The caloric stimulus. Temperature generation within the temporal bone.

Temperature measurements were performed on isolated cadaveric temporal bones subjected to caloric irrigation of the external meatus. This was undertaken in an attempt to clarify the pathways of heat transfer involved in thermal stimulation of the temporal bone. On average, the maximum temperature change produced across the lateral semicircular canal was 0.47 degrees C, occurring 74 s after the start of the 30-s irrigation. Temperature changes of approximately 1.5 degrees C were found to occur in the air of the middle ear cleft, indicating significant heat transfer by the process of natural convection. By comparison, removal of the bony ridge joining the external meatus with the inner ear had little effect on the temperature generated across the lateral semicircular canal. It is therefore considered that natural convection within the middle ear cleft plays an important role in caloric stimulation of the vestibular labyrinth. This view is supported by the observation that there is a lack of a direct solid connection between the outer ear and lateral limb of the lateral semicircular canal.