Structure of Ears
Ears can be divided into outer, middle, and inner ear. The outer ear mainly consists of the pinna which is made of ridged cartilage covered by skin and the external auditory canal. The eardrum is the thin membrane which separates the outer ear and the middle ear. The middle ear has three ossicles (ear bones) and the Eustachian tube and is filled with air. The inner ears mainly consists of the semi-circular canals, the otolithic organs and the cochlea and is filled with liquid.
(Image taken from (WebMD, 2009))
Function of Ears
Sound waves is first gathered by the pinna and directed by the external auditory canal to the eardrum. The eardrum converts sound waves into mechanical vibrations and the ossicles, which is connected to the eardrum, then amplify and transmit vibrations from the eardrum to the membrane of the oval window of the cochlea. Finally, the cochlea transforms mechanical energy into nerve impulses which can be transferred to the brain through the auditoury nerve. Through the pressure wave within the cochlea, the frequency and intensity of the sound can be indentified.
The function of balance is facilitated by the vestibular system within the inner ear which consists of the semi-circular canals and the otolithic organs. The semicircular canals are responsible for detecting changes in angular acceleration, and the otolith organs (the utricule and the saccule) are responsible for detecting changes in linear acceleration and gravity.
There are three interconnected semi-circular canals orientated at right angles to one another in each of the ears. They are filled with endolymph and contains sensory-hair-attached cupula. When the head is moved in the plane of a particular canal, the inertia of the endolymph acts so as to deflect the cupula in the opposite direction, causing the bending of the sensory hairs which would then transmit nerve impulses to the brain. Thus, the changes in the angular direction and the rate of movement of the head can be detected.
(Image taken from (McGraw-Hill, 2000))
The otolithic organs have a gelatinous structure covered by the otholiths which composed of calcium carbonate (CaCO3). Within the gelatinous structure, there are plenty of sensory hair cells. Gravity would cause the otoliths to distort the sensory hair in a direction determined by the position of the head which would then transmit nerve impulses to the brain. Thus, the vertical and lateral movements, including linear acceleration and deceleration, can be detected.
(Image taken from (Tamarkin, 2011))