specializes in the diagnosis and treatment of eye disorders and diseases with surgery, drugs, and corrective lenses. An optometrist is a specialist with a doctorate in
optometry who is trained to examine and test the eyes and treat defects in vision by prescribing corrective lenses. An optician is a technician who fits, adjusts, and dispenses corrective lenses that are based on the prescription of an ophthalmologist or optometrist.

Vision care personnel are vital to industry, public health, recreation, highway safety, education, and the community. Since 85 percent of learning is visual-based, good vision is extremely important in education, work, and play. Good vision enhances the production and morale of workers, and athletic performance is improved when vision problems are corrected. Vision care specialists work to promote the prevention of eye injuries and diseases while supporting practices that enhance good health and vision. Vision therapy may be used to correct many disorders of the eye such as amblyopia, reduced visual perception, reading disorders, poor eye coordination, and reduced visual acuity.

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