Vision, Hearing, and Balance

The eyes and ears provide the body with its greatest protection. Because these organs are sensitive to distant stimuli, they can give early warnings about possible dangers.

The Eyes

The eye is often compared to a camera, but it is more compli­cated than the most sophisticated camera. Humans have binocu­lar vision, the ability to view objects with two eyes. People also have stereoscopic vision, the ability to see objects in three di­mensions—height, width, and depth. With stereoscopic vision a person can assess the speed of a moving object and determine the distance of an object in space.

Structure of the Eye

The eye, or eyeball, is an almost per­fect sphere with a diameter of about 2.5 cm (1 in.). The eye is protected in a number of ways. A fatty layer within the orbit, a socket in the skull, cushions the eyeball. Eyelids and eyelashes also provide protection by preventing foreign particles from en­tering the front of the eye. If something touches the eyelashes or moves suddenly in front of the eye, the eyelid closes and reopens rapidly in a blinking reflex.

Each eye is moved by three sets of muscles and is lubricated by mucus and tears. The mucus is secreted by the conjunctiva, a delicate, blood-rich membrane that lines the inner eyelid and covers the front of the eye. Tears are produced by the lacrimal gland near the outer corner of the eye. When the eye closes, the eyelid spreads the mucus and tears, which moisten the eye and help remove foreign particles.

The eye has an outer wall that consists of three layers of tissue: the sclera, the choroid, and the retina. These three layers surround a jellylike substance, the vitreous humor that makes up two-thirds of the eyeball.

The sclera is tough, white connective tissue that forms the outermost layer. About 80 percent of the sclera, including the ''white'' of the eye, is opaque. The remainder is a transparent layer called the cornea at the front of the eye.

The choroid is the middle, darkly pigmented layer of tissue. It absorbs light and so prevents reflection, which would result in fuzzy images. The choroid contains many blood vessels that nourish the eye. Toward the front of the eye, the choroid forms a colored ring, the iris, which gives the eye its color. In the center of the iris is an opening called the pupil. In bright light, one set of muscles in the iris contracts and causes the pupil to become smaller. In dim light, a different set of iris muscles contracts, making the pupil larger.

Behind the pupil is the lens, a transparent, curved structure. By changing shape, the lens helps focus images onto receptor cells at the rear of the eye. The curvature of the lens is con­trolled by ciliary muscles attached to the choroid. A clear, wa­tery fluid called the aqueous humor fills the space between the lens and cornea. The vitreous humor fills the space behind the lens.

The innermost layer of the eye is the light-sensitive retina. The retina contains about 125 million receptors called rods and cones. The rods and cones are stimulated by light to generate nerve impulses. The rods are extremely light-sensitive and can detect various shades of gray even in dim light. However, they cannot distinguish colors, and they produce poorly defined images. The cones detect color, produce sharp images, and are important for seeing in bright light. In a tiny pit at the center of the retina is a concentration of cones. This area, the fovea, produces the sharpest image.

How You See

Light passes through the cornea, aqueous humor, pupil, lens, and vitreous humor on its way to the retina. Impulses generated by the rods and cones travel to the visual center in the occipital lobe of the brain by means of the optic nerve. The optic nerve from each eye consists of about 1 million nerve fibers. No rods or cones exist at the point where the optic nerve enters the retina. This area, called the optic disc, or blind spot, does not transmit impulses. Near the base of the brain, half of the nerve fibers from the left eye cross over and join half of the nerve fibers from the right eye. All these fibers go to the right side of the brain. Likewise, half the nerve fibers from the right eye join half from the left eye and go to the left side of the brain. Each side of the brain thus receives images from both eyes. The point at which the partial crossing-over of the fibers occurs is the optic chiasma.

Thinking About Biology: What Your Eyes Tell About You

A doctor can learn a great deal about the condition of your entire body by looking into your eyes. Using an ophthalmoscope, which has special lenses and a light, he or she can study the optic disc and the blood vessels of the retina. Studying the retina may reveal many disorders that do not directly involve the eyes. For example, high blood pressure can be identified by viewing the blood vessels of the retina. The increased pressure of the blood circulating through these tiny vessels may cause some of them to burst. Diabetes may also cause changes in these blood vessels and in the vitreous humor. Changes in the size and shape of the optic disc may indicate such serious disorders as glaucoma or even brain tumor.

Disorders of the Eye

Eye disorders affect more than 50 percent of the people in the United States. Among the most common of these disorders are myopia and hyperopia. In myo­pia, or nearsightedness, the eyeball is too long from the front to the back. Light focused by the lens falls at a point in front of the retina, resulting in a blurred image of distant objects. In hypero­pia, or farsightedness, the eyeball is too short from front to back. Light is focused at a point behind the retina, resulting in a blurred image of close objects. In astigmatism, irregularities in the curvature of the cornea result in fuzzy images. Prescribed eyeglasses or contact lenses can correct these conditions.

In a condition called glaucoma, the aqueous humor cannot drain into blood vessels around the eye. Because new aqueous humor is constantly produced by the choroid, failure to drain creates excess pressure within the eye. This pressure damages the retina and optic nerve and can lead to blindness.

The Ears

The eyes have only one function—vision. The ears, however, perform two vital functions—hearing and balance.

Structure of the Ear

The ear is divided into three major sections: the outer ear, the middle ear, and the inner ear. Each region has a specific function.

The outer ear consists of a cartilage flap called the pinna and the auditory canal, a tube leading to the middle ear. These structures channel sound to the eardrum, a tightly stretched membrane between the outer ear and the middle ear. The auditory canal is lined with cilia and special cells that secrete ceru­men, or earwax. Together, the cilia and earwax clear foreign particles from the auditory canal.

The middle ear lies within an air-filled space called the tympanic cavity inside the skull bone. A duct called the Eusta-chian tube connects the middle ear to the pharynx. Generally the tube is collapsed, but it opens when you yawn, swallow, cough, or blow your nose. Air pressure be­tween the middle ear and throat is then equalized. Air pressure around you varies with altitude and can change rapidly, as when you ride an elevator or airplane. If the pressure is not equalized, the eardrum can bulge, causing pain and difficulty in hearing. Lying across the middle ear cavity are three tiny bones called the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup. The stapes touches a membrane called the oval window, located between the mid­dle ear and the inner ear.

The inner ear contains the sensory receptors for hearing and balance. It consists of three main parts: the cochlea, the vesti­bule, and the semicircular canals. The organ of hearing is within the cochlea, a bony, coiled tube filled with fluid and lined with hair cells. A second membrane-covered opening is located in the cochlea below the oval window. Called the round window, it maintains a constant pressure within the inner ear. The upper part of the inner ear consists of three semicircu­lar canals, which are fluid-filled tubes positioned alright angles to each other. These canals help maintain balance by responding to head movement. A bony chamber called the vestibule lies between the semicircular canals and the cochlea.

How You Hear

Sound waves are generated when any object vibrates, or moves back and forth, in the air. The human ear can detect sounds between 20 and 20,000 vibrations per second.

The vibrations travel through the auditory canal to the eardrum; to the malleus, which touches the eardrum; and then to the incus and the stapes. The stapes touches the oval window. The oval window sets the fluid in the cochlea in motion. Stimulated by the fluid motion, hair cells in the cochlea generate nerve im­pulses that travel along the auditory nerve to the auditory center in the temporal lobe of the brain. Exactly how vibrations are transformed into impulses is not clear.

Hearing loss due to disease or injury of the auditory nerve or cochlea is called nerve deafness. It is the most common cause of total and permanent hearing loss. Deafness resulting from interference as vibrations pass to the inner ear is called conduc­tive deafness. This condition may be caused by several prob­lems, including excess earwax, infection, swelling and closing of the passage, rupture and inflammation of the eardrum, or immobility of the stapes due to bone overgrowth. Conductive deafness senerally can be treated.

How You Balance Yourself

Fluid in the semicircular canals flows when you change the angle of your head. A different canal in each ear is affected by any particular movement. For example, a movement to the right causes fluid in the right ear to flow toward the hair cells. As a result, many impulses are sent to the cerebellum from the right ear. At the same time, the movement causes fluid in the left ear to flow away from the hair cells. Few impulses are then sent to the brain from the left ear. The cerebellum interprets the two sets of impulses so you know which way your head is turned.

The saccule and utricle, the two sections of the vestibule, also help with balance. They are lined with hair cells covered by a gelatin-like membrane embedded with grains of limestone. Gravity pulls the limestone down onto the hair cells, causing them to generate impulses. The greater the pull on particular grains, the stronger the impulses. The cerebellum interprets the direction of gravity and lets you know the position of your head.

Smell, taste and touch

Smell and taste are closely associated senses. The fact that a stuffy nose makes food seem tasteless demonstrates the close relationship between these senses. Smell and taste seem to oper­ate more simply than sight and hearing, but biologists do not yet know precisely how the receptors for smell and taste discrimi­nate among various chemicals.

The skin, the largest organ of the body, contains several types of receptors. These receptors register touch, pressure, pain, heat, and cold. The receptors for these sensations vary in number and location over the body.

The Nose

The nose, the chief sense organ of smell, contains receptors embedded in mucous membrane. About 50 million of these spe­cial cells, called olfactory receptors, are located in each nasal passage. Airborne substances dissolve in the mucus that covers the olfactory receptors. The receptors produce nerve impulses that travel through olfactory nerves to the olfactory lobe in the cerebral cortex.

Some biologists think that the perception of smell occurs when a specialized molecule on the receptor surface reacts with a specific chemical in inhaled air. The reaction generates an impulse that results in a particular smell. Other scientists believe that the outline, or shape, of a molecule is the cause of its partic­ular odor. They think that a molecule of a specific shape fits into an olfactory receptor that will accept only that shape, just as a lock works with one key. These researchers believe that the thousands of odors humans can distinguish are simply combina­tions of seven basic odors.

The Tongue

The tongue is the major sense organ of taste. The chemical receptors for taste are clusters of sensory hair cells located in the taste buds. Each taste bud consists of about 40 receptor and supporting cells and an opening called the taste pore. The taste buds lie in bunches called papillae, which are visible as the bumps on your tongue. Although most of a person's 10,000 taste buds are on the tongue, a few also exist on the roof of the mouth and in the throat.

Taste buds produce one or a combination of four main taste sensations: sweet, sour, bitter, and salty. A receptor cell may be stimulated by only one taste, but most cells are stimulated by two or more tastes. This combination of different tastes may be what produces the wide variety of flavors you enjoy.

Like smell, taste depends upon chemical reactions that take place only in solution. Saliva constantly bathes taste buds, reaching receptor cells through the taste pores. Food molecules also enter the taste pores. The chemical reactions that take place somehow cause the receptor cells to generate nerve impulses. The impulses travel through three different nerves to the taste center in the cerebral cortex. No one knows precisely how taste receptors function. Some researchers think that sensory cells have sites that accept specific chemical molecules. Other re­searchers think that, as with smell, the shape of a molecule determines its taste. Molecules of a certain shape, they think, activate specific sites on a taste bud to produce one taste.

The Skin

The skin is considered the organ of touch. It actually contains five distinct senses, most with their own type of receptor. These five senses are touch, pressure, pain, heat, and cold. Impulses travel from the various sense receptors to different areas of the sensory cortex.

Touch receptors are the ends of certain nerve fibers .Many touch receptors are located at the base of hairs and generate impulses when the hairs move even slightly. However, touch is most sensitive in the fingertips, palms, lips, and other places where hair is not present. Other receptors react to pressure. Some are sensitive to deep pressure and vibration, while others are sensitive to lighter pressure.

Unlike the other senses of touch, the sense of pain has no specialized receptors. Pain receptors are free ends of unmyeli-nated nerve fibers. Pain appears to stem from a variety of stim­uli. Some parts of the body are almost pain-free. Other parts may sense only one type of pain. Sensitivity to pain may be related to other body conditions, such as mental attitude.

Temperature receptors may be either bare nerve endings or specially shaped cells. Different types of receptors detect heat and cold.