for example measuring the time intervals between the transmitted pulses and the received echoes and at the same time measuring the velocity of propagation in the bone accordingly in order to properly conduct a distance measurement it is important to measure simultaneously both the time between the transmitted and received waves as well as the velocity of propagation at the desired depth the us waves are transmitted through the jet of liquid which is directed toward the bottom part of the bore produced by the drill and into the bone under the bottom of the bore abrupt changes in the acoustic properties of the matter traversed by the us waves produce us echoes reflections fig1 illustrates in principle how this property of us waves is put to use in the present invention power signals 32 are sent from power 33 circuits to us transducer 68 the control electronics 31 controls the power circuits 33 in the following case us transducer 68 transmits us waves 34 through soft layers 26 and through a bone 22 us waves 34 encounter nerve canal 20 containing nerve 24 which generates a distinct mirror like reflection 36 when reflection 36 returns from canal 20 via.