CHAPTER 218

Biology of the Eyes

The structures and functions of the eyes are complex. Each eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.

Structure and Function

The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.

The eye has a relatively tough white outer layer (sclera or white of the eye). Near the front of the eye, the sclera is covered by a thin mucous membrane (conjunctiva), which runs to the edge of the cornea and also covers the moist back surface of the eyelids.

An Inside Look at the Eye

Light enters the eye through the cornea, a transparent dome on the front surface of the eye. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the iris), which is actually a hole through the iris. The iris—the circular, colored area of the eye—controls the amount of light that enters the eye so that the pupil dilates (enlarges) and constricts (shrinks) like the aperture of a camera lens. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.

The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors. The high density of photoreceptors in the macula makes the visual image detailed, just as high-resolution film has more tightly packed grains. Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye. The photoreceptors in the retina convert the image into electrical impulses, which are carried to the brain by the optic nerve.

There are two main types of photoreceptor: cones and rods. Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. The rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color. Rods are grouped mainly in the peripheral areas of the retina and do not contribute to detailed central vision as the cones do.

Tracing the Visual Pathways

Nerve signals travel from each eye along the corresponding optic nerve and other nerve fibers (called the visual pathway) to the back of the brain, where vision is sensed and interpreted. The two optic nerves meet at the optic chiasm, which is an area behind the eyes immediately in front of the pituitary gland and just below the front portion of the brain (cerebrum). There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side and continue to the back of the brain. Thus, the right side of the brain receives information through both optic nerves for the left field of vision, and the left side of the brain receives information through both optic nerves for the right field of vision. The middle of these fields of vision overlaps. It is seen by both eyes (called binocular vision).

An object is seen from slightly different angles by each eye so the information the brain receives from each eye is different, although it overlaps. The brain integrates the information to produce a complete picture.

The eyeball is divided into two sections, each of which is filled with fluid. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor. The pressure generated by these fluids fills out the eyeball and helps maintain its shape.

The anterior segment itself is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.

Muscles, Nerves, and Blood Vessels

Several muscles working together move the eye, allowing people to look in different directions without moving their head. Each eye muscle is stimulated by a specific cranial nerve (see page 835). The optic nerve (a cranial nerve), which carries impulses from the retina to the brain, as well as other cranial nerves, which transmit impulses to each eye muscle, travel through the orbit.

An ophthalmic artery and a central retinal artery (an artery that branches off of the ophthalmic artery) provide blood to each eye. Similarly, ophthalmic veins (vortex veins) and a central retinal vein drain blood from the eye. These blood vessels enter and leave through the back of the eye.

Protective Features

The bony structures of the orbit protrude beyond the surface of the eye. They protect the eye while allowing it to move freely in a wide arc.

The eyelashes are short, tough hairs that grow from the edge of the eyelid. The upper lashes are longer than the lower lashes and turn upward. The lower lashes turn downward. Eyelashes keep insects and foreign particles away from the eye by acting as a physical barrier and by causing the person to blink reflexively at the slightest provocation.

The upper and lower eyelids are thin flaps of skin that can cover the eye. They reflexively close quickly (blink) to form a mechanical barrier that protects the eye from foreign objects, wind, dust, insects, and very bright light. The reflex is triggered by the sight of an approaching object, the touch of an object on the surface of the eye, or the eyelashes being exposed to wind or small particles such as dust or insects. On the moist back surface of the eyelid, the conjunctiva loops around to cover the front surface of the eyeball, right up to the edge of the cornea. The conjunctiva protects the sensitive tissues underneath it.

Structures That Protect the Eye

When blinked, the eyelids help spread tears over the surface of the eye. Tears consist of a salty fluid that continuously bathes the surface of the eye to keep it moist and transfers oxygen and nutrients to the cornea, which lacks the blood vessels that supply these substances to other tissues. When closed, the eyelids help trap the moisture against the surface of the eye. Small glands at the edge of the upper and lower eyelids secrete an oily substance that contributes to the tear film and keeps tears from evaporating. Tears keep the surface of the eye moist. Without such moisture, the normally transparent cornea can become dried, injured, infected, and opaque. Tears also trap and sweep away small particles that enter the eye. Moreover, tears are rich in antibodies that help prevent infection. The eyelids and tears protect the eye while allowing clear access to light rays entering the eye.

The lacrimal glands, located at the top outer edge of each eye, produce the watery portion of tears. Mucous glands in the conjunctiva produce mucus, which mixes with the watery portion of the tears to create a more protective tear film. Tears drain from each eye into the nose through one of the two nasolacrimal ducts. Each of these ducts has openings at the edge of the upper and lower eyelids near the nose, called the punctum.

Effects of Aging

In middle age, the lens of the eye becomes less flexible and less able to thicken, and thus less able to focus on nearby objects, a condition called presbyopia. Reading glasses, or bifocal lenses, can help compensate for this problem.

In old age, changes to the sclera (the white of the eyes) include yellowing or browning due to many years of exposure to ultraviolet light, wind, and dust; random splotches of pigment (more common among people with a dark complexion); and a bluish hue due to increased transparency of the sclera.

The number of mucous cells in the conjunctiva may decrease with age. Tear production may also decrease with age, so that fewer tears are available to keep the surface of the eye moist. Both of these changes explain why older people are more likely to have dry eyes.

Arcus senilis (a deposit of calcium and cholesterol salts) appears as a gray-white ring at the edge of the cornea. It is common among people older than 60. Arcus senilis does not affect vision.

Some diseases of the retina are more likely to occur in old age, including macular degeneration, diabetic retinopathy, and retinal detachment. Other eye diseases, such as cataracts, also become common.

The muscles that squeeze the eyelids shut decrease in strength with age. This decrease in strength, combined with gravity and age-related looseness of the eyelids, sometimes results in the lower eyelid falling away from the eyeball, a condition called ectropion. In some older people, the fat around the orbit shrinks, causing the eyeball to sink into the orbit. Because of lax tissues in the eyelids, the orbital fat can also bulge forward into the eyelids making them appear constantly puffy.

The muscles that work to regulate the size of the pupils weaken with age. The pupils become smaller, react more sluggishly to light, and dilate more slowly in the dark. Therefore, people older than 60 may find that objects are not as bright, that they are dazzled initially when going outdoors (or when facing oncoming cars during night driving), and that they have difficulty going from a brightly lit environment to a darker one. These changes may be particularly bothersome when combined with the effects of a cataract.

Other changes in eye function also occur as people age. The sharpness of vision (acuity) is reduced despite use of the best glasses, especially in people who have a cataract, macular degeneration, or advanced glaucoma. The amount of light that reaches the back of the retina is reduced, increasing the need for brighter illumination and for greater contrast between objects and the background. Older people may also see increased numbers of floating black spots (floaters). Floaters usually do not significantly interfere with vision.