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passageway that leads from the outside of the head to the tympanic membrane, or eardrum membrane, of each ear. The structure of the external auditory canal is the same in all mammals. In appearance it is a slightly curved tube that extends inward from the floor of the auricle, or protruding portion of the outer ear, and ends blindly at the eardrum membrane, which separates it from the middle ear. The outside third of the canal wall consists of cartilage, and the inner two-thirds of the wall are made of bone. The canal is nearly 1 inch (2.5 cm) in length and is lined with skin that extends to cover the tympanic membrane. Tiny hairs directed outward and modified sweat glands that produce cerumen (earwax) help to discourage insects from entering the ear.