The results of experiments with frogs under a hydrostatic pressure demonstrate that cavitation is not an' important factor in the mechanism of production of paralysis of the hind legs of frog by ultrasonic (frequency one megacyde) irradiation over the lumbar enlargement region of the spinal cord. Experimental results indicate that a linear relation exists between the reciprocal of the minimum exposure time for paralysis and the acoustic amplitude. This result is readily described in terms of a

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... ne factor rate process. On the basis of this experimentally determined relation, it is shown that time rate of change of temperature cannot be correlated with the observations. It is concluded on the basis of a theoretical calculation that absorption of ultrasound at interfaces in the spinal cord does not result in minute hot regions. Further work on summation of subparalytic doses, spaced apart at various time intervals, indicates that the recovery process foRowing exposure to a subparalytic dose of ultrasonic radiation may not be a monotonic function of time. • ULTRASONICS AND LIVING SYSTEMS 365