Anatomical characteristics of the hypotympanum were tested on 50 temporal bones. The hypotympanum has the shape of an irregular bony groove which is surrounded by five walls. The outer wall is formed by the tympanic part of the temporal bone. In 65 per cent of the cases the inner wall is formed by part of the petrous bone which extends under the promontory; in 25 per cent it is formed only by the lower part of the promontory; and in 10 per cent it corresponds to the juncture of the promontory and the petrous bone. The lower wall is clearly defined in 48.2 per cent of cases and corresponds to the juncture of its inner and outer walls. In 65 per cent of cases a recess of the inferior hypotympanic sinus is found on the floor of the hypotympanum. In 25.3 per cent of cases the jugular bulb protrudes into the tympanic cavity. In 73.4 per cent of cases the front wall is formed by part of the petrous bone which extends from its floor towards the tympanic opening of the protympanum, and in 26 per cent of the cases it is formed by the wall of the internal carotid artery. On the front wall, in 22.4 per cent of cases, a recess of the anterior hypotympanic sinus is found. The back wall is formed by elements of the styloid complex and in five per cent of the cases a recess or posterior hypotympanic sinus is found on it.
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