7.2: Forming Images - Eye Components, Functions, and Age Changes

Conjunctiva

Each eye is nearly spherical in shape. The front surface consists of a smooth thin layer, the conjunctiva, which covers the part of the eye exposed to air (Figure 7.2). The conjunctiva also extends away from the edge of this region to form a lining on the inner surfaces of the upper and lower eyelids. It is transparent, and so it does not absorb or block any of the light that strikes the eye.

Figure 7.2 Structure of the eye. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The conjunctiva secretes a fluid that helps prevent eye damage from drying and lubricates the eye so that the eyelids slide over it easily. Small blood vessels in the conjunctiva that are visible over the white part of the eye help nourish the cornea.

Age Changes

Aging results in a small decrease in the smoothness of the conjunctiva, causing light entering the eye to be slightly disorganized and scattered. These changes make focusing the light into a clear image more difficult. The conjunctiva also diminishes slightly in transparency, causing it to absorb and block the passage of some light.

A more important age change is a gradual decline in the amount of fluid the conjunctiva secretes. In some individuals, fluid production is so low that the eyelids can no longer glide smoothly over the eye. The irritation and inflammation of the conjunctiva that result can be quite uncomfortable. Artificial tear solutions can relieve this discomfort in most individuals.

Cornea

Immediately behind the central region of the conjunctiva is another transparent structure, the cornea (Figure 7.2). Because the cornea is fairly thick and curved, it bends (refracts) the light that passes through it.

Age Changes

Aging results in a gradual decrease in the transparency of the cornea. This change is usually great enough to block a significant amount of light. Light at the blue end of the spectrum is blocked more than is light toward the red end; therefore, seeing blue objects or objects lit with blue light is reduced preferentially. The blue end of the spectrum has light of shorter wavelengths than does the red end.

Aging of the cornea also increases the degree of scattering of light that passes through the cornea. Much of the scattered light still reaches the retina, but since it strikes the retina in the wrong places and in a disorganized way, it causes the viewer to see bright areas in the wrong places in the field of view. This phenomenon, called glare, is often noticed by a person when he or she looks at outdoor objects on a bright but hazy day. An extreme example of glare can be created by looking at bright lights at night through a window covered with drops of water.

With aging, the cornea becomes flatter, reducing the amount of refraction it can cause and making it difficult to see close objects clearly. The curvature of the cornea also becomes irregular so that light from certain parts of the field of view is not focused properly. This condition, called astigmatism, decreases the clarity of images from some parts of the field of view. Eyeglasses or contact lenses can often compensate for astigmatism. Finally, the sensitivity of the cornea to pain from pressure on the eye diminishes, which in turn decreases a person's ability to ward off eye injury from external pressure.

Iris and Pupil

A short distance behind the cornea is the iris (Figure 7.2) which is shaped like a phonograph record or compact disk. Pigments in the iris give the eye its color.

The hole in the center of the iris is called the pupil. The pupil allows light to pass from the front of the eye into the rear region. Muscle cells in the iris regulate the size of the pupil. These cells are controlled reflexively by autonomic motor neurons.

In the presence of bright light, some muscle cells in the iris constrict the pupil to reduce light entering the eye; this helps protect the eye from being damaged by excess light. The pupil is also constricted when a person looks at an object close to the eye. This helps form a clear image by blocking out stray light.

When light is dim, other muscle cells in the iris dilate the pupil. This allows enough light into the eye to adequately stimulate the receptor neurons.

Age Changes

With aging, the number and strength of the muscle cells that cause dilation of the pupil diminish and the thickness and stiffness of the collagen fibers increase. As these processes continue, the size of the pupil for any light intensity decreases with each passing year, starting at age 20. The result is a continuous decline in light available to form images.

Age changes in the cells and fibers in the iris may also slow the rate at which the pupil dilates when changing from bright to dim light. This effect, combined with age‑related slowing of pupillary constriction, retards pupillary adaptation to changing light intensities.

Ciliary Body

The outer edge of the iris is attached to a thickened ring of cells called the ciliary body (Figure 7.2), which contains muscle cells that regulate the curvature of the lens. As in the iris, these muscle cells are controlled by reflexes. The ciliary body also secretes a fluid called aqueous humor, which is discussed below.

Age Changes

An important age change in the ciliary body is the slowing of its secretion of aqueous humor. The significance of this change is discussed below.

Lens and Suspensory Ligaments

A short distance behind the iris and the pupil is the transparent lens, which is nearly round (Figure 7.2). However, because the lens is elastic, its shape can be changed. The lens increases in thickness throughout life.

A ring of thin fibers called suspensory ligaments radiates outward from the lens much as spokes radiate from the center of a bicycle wheel. These ligaments reach and attach to the ciliary body much as spokes connect to the rim of a wheel.

Since the lens is a thick curved structure, it refracts the light passing through it. Unlike the cornea, however, the curvature of the lens can change so that the lens can refract light to a greater or lesser degree (Figure 7.3). Alterations in refraction are important because light coming to the eye from close objects must be refracted more than is light from farther objects. Therefore, to focus light from close objects, the lens becomes more rounded so that it bends the light more. The lens becomes flatter and bends light less to help the eye form a clear image of a distant object.

Figure 7.3 Focusing light from (a) distant object and (b) close object. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Age Changes

As the lens ages, it is altered in four ways. One alteration is a decline in transparency to all colors of light, especially blue light. The markedly declining transparency of the lens blocks more light than does the reduction in the transparency of any other part of the eye. This change begins during the third decade of life and increases exponentially with age.

The second alteration is the development of opaque spots, which begins during the fifth decade. Usually these opacities are toward the periphery of the lens. Lens opacities block the passage of light and cause a great increase in light being scattered. They account for more scattering of light and more glare than do age changes in any other eye component.

If many opacities form close to the center of the lens, vision is greatly impaired and the condition is called cataracts. While cataracts categorized as a disease, their development is part of aging of the lens. Everyone who lives long enough will eventually develop cataracts.

The third alteration of the lens is a reduction in its ability to refract light. This change results from accumulation of damaged proteins plus age-related formation of abnormal proteins. These two changes offset the age-related increase in lens thickness and curvature. With diminishing refractive power, it becomes increasingly difficult to see close objects clearly. Individuals who are nearsighted during youth may benefit from this age change since their ability to see distant objects improves.

Many individuals have lenses so flat that they cannot focus light even from far objects. This condition may eventually begin to improve because thickening of the lens increases its ability to refract light.

The fourth age change is a decrease in elasticity, which may result partly from an increase in the cross‑linkages among collagen fibers. As the lens loses elasticity, its shape changes more slowly when it adjusts to near or distant objects. Declining elasticity actually begins some time before age 10 and continues at a steady rate until about age 50. The decline in elasticity decreases the amount of curvature the lens can achieve, and objects must be farther away from the face to be seen clearly. Thus, the smallest distance from the eye at which an object can be seen clearly - the near point of accommodation - increases.

This increase is usually not noticed until about age 40 because most objects used in daily living are located beyond the near point. After age 40, the reserve capacity of the lens for elasticity has dwindled sufficiently and the near point becomes large enough to interfere with ordinary activities such as reading and writing. This condition, called presbyopia, involves farsightedness caused by stiffening of the lens. Difficulties and limitations caused by presbyopia can be reduced by wearing eyeglasses or contact lenses.

The increase in the near point is rapid from about ages 40 to 50, but the rate of change slows during the sixth decade of life. By age 60 there is usually no further increase in the near point because the lens has lost all ability to change its curvature.

Because adjusting the curvature of the lens requires the use of the aqueous humor and vitreous humor, a more complete explanation of how the curvature of the lens adjusted and how this process is affected by aging follows the description of those two humors.

Aqueous Humor

As mentioned above, the ciliary body produces a liquid called aqueous humor (Figure 7.2). The aqueous humor flows forward from the ciliary body, passes through the pupil, and is removed from the eye by a special tube located around the edge of the cornea.

Since aqueous humor is produced and removed continuously, its steady flow delivers nutrients and removes wastes. These services are important for the cornea and lens, which have no blood vessels.

The aqueous humor fills the cavity between the cornea and the lens. The fluid provides a slight outward pressure that helps keep the cornea curved outward so that it refracts light properly. By causing the entire eye to bulge outwardly, this pressure also helps provide tension on the suspensory ligaments. The tension helps to hold the lens in place and pull it into a slightly flattened shape.

Age Changes

As a person ages, the rate at which aqueous humor is produced declines. This change reduces the rate at which the cornea and lens are serviced. There is also a decrease in the amount of aqueous humor present, and this may contribute to the flattening and irregular curvature of the cornea that develop with aging.

Vitreous Humor

Another eye humor, the vitreous humor, fills the cavity in the eye behind the lens (Figure 7.2)The transparent vitreous humor is composed of a soft gel that has the consistency of partially solidified gelatin. The center of the gel becomes liquefied early in childhood. Thereafter, the liquefaction slowly and continuously spreads outward.

The vitreous humor is held in place by a ring of attachment on the front edge of the retina and an attachment point at the back of the eye where the optic nerve begins.

The vitreous humor produces an outward pressure so that, like the aqueous humor, it puts tension on the suspensory ligaments. The soft consistency of the vitreous humor allows it to protect eye structures by absorbing shock. It also holds the retina and choroid layers in place by pushing against them.

Age Changes

With aging, alterations in the chemistry of the vitreous humor make it lose transparency and cause more scattering of light. Blocking and scattering of light are increased by small areas of vitreous humor that become opaque and often grow large enough to be visible in the field of view. These areas, which appear as objects of varying sizes, shapes, and textures, are called floaters because they are seen to move, especially when the eye moves. Though floaters are not dangerous, they decrease the quality of the images that are formed, are often distracting, and can obscure parts of the field of view. Some floaters are pieces of vitreous humor that have broken off from the main mass and float between the vitreous humor and the retina.

Chemical changes caused by aging of the vitreous humor also make it decrease in size and shrink away from the retina. These alterations reduce the amount of support provided for the retina. At the same time, more of the central region is becoming liquefied, making the vitreous humor move about when the eye moves. As the vitreous humor shifts, it pulls on the retina, especially during rapid eye or head movements, and causes the person to perceive flashes of light called flashers.

The presence of a few flashers may be distracting but is of little importance. However, as the vitreous humor ages, the tension it places on the retina increases. If the tension becomes great enough, the person may perceive many flashes and part of the field of view may become darkened. These symptoms constitute a warning that the vitreous humor may have detached the retina from the back of the eye. If not treated and corrected immediately, detachment of the retina usually causes some degree of blindness.