Coloured 3D computed tomography (CT) scan of the outer, middle and inner components of a healthy adult ear. The outer ear's pinna (left) is the external part of the ear. The auditory canal leads from here to the eardrum (tympanic membrane, green). Surrounding the auditory canal are mastoid air cells (purple) that provide protection and regulate air pressure. The eardrum transmits sounds from the air to the bones of the middle ear (malleus, incus, stapes, all red). These mechanically transmit sounds to the fluid-filled organs (green) of the inner ear, the semicircular canals and the cochlea. Hair cells in the cochlea trigger nerve signals that are transmitted to the brain.