The ear is divided into 3 parts: the external ear, the middle ear, and the inner ear. The external ear includes the visible part of the ear (the auricle) and the ear canal. This system allows the air vibrations of sounds to pass from the environment to the ear drum. The middle ear includes the eardrum (tympanic membrane) and the air-filled chamber behind it. The middle ear space contains the three bones of hearing, the malleus ("hammer"), incus ("anvil") and stapes ("stirrup"). In the normal ear, the tympanic membrane vibrates from sound, the three bones (or ossicles) move as a unit to transmit the sound energy to the inner ear. The middle ear space is connected to the back of the nose through the eustachian tube, this allows air to enter the middle ear and equalize pressures with the outside environment. The inner ear consists of the cochlea (organ of hearing), the vestibular system (organ of balance), and the nerves that travel to the brain. The cochlea converts mechanical sound vibrations to electrical nerve impulses. Within the cochlea there are about 16,000 "hair cells" embedded in a system of flexible membranes. When sound vibrations cause the membranes to move, the microscopic hairs on these hair cells bend. This bending causes an electrical signal to be sent to the hearing nerve. The electrical signal from the hearing nerve is then processed in the brain, where the sound is perceived.