Abstract
The perceptive mechanism of the internal ear may be said to occupy a position of reasonable physiological quiet so far as air sounds are concerned because the end organ is immersed in liquid and because of the depth to which the apparatus is buried away from the surface. The writer has shown elsewhere 1 that this acoustic insulation amounts to about 30 sensation units or a factor of 1000 times the normal threshold intensity. This indicates the functional capacity of the sound transmission apparatus which may be likened to a matching transformer with the larger area at the low impedance end of the system (drum membrane) and the smaller area (stapes footplate) at the high impedance end.
If, however, the vibrations are transmitted directly to the cranial bones through the stem of a fork or with a bone telephone, then a somewhat different physiological condition obtains. The vibrations necessarily pass through all structures including the cochlea and the sound transmission system. The evidence indicates that the activating factor under these conditions is a direct transmission of the vibrations through the wall of the otic capsule (cranial transmission). This means that the vibrational thrusts through the air sound transmission apparatus do not contribute enough increased intensity to that already developed within the cochlea by direct transmission to come to consciousness. The result is that bone acuity is not influenced by the functional condition of the air sound transmission system proper. This point is of considerable diagnostic significance.
When the external auditory canal is occluded, or when the drum membrane has been loaded with liquid, the mechanical conditions for the transmission of both air and bone sounds are changed. The air acuity is more markedly lowered at the high frequency end and the normal individual presents a curve which is quite similar to a typical internal ear disability.
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