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Ears & Diving
Anatomy of the Ear
The ear is the organ of hearing and balance. It consists of a cavity in the skull structure lined with soft tissue, which encloses three distinctive spaces filled with air or liquid (external, middle and inner ear); these distinctive spaces host both sound transmission mechanisms and sensory apparatuses.
The external ear includes the pinna (auricle) and the ear canal up to the eardrum (tympanic membrane), which separates it from the middle ear. The lining of the external ear is skin rich with glands that produce earwax.
The middle ear is a cavity in a temporal bone lined with a thin layer of tissue similar to that found in the nose and throat. It is separated from the ear canal by the eardrum and connected to the throat via the Eustachian tube. It includes three tiny bones (auditory ossicles) forming the chain attached to the eardrum on one side and to the oval window membrane on the inner-ear side. The middle-ear space is filled with air at ambient pressure, which needs to be equalized when ambient pressure changes (as occurs in diving or flying). This is accomplished by moving air in or out through the Eustachian tubes, which connect the throat to the middle ear, using equalization techniques such as the Valsalva maneuver.
The inner ear, or labyrinth, includes the cochlea (hearing organ) and the vestibule and semicircular canals (balance organs). The cochlea and the vestibule are the origin of the auditory and vestibular nerves.
Pressure waves transmitted by air or water are funneled by the pinna and the ear canal to the tympanic membrane. The pressure waves cause the tympanic membrane to vibrate, which causes the auditory ossicles to move simultaneously in response. The stapes (the last bone in the chain) strikes the oval window of the cochlea. Since this is a closed system, when the oval window is pushed inward, the round window pushes outward. The fluid within the cochlea transmits the pressure waves to the auditory nerve, which in turn, sends signals to the brain that are interpreted as sound.
Parts of the vestibule are projections known as the semicircular canals. The fluid within this system moves correspondingly with head movement. Inside the semicircular canals are hairlike structures called cilia. The cilia detect movement of the fluid through the canals and send the signals through the vestibular nerves to the brain, where the movement is interpreted and used to help determine the position of the head in three-dimensional space.