7.5: Other Eye Components - Structure, Functions, and Age Changes
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Choroid
The choroid lies in contact with Bruch's membrane on the outer surface of the retina (Figure 7.2). As has been mentioned, it contains many blood vessels that nourish and remove wastes from the photoreceptor layer in the sensory retina.
The choroid also contains a large amount of black pigment that absorbs light that has passed through the retina. Therefore, reflection of the light back through the retina is prevented. If the light were reflected, it would pass through the vitreous humor and strike other parts of the retina, causing foggy or blurry images.
Age Changes
With advancing age, the blood vessels in the choroid become irregular, decreasing their ability to service the retina. Other age changes in the choroid include thickening, weakening, declining elasticity, and the formation of irregularities. These changes cause irregularities in the retina and therefore interfere with the focusing of light on the retina. These changes also allow the choroid to be more easily torn as a result of trauma.
Sclera
The outermost layer of the eye is the sclera, which is the white part (Figure 7.2). It extends from the edge of the cornea at the front of the eye around to the optic nerve at the back.
The sclera consists of a tightly woven mat of collagen fibers. Its strength allows it to support eye structures and protect them from trauma. The sclera also serves as a firm attachment point for the external muscles of the eye, which allow a person to turn the eye in its socket.
External Muscles
Six external muscles attach to each eye (Figure 7.6). The arrangement of these muscles allows a person to turn the eye in any direction. Turning the eyes serves several purposes. First, it permits the eye to be pointed directly at a stationary object. Second, it allows the eye to keep a moving object centered in the field of view. Maximum visual acuity is achieved when the eye follows the object smoothly. Third, it permits the scanning of a wide field of view.
Rotating the eyes in their sockets also allows both eyes to be aimed at the same object simultaneously so that a person does not have double vision. This goal cannot be achieved by any other means. For example, since the eyes are positioned for distant viewing when at rest, they must be rotated inward toward the nose (converged) when a person wishes to view a close object. By contrast, they must be rotated outward (diverged) when the person wants to view a distant object again.
To observe divergence and convergence, have a volunteer hold a pencil away from his or her face at arm's length. Then watch the volunteer's eyes as he or she looks first at the pencil, then at a distant wall, and then back at the pencil.
Eye movements produced by the external eye muscles are controlled voluntarily at some times and occur under reflex control at others. For example, a person may voluntarily contract the muscles to move the eye to the right and left. By contrast, convergence and divergence, which are caused by contraction of the same muscles, usually occur reflexively.
Age Changes
The ability of the external eye muscles to turn the eye smoothly declines with age. This may be due to age changes in the muscles or to alterations in the nerve pathways that control the muscles.
The decline in the smoothness of eye movements greatly decreases visual acuity both when one is viewing moving objects and when one is viewing stationary objects while moving. Decline in vision becomes greater as the speed of movement increases. The reduction in visual acuity caused by this age change results in difficulties in activities such as participating in sports, driving a vehicle, and watching motion pictures.
Adipose Tissue
The space between the eye and the surrounding skull contains much fat tissue (adipose tissue). This tissue cushions the eye from trauma, supports the eye in position, and helps maintain its round shape. The skull bones forming the eye socket also protect and support the eye.
Age Changes
The amount of adipose tissue around the eye decreases with aging. This allows the eye to sink deeper into its socket, causing a cosmetic change in the face and a decrease in the size of the field of view. The decrease in fat tissue also reduces the support the eye receives; therefore, the eye may become misshapen. Depending on the nature of the change in eye shape, the position and functioning of several structures, including the cornea, vitreous humor, retina, and choroid may be affected. Any alteration in these structures will adversely affect vision.
Eyelids
The upper and lower eyelids each consist of a fold of skin (Figure 7.7). Part of the conjunctiva covers the inner surface of each eyelid.
A row of hairs is present along the edge of each eyelid.
The front of the eye must be kept moist because it consists of living cells. If the cells on the surface of the eye were to die, as do those on the surface of the skin, light could not be properly focused. The eyelids keep the eye moist by secreting fluid from the conjunctiva, spreading the conjunctival and lacrimal fluids over the eye surface, and closing to prevent evaporation of moisture. The eyelids also protect the eye from traumatic injury and prevent exposure to dust, noxious chemicals, microbes, and excess light.
The functioning of the eyelids depends on their positioning and movement, both of which are determined mostly by the contraction of muscles. Closing the eyelids is accomplished by contracting a ring of facial muscle that surrounds the eye. Opening the eyelids occurs through contraction of a muscle attached to the upper eyelid. Contraction of other facial muscles can also influence the position of and movement of the eyelids.
Contraction of muscles affecting the eyelids occurs under voluntary control at some times and under reflex control at others. For example, a person may blink any time he or she chooses, but blinking also occurs reflexively. Closing the eyes when falling asleep or sneezing is another example of reflex control of the eyelids.
Age Changes
Age changes in the skin of the eyelids are the same as those that occur in other parts of the facial skin. The discomfort and possible injury resulting from the decrease in secretion by an aging conjunctiva were described earlier in this chapter. Finally, age changes in the muscles of the eyelids are the same as those that occur in other voluntary muscles.
Lacrimal Gland
The upper outer portion of the eye socket contains a lacrimal gland, which secretes lacrimal fluid. The eyelids spread this fluid over the front of the eye and the fluid moves gradually toward the corner of the eye near the nose. There it drains through an opening into lacrimal ducts and is eventually carried by ducts to the nasal cavity.
Besides moistening the eye, lacrimal fluid washes away irritating objects, chemicals, and microbes. It also prevents infections by killing certain bacteria through the action of a chemical called lysozyme.
Age Changes
The production of lacrimal fluid decreases with aging. Many people produce enough regardless of age, but production becomes so low in some individuals that they develop dry and irritated eyes. The use of artificial tear solutions significantly reduces these problems, just as it does when secretion by the conjunctiva is inadequate.