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.