Anatomy of the Ear

Sound waves impinge on the auricle (pinna) of the outer ear and travel through the external ear canal (external auditory meatus) to the tympanic membrane (eardrum). The eardrum vibrates according to the frequency of the sound e.g. 1000 cycles per second if it is a 1000 Hz signal.

Sound travels through the three ossicles (bones), the malleus (hammer), incus (anvil), and stapes (stirrup) of the middle ear. The eustachian tube, which opens into the middle ear, is responsible for equalizing pressure between the air outside the ear to that within the middle ear. The stapes attaches to the oval window, which connects the middle ear to
the inner ear.

Two important structures are located within the inner ear: the vestibular organ of balance, and the organ of hearing, the cochlea (with the organ of Corti). The cochlea is a spiral-shaped structure with a base and an apex. The organ of Corti lies on the basilar membrane within the scala media of the cochlear duct. Outer and inner hair cells (OHC and IHC) are found on the basilar membrane and are coupled to two types of nerve fibers.

The nerve fibers form the auditory section of the eighth nerve lead from the brainstem to terminate in the auditory cortex (cochlear nucleus, superior olivary complex, inferior olivary complex, medial geniculate body and AI and AII of the cortex).

27-Pinna 29-External Auditory Meatus (external ear canal) 31-Tympanic Membrane (ear drum)

The pinna (27) collects sound and directs it into the ear canal (29). In adults, the ear canal is about 25 mm long and 7 mm wide. The canal narrows near the middle and then widens again at the ear drum (31). The first part of the ear canal is cartilaginous and can be changed in shape by tugging on the pinna.

Cerumen (wax) is produced by glands in the lateral half of the ear canal. Wax keeps the ear canal moist, inhibits the formation of bacteria, and prevents insects from migrating towards the eardrum. The wax also serves as a cleaning mechanism for the ear, which is why doctors normally discourage any manual cleaning (for example, with cotton swabs) because this may push the wax too far into the ear.

17-Malleus 19-Incus 21-Eustachian Tube

23-Tensor Tympani 25-Stapes

31-Tympanic Membrane (ear drum) -part of the outer ear

The middle ear cavity is filled with air and contains several small bones then transfer vibrations from the ear drum to the cochlea of the inner ear. The malleus is the largest bone in the middle ear, with the stapes being the smallest bone in the human body (the stapes is also fully developed at birth). The eustachian tube connects the middle ear to the pharynx and respiratory system.