FIGURE 25.2 • Schematic drawings showing development of the ear. a. This drawing shows the relationship of the surface ectodermderived otic vesicle to the first pharyngeal arch during the fourth week of development. b. The otic vesicle sinks deep into the mesenchymal tissue and develops into the membranous labyrinth. Note the development of the tubotympanic recess lined by endoderm into the future middle-ear cavity and auditory tube. In addition, accumulation of mesenchyme from the first and second pharyngeal arches gives rise to the auditory ossicles. c. At this later stage of development, the first pharyngeal groove grows toward the developing tubotympanic recess. The auditory ossicles assume a location inside the tympanic cavity. d. This final stage of development shows how the tympanic membrane develops from all three germ layers: surface ectoderm, mesoderm, and endoderm. Note that the wall of the otic vesicle develops into the membranous labyrinth.
Perforation of the tympanic membrane may cause transient or permanent hearing impairment.
The auditory ossicles connect the tympanic membrane to the oval window.
The three auditory ossicles, or bones—the malleus, the incus, and the stapes—cross the space of the middle ear in series (Fig. 25.6) and connect the tympanic membrane to the oval window. These bones work like a lever system that increases the force transmitted from the vibrating tympanic membrane to the stapes by decreasing the ratio of their oscillation amplitudes. The ossicles help to convert sound waves to mechanical (hydraulic) vibrations in tissues and fluid-filled chambers. Movable synovial joints connect the bones, which are named according to their approximate shape:  The malleus (hammer) attaches to the tympanic mem brane and articulates with the incus.
 The incus (anvil), is the largest of the ossicles and links the malleus to the stapes. The stapes (stirrup), the footplate of which fits into the oval window, acts like a small piston on the cochlear fluid.
Diseases that affect the external acoustic meatus, tympanic membrane, or ossicles are responsible for the conductive hearing loss (see Clinical Folders 25.1 and 25.2).
Two muscles attach to the ossicles and affect their movement.
The tensor tympani muscle lies in a bony canal above the auditory tube; its tendon inserts on the malleus. Contraction of this muscle increases tension on the tympanic membrane. The stapedius muscle lies in a bony eminence on the posterior wall of the middle ear; its tendon inserts on the stapes. Contraction of the stapedius tends to dampen the
FIGURE 25.3 • Horizontal section of a human temporal bone. The relationships of the three divisions of the ear within the temporal bone are shown. The tympanic membrane (TM) separates the external acoustic meatus from the tympanic cavity (TC). Within the tympanic cavity, sections of the malleus (M) and incus (I) can be seen. The posterior wall of the tympanic cavity is associated with the mastoid air cells (AC). The lateral wall of the cavity is formed principally by the tympanic membrane. The opening to the internal ear or oval window (arrowhead) is seen in the medial wall of the cavity (the stapes has been removed). The facial nerve (F) can be observed near the oval window. The cochlea (C), the vestibule (V ), and a portion of the lateral semicircular canal (LSC) of the bony labyrinth are identified. The cochlear and vestibular divisions of cranial nerve VIII (N) can also be observed within the internal acoustic meatus. 65.
lateral process of malleus manubrium of malleus