FIGURE 25.4 • The tympanic membrane in otoscopic examination of the external ear. This photograph shows the left tympanic membrane seen with otoscope during examination of the external acoustic meatus. The landmarks of tympanic membrane include the manubrium of the malleus with its visible attachment to the tympanic membrane, umbo at the tip of the manubrium, and projecting lateral process of the malleus. Note the cone of light (light reflex) that is usually seen extending anteroinferiorly from the umbo of tympanic membrane. (Courtesy of Dr. Eric J. Moore, Mayo Clinic, Rochester, MN.) movement of the stapes at the oval window. The stapedius is only a few millimeters long and is the smallest skeletal muscle.
The two muscles of the middle ear are responsible for a protective reflex called the attenuation reﬂex. Contraction of the muscles makes the chain of ossicles more rigid, thus reducing the transmission of vibrations to the internal ear. This reflex protects the internal ear from the damaging effects of very loud sound.
The auditory tube connects the middle ear to the nasopharynx.
The auditory (Eustachian) tube is a narrow flattened channel approximately 3.5 cm long. This tube is lined with ciliated pseudostratified columnar epithelium, about one fifth of which is composed of goblet cells. It vents the middle ear, equalizing the pressure of the middle ear with atmospheric pressure. The walls of the tube are normally pressed together but separate during yawning and swallowing. It is common for infections to spread from the pharynx to the middle ear via the auditory tube (causing
FIGURE 25.5 • Cross section through a human tympanic membrane. This photomicrograph shows the tympanic  membrane, external acoustic meatus, and tympanic cavity. 9. Inset. Higher magnification of the tympanic membrane. The outer  epithelial layer of the membrane consists of stratified squamous epithelium (SSE), and the inner epithelial layer of the membrane consists of low simple cuboidal epithelium (SCE). A middle layer of connective tissue (CT ) lies between the two epithelial layers. 190.
otitis media). A small mass of lymphatic tissue, the tubal tonsil, is often found at the pharyngeal orifice of the auditory tube.
The mastoid air cells extend from the middle ear into the temporal bone.
A system of air cells projects into the mastoid portion of the temporal bone from the middle ear. The epithelial lining of these air cells is continuous with that of the tympanic cavity and rests on periosteum. This continuity allows infections in the middle ear to spread into these cells, causing mastoiditis. Before the development of antibiotics, repeated episodes of otitis media and mastoiditis usually led to deafness.
The internal ear consists of two labyrinthine compartments, one contained within the other.
The bony labyrinth is a complex system of interconnected cavities and canals in the petrous part of the temporal bone. The membranous labyrinth lies within the bony labyrinth and consists of a complex system of small sacs and tubules that also form a continuous space enclosed within a wall of epithelium and connective tissue.
There are three fluid-filled spaces in the internal ear:  Endolymphatic spaces are contained within the membranous labyrinth. The endolymph of the membranous labyrinth is similar in composition to intracellular ﬂuid (it has a high K concentration and a low Na concentration).
 The perilymphatic space lies between the wall of the bony labyrinth and the wall of the membranous labyrinth. The perilymph is similar in composition to extracellular ﬂuid (it has a low K concentration and a high Na concentration).