This study was performed to elucidate the anatomic variations of the jugular fossa (JF) on the basis of examination of 120 human temporal bones. Observations were made of temporal bones sectioned along a plane including the cochlea, the JF, and long axis of the internal auditory canal.
The position of the upper margin of the JF was classified according to its relation to the tympanic cavity, the cochlea, and the internal auditory canal.
The height of the upper margin of the JF was classified as follows. Low type; inferior to the level of the external auditory canal. Middle type; between the level of the external auditory canal and the cochlea. High type; superior to the level of the cochlea.
The results revealed that 58 ears were of low type, 46 middle type, and 16 high type. Middle and high type comprised 62 ears, thus in 52% of ears the upper margin of the JF was situated superior to the external auditory canal.
The relation between the JF and the cochlea was classified as follows. Medial type; medial to the medial margin of the cochlea. Lateral type; lateral to the medial margin of the cochlea.
The results showed that 74 ears were of medial type, and 46 lateral type. Therefore, in 62% of ears the upper margin of the JF was situated medial to the medial margin of the cochlea. Fortyseven ears of medial type were of middle or high type. Ears of lateral type included none of high type.
The jugular bulb diverticulum was observed in 32 ears, which consisted of 3 of low type, 13 middle type, 16 high type. When the upper margin of the JF was positioned higher, the JF was in a more medial position. However, no defect of the bony labyrinth was observed. In conclusion, it is considered that protrusion of the jugular bulb into the tympanic cavity was not caused by the abnormally high position of the JF, but by its lateral displacement.
Distances from the JF to the surrounding structures were as follows; to the tympanic membrane 5.58±2.43mm (mean±S.D.), to the tympanic cavity 2.94±1.92mm, to the cochlea 4.93±2.20mm, and the internal auditory canal 5.82±2.38mm.