The eye has three layers, or coats (Fig. 48-1). The external layer is the sclera (the opaque tissue making up the “white” of the eye) and the transparent cornea on the front of the eye.

FIG. 48-1 Anatomic features of the eye.

The middle layer, or uvea, is heavily pigmented and consists of the choroid, the ciliary body, and the iris. The choroid, a dark brown membrane between the sclera and the retina, lines most of the sclera. It has many blood vessels that supply nutrients to the retina.

The ciliary body connects the choroid with the iris and secretes aqueous humor. The iris is the colored portion of the external eye; its center opening is the pupil. The muscles of the iris contract and relax to control pupil size and the amount of light entering the eye.

The innermost layer is the retina, a thin, delicate structure made up of sensory photoreceptors that begin the transmission of impulses to the optic nerve. The retina contains blood vessels and two types of photoreceptors called rods and cones. The rods work at low light levels and provide peripheral vision. The cones are active at bright light levels and provide color and central vision.

The optic fundus is the area at the inside back of the eye that can be seen with an ophthalmoscope. This area contains the optic disc, a creamy pink or white depressed area where the fine nerve fibers that synapse with the photoreceptors join together to form the optic nerve and exit the eyeball. The optic disc is sometimes called the “blind spot” because it contains only nerve fibers and no photoreceptor cells. To one side of the optic disc is a small, yellowish pink area called the macula lutea. The center of the macula is the fovea centralis, where vision is the most acute.

Refractive Structures and Media

Light waves pass through these structures on the way to the retina: cornea, aqueous humor, lens, and vitreous humor. Each structure has a different density, which causes the light waves to bend (refract) and focus images on the retina. Together, these structures are the eye’s refracting media.

The cornea is the clear layer that forms the external bump on the front of the eye (see Fig. 48-1). The aqueous humor is a clear, watery fluid that fills the anterior and posterior chambers of the eye. Aqueous humor is continually produced by the ciliary processes and passes from the posterior chamber, through the pupil, and into the anterior chamber. This fluid drains through the canal of Schlemm into the blood to maintain a balanced intraocular pressure (IOP), the pressure within the eye (Fig. 48-2).

FIG. 48-2 Flow of aqueous humor.

The lens is a circular, convex structure that lies behind the iris and in front of the vitreous body. It is normally transparent and bends the rays of light entering through the pupil so that they focus properly on the retina. The curve of the lens changes to focus on near or distant objects. A cataract is a lens that has lost its transparency.

The vitreous body is a clear, thick gel that fills the vitreous chamber (the space between the lens and the retina). This gel transmits light and maintains eye shape.

The eye is a hollow organ and must be kept in the shape of a ball for vision to occur. To maintain this shape, the gel in the posterior segment (vitreous humor) and the fluid in the anterior segment (aqueous humor) must be present in set amounts that apply pressure inside the eye to keep it inflated (McCance et al., 2010). This pressure is known as intraocular pressure or IOP. IOP has to be just right. If the pressure is too low, the eyeball is soft and collapses, preventing light from getting to the light-sensitive photoreceptors in the back of the eye on the retina. If the pressure becomes too high, the extra pressure compresses capillaries and nerve fibers in the eye. Pressure on retinal blood vessels prevents blood from flowing through them, and as a result the photoreceptors and nerve fibers become hypoxic. Compression of the fine nerve fibers prevents intracellular fluid flow, which also reduces nourishment to the distal portions of these thin nerve fibers. The increased intraocular pressure and resulting hypoxia of the photoreceptors and their synapsing nerve fibers is a condition called glaucoma. Continued hypoxia inside the eye result in photoreceptor necrosis and death and permanent nerve fiber damage. With extensive photoreceptor loss and nerve fiber damage, vision is lost and the person is permanently blind.

External Structures

The eyelids are thin, movable skinfolds that protect the eyes, shut out light during sleep, and keep the cornea moist. The upper eyelid is larger than the lower one. The canthus is the place where the two eyelids meet at the corner of the eye.

The conjunctivae are the mucous membranes of the eye. The palpebral conjunctiva is a thick membrane with many blood vessels that lines the undersurface of each eyelid. The thin, transparent bulbar conjunctiva covers the entire front of the eye.

Tears are produced by a small lacrimal gland, which is located in the upper outer part of each orbit (Fig. 48-3). Tears flow across the front of the eye, toward the nose, and into the inner canthus. They drain through the punctum (an opening at the nasal side of the lid edges), into the lacrimal duct and sac, and then into the nose through the nasolacrimal duct.

FIG. 48-3 Front view of the eye and adjacent structures.

Muscles, Nerves, and Blood Vessels

Six voluntary muscles rotate the eye and coordinate eye movements (Fig. 48-4 and Table 48-1). Coordinated eye movements ensure that the retina of each eye receives an image at the same time so only a single image is seen.

FIG. 48-4 The extraocular muscles.

TABLE 48-1 FUNCTIONS OF OCULAR MUSCLES

Superior Rectus Muscle

• Together with the lateral rectus, this muscle moves the eye diagonally upward toward the side of the head.

• Together with the medial rectus, this muscle moves the eye diagonally upward toward the middle of the head.

Lateral Rectus Muscle

• Together with the medial rectus, contraction of this muscle holds the eye in a straight position.

• Contracting alone, this muscle turns the eye toward the side of the head.

Medial Rectus Muscle

• Contracting alone, this muscle turns the eye toward the nose.

Inferior Rectus Muscle

• Together with the lateral rectus, this muscle moves the eye diagonally downward toward the side of the head.

• Together with the medial rectus, this muscle moves the eye diagonally downward toward the middle of the head.

Superior Oblique Muscle

• Contracting alone, this muscle pulls the eye downward.

Inferior Oblique Muscle

• Contracting alone, this muscle pulls the eye upward.

The muscles around the eye are innervated by cranial nerves (CNs) III (oculomotor), IV (trochlear), and VI (abducens). The optic nerve (CN II) is the nerve of sight, connecting the optic disc to the brain. Part of the trigeminal nerve (CN V) stimulates the blink reflex when the cornea is touched. The facial nerve (CN VII) innervates the lacrimal glands and muscles controlling lid closure.

The ophthalmic artery brings oxygenated blood to the eye and structures in the orbit. This artery branches to supply blood to the retina. The ciliary arteries supply the sclera, choroid, ciliary body, and iris. Venous drainage occurs through the two ophthalmic veins.

Function

The four eye functions that provide clear images and vision are refraction, pupillary constriction, accommodation, and convergence.

Refraction involves bending light rays from the outside world into the eye. The different curved surfaces and refractive media of the eye allow light to pass through to the retina. Each surface and media bends (refracts) light differently to focus an image on the retina. Emmetropia is the perfect refraction of the eye: with the lens at rest, light rays from a distant source (20 feet [6 m] or more) are focused into a sharp image on the retina. Fig. 48-5 shows the normal refraction of light within the eye. Images fall on the retina inverted and reversed left to right. For example, an object in the lower nasal visual field strikes the upper outer area of the retina.

FIG. 48-5 Refraction and correction in emmetropia, hyperopia, and myopia.

Errors of refraction are common. Hyperopia (farsightedness or hypermetropia) occurs when the eye does not refract light enough. As a result, images actually fall (converge) behind the retina (see Fig. 48-5). Vision beyond 20 feet is normal, but near vision is poor. Hyperopia is corrected with a convex lens in eyeglasses or contact lenses.

Myopia (nearsightedness) occurs when the eye overbends the light and images converge in front of the retina (see Fig. 48-5). Near vision is normal, but distance vision is poor. Myopia is corrected with a biconcave lens in eyeglasses or contact lenses.

Astigmatism is a refractive error caused by unevenly curved surfaces on or in the eye, especially of the cornea. These uneven surfaces distort vision.

Pupillary constriction and dilation control the amount of light that enters the eye. If the level of light to one or both eyes is increased, both pupils constrict (become smaller). The amount of constriction depends on how much light is available and how well the retina can adapt to light changes. Pupillary constriction is called miosis, and pupillary dilation is called mydriasis (Fig. 48-6). Drugs can alter pupillary constriction.

FIG. 48-6 Miosis and mydriasis.

Accommodation allows the healthy eye to focus images sharply on the retina whether the image is close to the eye or distant. The process of maintaining a clear visual image when the gaze is shifted from a distant to a near object is known as accommodation. The eye can adjust its focus by changing the curve of the lens.

Convergence is the ability to turn both eyes inward toward the nose at the same time. This action helps ensure that only a single image of close objects is seen.

Eye Changes Associated with Aging

Changes inside the eye cause visual acuity to decrease with age (Touhy & Jett, 2010). Age-related changes of the nervous system and in the eye support structures also reduce visual function (Chart 48-1).

Chart 48-1 Nursing Focus On The Older Adult

Changes in the Eye and Vision Related to Aging

STRUCTURE/FUNCTION	CHANGE	IMPLICATION
Appearance	Eyes appear “sunken.”	Do not use eye appearance as an indicator for hydration status.
	Arcus senilis forms.	Reassure patient that this change does not affect vision.
	Sclera yellows or appears blue.	Do not use sclera to assess for jaundice.
Cornea	Cornea flattens, which blurs vision.	Encourage older adults to have regular eye examinations and wear prescribed corrective lenses for best vision.
Ocular muscles	Muscle strength is reduced, making it more difficult to maintain an upward gaze or maintain a single image.	Reassure patient that this is a normal happening and to re-focus gaze frequently to maintain a single image.
Lens	Elasticity is lost, increasing the near point of vision (making the near point of best vision farther away).	Encourage patient to wear corrective lenses for reading.
Lens	Lens hardens, compacts, and forms a cataract.	Stress the importance of yearly vision checks and monitoring for when intervention is needed.
Iris	Decrease in ability to dilate results in small pupil size and poor adaptation to darkness.	Teach older adults the need for good lighting for best vision to avoid tripping and bumping into objects.
Pupil	Pupil size is smaller, reducing the ability to see in dim light.	Teach older adults the need for good lighting for best vision to avoid tripping and bumping into objects.
Color vision	Discrimination among greens, blues, and violets decreases.	The patient may not be able to use “dipstick” or other color-indicator monitors of health status.
Tears	Tear production is reduced, resulting in dry eyes, discomfort, and increased risk for corneal damage or eye infections.	Teach patient to use saline eyedrops on a schedule to reduce dryness. Teach patient to increase humidity in the home.