Sound works by traveling through air in waves; these compress, then stretch, and the process then repeats. The compression pushes on such objects as ear tissue. As a wave then stretches out again, it pulls upon the tissue. These pushing and pulling wave aspects cause a vibration in whatever the sound hits.
Waves of sound first strike the outer ear. It is known more formally as the auricle or pinna. The shape of the outer ear assists in collecting sound and directing it within the head; it travels then toward the middle ear and inner ear. The ear’s shape assists in the amplification of the sound along the way, increasing the sound’s volume, as well as determining where the sound is originating.
Sound waves travel from the outer ear through the tube known as the ear canal. In humans, this tube is tiny. It is only about an inch long. While humans have both an outer ear and an ear canal, not all animals share these features. Many frogs simply have a flat spot located behind their eyes: this is the frog’s eardrum. In animals that do have both an ear canal and an outer ear, the tympanum, or the eardrum, is located inside the head. A tight membrane, it stretches across the ear canal’s end. When sound waves impact the eardrum, they cause a vibration in the membrane. This causes a triggering of pressure waves which then swell, passing into the middle ear.
The Anatomy of an Ear
Within the middle ear is a small cavity. This has three tiny bones: the malleus, which in Latin means hammer; the incus, which in Latin means anvil; and the stapes, which in Latin means stirrup. In humans, this trio of bones is known as the ossicles. The smallest bones in the human body, one of them, the stapes, is only a tenth of an inch long. This trio of bones works together in receiving sound waves, transmitting them along to the inner part of the ear. Not all animals possess ossicles. Snakes, for example, lack outer and middle ears. In a snake, the jaw transmits the vibrations of sound directly to the snake’s inner ear.
Within the inner ear is a structure shaped like a snail and filled with fluid. This is known as the cochlea. Within the cochlea stand ranks of microscopic hair cells. They contain collections of quite small strands that are like hair and are embedded in a membrane that is like gel. When the vibrations of sound enter within the cochlea, they cause the membrane, along with its hair cells, to sway back and forth. The movements of these cells and membrane send messages along to the brain. The messages register sounds like any of an abundance of distinct pitches.
The hair cells in the ear are quite fragile. The death of one means that it is gone forever. This means that over time, as these hair cells disappear, people begin losing the ability to detect particular sounds within the functions of the ear. The hair cells that die off first tend to be those that respond to high-pitched sounds. A teen, for example, may be capable of hearing a sound with a distinctly high frequency of around 17,000 hertz, while an older person may not.
Putting it all Together to Create Hearing!
A sound is transmitted in the form of sound waves that come from the environment. These sound waves are collected by the outer ear, then sent down along the ear canal until they strike the eardrum. The eardrum is caused to vibrate by these sound waves, which in turn sets into motion the trio of tiny bones, the ossicles, set within the middle ear. The bones’ motion then causes the movement of the fluid in the cochlea or inner ear. This movement of the fluid in the inner ear, in turn, causes the cochlea’s hair cells to bend. The movement of the hair cells is changed into electrical pulses. These electrical pulses are then transmitted to the auditory, or hearing, nerve and sent along up to the brain. They are then interpreted as sound.
The anatomy of the auditory system is quite complex but can be categorized broadly in two parts: one is peripheral and the other central. The former is made up of a trio of parts: the outer ear, then the middle ear, and finally the inner ear. The outer ear is made up of the auricle or pinna, the ear canal, and the eardrum. The middle ear, which is small and filled with air, contains the ossicles. The malleus, which connects to the eardrum, links the eardrum to the outer ear. The stapes connects then to the inner ear. The inner ear is special, containing both organs for hearing and balance. The hearing part is the cochlea, which means snail in Greek. It has a shape that is distinctively coiled. The central hearing system does not have as many parts but is incredibly complex. It is made up of the auditory nerve and the complicated pathways through the stem of the brain and on to the brain’s auditory cortex.
Regardless of size or shape, the ear of vertebrates is a fascinating and complex sensory apparatus. Sounds tell creatures much about the environment that surrounds them. Ears interpret complex waves and turn them into signals that the brain can interpret. The functions of the ear, when understood, allow the appreciation of what otherwise simply appears to be flaps of skin at the side of the head.