The ears are the organs responsible for our ability to hear and maintain balance. Hearing is an important special sense as it allows us to communicate and perceive sounds and speech. In terms of function, the ear can be split into two systems: the auditory and vestibular systems.
The auditory system is responsible for audition (hearing) and begins as soon as sound waves enter the ear. The auditory system is composed of the external, middle, and inner ear.
The external ear is made up of the auricle (or pinna) and the external acoustic meatus (or auditory canal). The auricle is the visible part of the ear that sits on the side of the head and directs sound into the external acoustic meatus. The external auditory meatus is an S-shaped canal that directs sound onto the tympanic membrane.
The inner ear contains the sensory structures for both hearing and balance. It consists of a network of bony canals contained deep within the temporal bone. The parts of the bony labyrinth are the cochlea and the semicircular canals, which are connected together by a centrally-placed vestibule. Only the cochlea is involved with hearing.
The auricles sit on either side of the head and are composed of, and shaped by, elastic cartilage that is covered with skin. The shape of the auricles causes sound to be directed into the external acoustic meatus.
The external acoustic meatus is an S-shaped passage that begins as an opening in the auricle. It travels about 4 cm through the tympanic portion of the temporal bone to terminate at the tympanic membrane.
The scala vestibuli is the upper, triangular canal, bound by the roof of the bony cochlea above and the vestibular membrane below. It passes through the entire length of the cochlea. The oval window sits at the base of the canal, where it connects to the tympanic cavity. The helicotrema, an opening that joins the scala vestibuli to the scala tympani, sits at the apex. The scala vestibuli contains perilymph, which is maintained by the secretory epithelial cells lining the canal.
The scala tympani is the lower, triangular canal comprising the space beneath the basilar membrane. It passes through the entire length of the cochlea. At its base sits the round window, which connects it to the tympanic cavity, and at its apex the helicotrema joins it to the scala vestibuli. The scala tympani also contains perilymph.