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The tympanic cavity is a narrow, vertical space with the eardrum (AD1) obliquely placed in its lateral wall. Its medial wall has two openings leading to the internal ear, namely, the oval window, or vestibular win-dow(D2), and theround window, or cochlear window (D3). The roof of the tympanic cav-ity, the tegmental wall, is relatively thin and borders on the surface of the petrous pyra-mid. The floor of the tympanic cavity is formed by a thin layer of bone; beneath it runs the jugular vein.
The tympanic cavity continues in anterior direction as the auditory tube (A4). Its superior part opens posteriorly into the mastoid antrum (A5), a round space into which numerous small cavities open, the mastoid air cells (A6). These air-containingcavities are lined with mucosa and form a system of chambers that penetrates the en-tire mastoid bone; they may even extend into the petrous bone.
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