 The Sense of Hearing The external, middle, and inner structures of the ear provide hearing and balance. The external ear includes the oracle to collect sound waves and a 2.5 centimeter tube, the external auditory meadus, which passes into the temporal bone. The tympanic membrane, or eardrum, is the first part of the middle ear. The eardrum is semi-transparent and is covered by a thin layer of skin on its outer surface and a mucous membrane on its inner surface. The eardrum is cone-shaped with an oval margin. The apex of the cone faces inward. Sound waves travel through the ear canal to the tympanic membrane, which vibrates in response. Other middle-ear structures include the three middle-ear bones called the auditory ossicles and an air-filled space called the tympanic cavity. The three small bones, the malleus, incus, and stapes, are covered by mucous membranes and are attached by ligaments to the tympanic cavity. The three auditory ossicles vibrate to transmit to the inner ear through an opening, the oval window, in the wall of the tympanic cavity. The malleus conducts to the incus and the incus to the stapes, and in doing so, the force of the vibrations is magnified and the force from the larger eardrum membrane surface is concentrated to the smaller surface of the oval window. Two small muscles in the middle ear serve as effectors in the tympanic reflex to protect the eardrum and ossicles from potential damage from loud noises. Another tube in the middle ear connects with the throat and mouth. This, the oestation tube, helps to maintain air pressure on both sides of the eardrum. The complicated inner ear looks like a distorted snail. It has a system of tubes, chambers, and fluids. The chambers and tubes are called the labyrinth, a bony structure enclosing a membranous one. The perilymph fluid flows between the bony and membranous layers, and the endolymph fluid fills the membranous labyrinth. The labyrinths form a cochlea for the sense of hearing and three semicircular canals for the sense of balance. The cochlear branches of the eighth cranial nerve carry auditory sensations to the medulla oblongata. The sensations proceed to the thalamus for a final destination of the auditory cortices of the temporal lobe. Because fibers cross over, sounds are interpreted by both sides of the brain. The vestibular branches of the eighth cranial nerve conduct impulses for the sense of balance. Parts of the cerebellum interpret impulses for balanced movements. We have completed this learning activity, the sense of hearing.