ANATOMICAL AND PHYSIOLOGICAL CONSIDERATIONS

The middle ear comprises the tympanic cavity and auditory tubes lined by mucous membrane. The tympanic cavity, located between the tympanic membrane and internal ear, consists of three parts: the atrium, the epitympanic recess—which contains most of the auditory ossicles—and the large tympanic bulla (Figure 16-1A and B). The function of the middle ear is to transmit sound waves that reach the tympanic membrane through the auditory ossicles to the internal ear. The internal ear consists of two cavities (membranous and osseous labyrinth) in the petrous part of the temporal bone that enclose a complex membranous membrane containing the auditory cells and distal ramification of the auditory nerve. The osseous labyrinth, immediately medial to the tympanic cavity, has three parts: the cochlea, vestibule, and semicircular canals. The membranous labyrinth lies within the osseous labyrinth; it contains supporting cells and hair cells. The distal extremities of the cochlear nerves are located at the base of the hair cells.

Figure 16-1 Anatomical specimen that shows position and relationship of tympanic bullae in calves: A, lateral view; B, ventral view.

Chronic suppurative otitis media results first from a bacterial infection. The latter may be primary or secondary to a viral infection. This causes inflammation, ulceration, and production of granulation tissue within the middle ear (Figure 16-2B). The cycle of inflammation described above leads to destruction of the bony margins of the middle ear (Figure 16-2A and B).

Figure 16-2 CT scan of a calf with otitis media. This is a transverse CT image of the temporal region, acquired using a standard algorithm. A is displayed in a bone window (3200 window, 250 level) and B in a soft tissue window (375 window, 40 level). This examination illustrates bilateral otitis media with bilateral bulla osteitis and right otitis interna. Evidence of otitis media: note the lysis of the wall of the tympanic bullae (A) and presence of purulent exudate and granulation tissue (soft tissue density instead of air density) in the affected tympanic bulla (B). Evidence of otitis interna: lysis right petrous temporal bone (arrow, A).

(Courtesy of Dr. Anthony Pease; Cornell University.)

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