Structure of the ear
There are three parts to the structure of the ear (see Figure 1):
- outer ear
- middle ear
- inner ear
Figure 1. Anatomy of the Ear (click for an enlarged image)
This is the visible part of the ear and consists of three element:
- pinna (or auricle): pinna is Latin for wing or flap – so, the pinna is the visible cartilaginous wing/flap that projects from the head.
- meatus: a meatus is simply an opening or passageway – the acoustic meatus is, therefore, the opening of the passage through the temporal bone of the skull.
- canal: the narrow tube which leads to the middle ear.
The function of the outer ear is simply to direct air vibrations onto the ear drum of the middle ear.
When a sound wave arrives at a listener’s ear it causes the eardrum (tympanic membrane) to vibrate. This, in turn, causes the movement of three small bones (ossicles) within the cavity of the middle ear:
- hammer (malleus)
- anvil (incus)
- stirrup (stapes)
Their function is to amplify the vibration of the sound wave.
In addition to these bones, there is a tube (Eustachian tube) which connects to the pharynx (the cavity that leads from the back of the mouth and the nasal passages to the larynx and esophagus). The Eustachian tube aerates and drains the middle ear. It allows the gas pressure in the middle ear to equalize with the external air pressure. The ‘popping’ sensation you sometimes experience when descending in an airplane is caused by the Eustachian tube opening. If the Eustachian tube becomes blocked this can lead to middle ear inflammation and intermittent hearing loss.
Within the inner ear there is a bony labyrinth. This is a complex system of cavities and tubes, divided into three parts: the vestibule (central cavity), semicircular canals, and cochlea (a spiral, snail-shaped bony tube filled with tiny hair cells).
There is a similar membranous labyrinth, situated inside the bony labyrinth, which is filled with fluid (endolymph). The vestibule of the membranous labyrinth consists of two cavities. The largest of these is the utriculus. This is the main organ of the so-called vestibular system, which works to helps us maintain postural balance and to provide us with information about the orientation of our body in space. The smaller cavity is the sacculus, which connects by a membranous tube to the cochlea.
The stapes of the middle ear is connected to a membrane in the wall of the cochlea. Overall, the cochlea functions as a so-called transducer, i.e. a device that converts energy from one form to another. For example, a loudspeaker is a transducer that converts electrical signals into sound energy. In the case of the ear, the cochlea transforms mechanical vibrations into electrical signals. These electrical signals are subsequently transmitted along the 30,000 or so fibers (neurons) that make up the auditory nerve to the brain of the listener.
The auditory nerve feeds signals to the cortex of the brain, specifically the auditory cortex which is situated in the temporal lobe. Here the neurons are sensitive to different frequencies, some being responsive to high frequencies, some to low, and so on. The auditory cortex allows us to perceive sound, identifying such things as pitch, loudness and rhythmic patterns of sound and music.