The ear consists of three parts: the outer ear, the middle ear and the inner ear. The outer ear includes the external cartilaginous structures of the ear (the auricle) and the ear canal. The ear canal is irregularly curved and is approximately 2.5 cm long and 8 mm in diameter. The physiological curvature of the ear canal effectively prevents the intrusion of water and foreign bodies into the ear canal. The external part of the ear canal is relatively soft, while the inner end is a harder bony structure. The ear canal terminates at the tympanic membrane. The inner lining of the ear canal has many glands that secrete cerumen. The role of the outer ear is to locate, pick up, conduct and amplify sound. It also serves as a natural barrier to protect the middle ear. The middle ear is the air-containing cavity located behind the tympanic membrane. The middle ear maintains stable air pressure in the middle ear through the eustachian tube and pharynx. The middle ear cavity contains three tiny bones called the auditory bones: the hamate bone, the anvil bone, and the stapes bone. This chain of auditory bones amplifies air vibrations and transmits them to the inner ear, causing vibrations in the inner ear fluid. The stapedial and tympanic muscles are attached to the auditory chain. These two muscles are activated by reflexes when loud sounds are transmitted in. Once activated, they protect the inner ear by blocking the transmission of sound through the auditory chain. The inner ear, also known as the labyrinth, is deep within the temporal bone rock and includes the sensory endpoints of the auditory and balance systems. The inner ear is divided by anatomical location into three parts: the cochlea, vestibule and semicircular canal, and by histological structure into two parts: the bony vagus and the membranous vagus, and by physiological function into the auditory vagus (cochlea) and the vestibular vagus (including the vestibule and the three semicircular canals). The cochlea is a 30 mm long canal lumen coiled in 2 and 3/4 turns and filled with fluid. The vestibular and basilar membranes divide it into three parallel sections along the length of the canal: the vestibular, middle and tympanic steps. The cochlea plays a major role in the transmission and transformation of sound waves. The hair cells in the cochlea convert mechanical vibrations from the middle ear into electrical nerve impulses, which are then transmitted by the cochlear nerve to the central auditory pathway.