The external ear funnels sound waves to the external auditory meatus (Figure 1). In some animals, the ears can be moved like radar antennas to seek out sound. From the external auditory meatus, sound waves pass inward to the tympanic membrane (eardrum).
Fig1. The structures of the external, middle, and inner portions of the human ear. Sound waves travel from the external ear to the tympanic membrane via the external auditory meatus. The middle ear is an air-filled cavity in the temporal bone; it contains the auditory ossicles. The inner ear is composed of the bony and membranous labyrinths. To make the relationships clear, the cochlea has been turned slightly and the middle ear muscles have been omitted. (Reproduced with permission from Fox SI. Human Physiology. New York, NY: McGraw-Hill; 2008.)
The middle ear is an air-filled cavity in the temporal bone that opens via the eustachian (auditory) tube into the nasopharynx and through the nasopharynx to the exterior. The tube is usually closed, but during swallowing, chewing, and yawning it opens, keeping the air pressure on the two sides of the eardrum equalized. The three auditory ossicles, the malleus, incus, and stapes, are located in the middle ear (Figure 2). The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes. The stapes is named for its resemblance to a stirrup. Its footplate is attached by an annular ligament to the walls of the oval window. Two small skeletal muscles, the tensor tympani and the stapedius, are also located in the middle ear. Contraction of the former pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the latter pulls the footplate of the stapes out of the oval window. The functions of the ossicles and the muscles are considered in more detail below.
Fig2. The medial view of the middle ear containing three auditory ossicles (malleus, incus, and stapes) and two small skeletal muscles (tensor tympani muscle and stapedius). The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes. The footplate of the stapes is attached by an annular ligament to the walls of the oval window. Contraction of the tensor tympani muscle pulls the manubrium medially and decreases the vibrations of the tympanic membrane; contraction of the stapedius muscle pulls the footplate of the stapes out of the oval window. (Reproduced with permission from Fox SI. Human Physiology. New York, NY: McGraw-Hill; 2008.)
