Tomograms including 127 frontal projections and 60 lateral projections from the normal temporal bones were examined and the following results were obtained.
1. The ossicles were best visualized at a level of 1.7±5.0mm posterior to the porus acusticus externus on frontal projection, and at a level of 31.3±5.1mm medial to the auricle on lateral projection.
2. On the frontal projection, all information, concerning the middle ear cleft was possible to be obtained from the four different planes; cochlear, transitional, vestibular and mastoid planes which were taken with the distance of 2.5mm between each cut.
3. On the frontal projection with 4 different planes, the ossicles, spur, the horizontal portion of the facial nerve, the external auditory canal, the middle ear cavity, the cochlea, the vestibule, the internal auditory canal, the lateral and anterior semicircular canals, the mastoid antrum and mastoid cells were identified in all of the cases examined. However, the figue differences of each structure described above temporal bones were inevitable.
The vestibular window was observed in 96% of the cases examined, mastoid tegmen in 94%, the pyramidal eminence in 90%, the Körner's septum in 97%. On the other hand, the tegmen of the attic was observed only in 48%, and the stapes in 14%, and then these figures did not seen to offer sufficient information for the diagnosis the middle ear cleft.
4. In 60 tomograms with the laternal projection, one cut of the ossicular plane may include all information pertaining to the important structures in the middle ear cleft, The head and handle of malleus, the incus body, the long process of incus, tegmen tympani, the vertical portion of the facial canal were identified in 100% of the cases. With the slight difference in their shape. The anterior tympanic spine was observed in 85% and the mastoid clearly visualizd in 75% of the cases.
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