The Anatomy of the Middle Ear
The middle ear takes up the space behind the tympanic membrane, inside the temporal bone. The space is called the tympanic cavity, and it has two parts: the tympanic cavity proper directly behind the membrane and the epitympanic recess, which is located just above the membrane.
The tympanic cavity is connected to the nasopharynx anteriorly and medially by the pharyngotympanic tube and posteriorly and superiorly by the mastoid antrum, a cavity in the mastoid process of the temporal bone, by the aditus to the mastoid antrum. The tympanic cavity is lined with mucous membrane and has the following six walls:
Tegmental wall: This roof is formed by the tegmen tympani, which is part of the temporal bone.
Jugular wall: This floor is formed by a bone that separates the middle ear from the internal jugular vein.
Membranous wall: This lateral wall is formed mostly by the tympanic membrane and the bony wall of the epitympanic recess.
Labyrinthine wall: This medial wall separates the middle ear from the inner ear. It includes the promontory of the labrynthine wall and the oval and round windows.
Carotid wall: This anterior wall separates the tympanic cavity from the carotid canal and artery. It has the opening for the pharyngotympanic tube and the canal for the tensor tympani muscle.
Mastoid wall: This posterior wall has an opening called the aditus to the mastoid antrum that connects the tympanic cavity (epitympanic recess) to the mastoid air cells (sinus).
The tympanic cavity is home to the auditory ossicles and muscles as well as the chorda tympani and the tympanic plexus of nerves. The auditory ossicles are a chain of mobile bones that cross the tympanic cavity from the tympanic membrane to the oval window on the labrynthine wall. Their function is to transmit sound from the air of the external acoustic meatus to the fluid in the labyrinth. The ossicles are small bones covered with mucus membrane, but unlike other bones, they have no periosteum:
Malleus: Also known as the hammer, this bone is attached to the tympanic membrane. It has a rounded head superiorly, a neck that lies against the flaccid part of the tympanic membrane, and a handle that’s embedded in the membrane. The head articulates with the incus.
Incus: This bone is also known as the anvil. It articulates with both the malleus and the stapes. Its body lies in the epitympanic recess (along with the head of the malleus), and a long limb lies parallel to the handle of the malleus. A short limb is connected to the posterior wall of the tympanic cavity. The lenticular process of the long limb articulates with the stapes.
Stapes: The base of this bone (also known as the stirrup) is attached to the oval window.
Tensor tympani: This muscle originates superior to the pharyngotympanic tube, the sphenoid, and the temporal bone and inserts into the handle of the malleus. It tenses the tympanic membrane and dampens the movements of the ossicles to prevent ear damage from loud sounds. It is innervated by the trigeminal nerve (CN V3).
Stapedius: This muscle is found in the pyramidal eminence on the posterior wall of the tympanic cavity. Its tendon inserts onto the neck of the stapes. Like the tensor tympani, it helps to dampen loud sounds. It is innervated by the facial nerve (CN VII).
The pharyngotympanic tube has both a bony and cartilaginous section and is lined with mucous membrane. The function of the tube is to equalize pressure between the middle ear and the air pressure outside the body. Its diameter is controlled by the levator veli palatini that pushes on one wall when it contracts and the tensor veli palatini muscles that pull on the other wall.