Ears have been around a long time, at least 400 million years, in fact.

But hearing was not, and is not, their primary function. A sense of balance is what they're all about and that is regulated by the highly specialized mechanisms of the inner ear.

Hearing is simply an offshoot of this sense, a bonus enjoyed only by certain species of insects and vertebrates, ranging from fish through amphibians and reptiles to birds and mammals. Including humans.

If the human ear was meant to fly God shouldn't have given us eustachian tubes. They connect ear, nose and throat and equalize pressure on the eardrum, which lies between the outer and middle ear.

The rapid changes in cabin air pressure during a plane's descent can affect the tubes, especially if they're blocked by a cold, allergy or sinus infection. tSymptoms of aerotitis range from a mild "pop" or plugged-up-feeling to acute pain and temporary hearing loss.

Yawning, swallowing, chewing gum and pinching the nose while blowing through it should help equalize the pressure on the eardrum. If not, try an antihistamine tablet an hour before landing.

Avoid wines while aloft, particularly sherry and port. They contain histamines, which only aggravate the problem.

Infants and children suffer, too. Make sure they're awake and sitting up well before the plane lands. If they're too young to follow the yawn-and-swallow advice, give them water (not milk) to drink, or a pacifier to suck.

But the basic rule for all ages: Avoid flying when you have any sort of upper respiratory infection. The same rule applies when diving deep into water; some people even have a pressure problem in high-speed elevators.

Ignore tradition; perfume is lost behind the ears. Dab some on pulse points instead (for instance on the throat just below each earlobe) and let their extra warmth speed your fragrance on its way.

Never mind Clark Gable: Children still hate it when their ears stick out. They can be helped by plastic surgery, best done around 5 or 6 years. Cauliflower ears and other deformities can be remedied too. Have a talk with your doctor.

Probably the most famous ear on earth belonged to Vincent van Gogh, who, according to erstwhile firend Paul Gauguin, went home after a cafe set-to in France and "immediately cut his ear off level with the skull." As usual, Gauguin exaggerated. Van Gogh, in fact, sheared off the lobe and presented it (possibly washed and in an envelope) to a girl at an Arles brothel. Vincent never explained why.

Conductive hearing loss -- the simpler type -- occurs when sound is blocked on its way from the outer ear canal through the eardrum to the inner ear and auditory nerve, its pathway to the brain. The causes are numerous.

In otosclerosis, perhaps the most serious type of hearing loss, a stirrup-shaped bone smaller than a grain of rice becomes rigidly attached to the wall of the middle ear. Surgery now offers a 90 percent chance of relief.

More commonly, hearing is affected by an over-secretion of wax, or a blocked eustachian tube, or some small object pushed into the ear. While pain or discharge should speed you to a doctor -- never home-treat your own or children's ears without getting a medical go-ahead -- plugs of wax or other obstructions should get medical attention, too.

Don't poke away at them yourself. The eardrum is as thin as a sheet of tissue paper and nearly as delicate. Clean the outer ear with a washcloth, but resist the temptation to venture further in. For the most part, the ear is admirably adapted to take care of itself.

Perceptive hearing loss, or "nerve deafness," may obliterate sounds in the high frequency range or wipe out hearing entirely. Noise pollution can be a significant cause; permanent impairment can begin with long exposure to sounds at 85 decibels.

Heavy city traffic can be 90 decibels; motorcycles, 95 at 50 feet; power lawn-mowers, 100; pneumatic drills, 110 and a jet, 500 feet overhead, 115 Exposure to 100 decibels for 15 minutes should be followed by at least 20 minutes of peace and quiet, for your nerves as well as for your hearing.

Other causes of perceptive hearing loss: blows to the ear, drug reactions (which also can cause unpleasant ringing sensations) and alcoholism.

And then there's Meniere's syndrome (a serious disturbance of the inner ear that's one of the few forms of deafness ffecting both hearing and balance), brain or nerve tumors, measles, mumps, meningitis, strep infections . . . and cities in general

The way things are going, says a United Nations study, urban noise, pollution will have caused some degree of hearing loss in a majority of the world's citizens by the year 2000.

Industrial-type earmuffs are the best protection against noise. Earplugs sold over the counter won't be of much help, and they can cause infection. (never borrow anyone else's.) Plugs should be fitted properly and their care and use monitored by a professional.

If you're convinced everyone mumbles, if you need the TV and radio volume higher than everyone else in the room, you may need a hearing aid. None can restore loss hearing, however, they merely amplify sounds. (All sounds, not just the ones you want to hear.)

Don't head for the nearest dealer to buy the first device you see. Dresser drawers coast to coast are hideouts for aids purchased in haste and discarded in frustration.

Have your family doctor refer you to an otologist (ear specialist) or otolaryngologist (ear, nose and throat specialist -- although the proper word is otorhinolryngologist). He or she can ascertain the type and cause of impairment.

Then visit an audiologist for a thorough hearing evaluation. If a hearing aid can help, you'll be referred once more for proper fitting, followed up by extensive instruction and counseling.

Deafness in children has been mistaken for everything from mental retardation to cussedness, with regrettable implications for the child. If you suspect your child isn't hearing all he or she should, don't be put off by well-meaning reassurances, even from your family doctor. "The parents are almost always right," says one audiologist.

Audiologists suggest you try these simple tests:

Clap your hands smartly 3 to 6 inches behind a baby under 3 months. There should be a startled response.

At 3 to 6 months, the baby should stop moving when you call or make an unfamiliar noise.

From 9 to 12 months, the infant should turn if called from behind.

By 2 years, a toddler should be talking in short phrases.

While lack of response isn't absolute proof of hearing loss, it does mean you should waste no time having the child professionally tested. Risk-prone infants (identified by a mother's bout with German measles during pregnancy, deafness in both parents or ear deformation) can be tested from birth.

Older children who ask for things to be repeated, or who respond to questions with unrelated answers also should be tested.

The ear, say doctors, is one of the more sensitive parts of the body. No wonder. Four of the 12 cranial nerves have branches to the ear, and those connect to the face, tongue, throat, main digestive organs, heart and lungs.

It may not be the sound of music, but one man who lectures on the fine points of wine tasting, advises students to listen as they pour. "Wine should involve all the senses. Appreciate the cork's pop. Put your ear down to hear the fizz of bubbles in champagne."

He's right. At the prices we pay nowadays, we can't afford to waste a single thing.