ALTHOUGH we largely take it for granted until it fails or gives us distorted sense of sounds, hearing is one of our most important senses! But did you ever stop to think about what the sensory world would be like without hearing? To begin with, the normal ear identifies about 400,000 different sounds. Imagine the loss from taking out these rich sensory experiences sounds created by footsteps, wind and rain, drums, a child, a voice. Many sounds warn us of danger or soothe the emotions.
But sounds are more important than sirens or music. In fact, it might surprise you to realize that our hearing is a major part of what makes us unique as humans. Our mental and communicative skills depend largely on this sense. Without hearing we cannot learn to talk well. Language, which sets us apart from the animals, is very difficult to learn without hearing. Helen Keller, herself both blind and deaf from baby hood, wrote, "The problems of deafness are deeper and more complex, if not more important, than those of blindness. Deafness is a much worse misfortune, for it means the loss of the most vital stimulus the sound of the voice that brings language, sets thoughts astir, and keeps us in the intellectual company of man."
If you have had hearing and suddenly it is taken away from you, the eerie loss and isolation has profound psychological impact. "When your ears have been busy bringing you the sounding world for almost nineteen years," wrote Mary Heiner, who went deaf in college, "the abrupt absence of that part of consciousness is too astounding, too bewildering, too frightening to be summed up in the one word, 'deaf.' I'd gone to sleep in a secure world full of sound wonderful and dancing sound and I awakened in a silence as woolly and obliterating as deep snow in the country."
For sound detection, the ear is a most incredible device! It inspires admiration in an engineer acquainted with the most intricate machinery known to man. As a pressure sensitive transducer, it generates nerve messages from sound waves; the nerve messages are interpreted as sound by the brain. The ear is so sensitive that weak sounds causing the eardrum to move less than the diameter of a hydrogen atom (25.5 billionths of an inch) can be heard, yet loud sounds several million times stronger will not destroy the sensitive mechanisms of hearing. With this device we can detect sounds ranging in vibrations from 20 to 20,000 times a second. A new born infant can hear sound waves up to 40,000 per second, and a bat can detect 120,000 vibrations per second well beyond the range of humans. How does this magnificent receiver set work?
The ear can be divided into three sections, though, of course, it functions as one unit. These parts are called the outer, middle, and inner ears. A sound wave impacts the outer ear and is captured by the ear lobe and conducted into the head through a small canal called the external auditory meatus, or ear canal. The ear canal is about one inch long and about one-half inch in diameter. The sound waves are focused at the base of the ear canal on a taut membrane called the eardrum, or tympanic membrane. The eardrum vibrates with the sound, oscillating slowly for low pitches and faster for higher ones. These three structures, the ear lobe, canal, and drum make up the so-called outer ear.
The middle ear amplifies sound waves from the eardrum about a hundred times before sending the pressure wave to the inner ear. Most of the amplification takes place simply because the ear drum has about twenty times as much surface area as does the oval window of the inner ear. However, some amplification takes place by lever action of three very small bones, the auditory ossicles, in the middle ear. These are called the hammer, anvil, and stirrup. The hammer is attached to the eardrum and is first to receive the sound wave, conveying it through the anvil to the stirrup. The stirrup moves the oval window as the sound pressures pulsate the eardrum. The smallest of the three, it is about half the size of a grain of rice. The three bones form a pathway for mechanical movements through the middle ear.
Another function of the middle ear is to protect the inner ear from over loading and possible destruction by loud noises, like those from a jet air craft. Two very small muscles associated with the small bones in the inner ear are triggered by loud noise to con tract, tighten the eardrum, and with draw the stirrup from coupling the inner to the outer ear. Thus the sound waves are made less forceful. A safety role is also played by the external ear canal, the Eustachian tube, in equalizing air pressure changes between the middle ear and the mouth. The popping sound in your ear when you descend via an elevator in a tall building is caused by these pressure changes. The most important functions of the middle ear are summarized in two words, amplification and protection all in a volume of space as big as a sugar cube!
The actual translation of mechanical pressure to nervous system coding takes place in the inner ear, a small three-chambered tube embedded in the temporal bone of the skull. It is a little more than an inch long and coiled like a snail shell with about three turns. From this it gets its scientific name cochlea, the Greek word for snail.
When a sound wave hits the eardrum, the oval window of the cochlea, set in vibration by the stirrup, imparts movement to the fluid in one of the canals. This canal of the inner ear, the tympanic duct, joins a second canal at the top of the spiral, so hydraulic movement in one canal causes fluid motion in the other. A round window dissipates this hydraulic motion. Between these systerns of canals lies the cochlear duct. The ducts are very small the fluids from all three combined is only about a drop and a half! In fact, the miniaturization of all the various parts is one of the impressive features of the ear.
An airborne sound is directed, amplified, and sent to the organ of Corti. The organ of Corti, housed in the cochlear duct, is an intricate and beautifully designed structure that permits man to distinguish sounds as varied as a pipe organ and the bark of a dog. Amplified sound waves are sent here, and wavelike ripples in the cochlear duct cause sensitive hair cells to code their frequency and intensity.
The hair cells, which number about 25,000, are arranged in pyramids that rest on a structure called the basilar membrane. Projecting hairs from the hair cells contact a highly flexible membrane called the tectorium. With the slightest motion of the basilar membrane these hairs are stressed and be come electrically excited in a fraction of a second. The electrical charges set off coded messages to the brain over the auditory nerve. Within the brain these messages ascend enormously complex pathways, but eventually arrive at the cortical mantle, where the sound is decoded and compared with stored memories of previous sounds heard and learned on other occasions. It is ultimately the brain that relates these sounds to joy, sorrow, laughter, music, and communication.
The ear is one of the most impressively engineered sensing devices known to man. Although we still do not understand all there is to know about our ability to hear, what we already know strongly supports the creative role played by our Master Craftsman in its design. In its grace and intricacy, the ear of man is another witness to His wisdom, power, and love.
