1. Apparatus for generating a triggering signal to an X-ray machine comprising: operator actuated means for selectively generating an ARM signal indicative of a desired armed condition; means for generating an analog respiration signal directly representative of the respiration of a subject; means, responsive to said analog respiration signal, for periodically sampling said analog respiration signal and generating sample indicative of said samples; amplitude discriminator means, responsive to said sample signal, for generating a signal indicative of the difference between the instantaneous sample and a mean sample, said mean sample being the last preceding sample differing from the preceding mean sample by a predetermined value; means, cooperating with said amplitude discriminator means for generating a signal change signal indicative of a change in sign of said difference in a predetermined direction; means for storing indicia of the modified value of the next successive sample occurring after generation of said sign change signal, the value of said next successive value being modified by said predetermined value; time discriminator means, responsive to said sign change signal, for generating an extrema signal, said time discriminator means including means for inhibiting generation of said extrema signal in response to sign change signals within a prescribed time period after a preceding sign change signal, said first prescribed time period being in accordance with the maximum rate of physiological respiration of said subject; means, responsive to said extrema signal, for generating a signal indicative of a second prescribed time period after generation of said extrema signal, said second prescribed period being in accordance with the inspiratory to expiratory action of said subject; means, responsive to said sample signal and a signal indicative of said stored indicia of modified value, for generating an actuation signal when the value of the instantaneous sample approximately equals said modified value; and logic means, responsive to said actuation signal, for selectively generating, if not inhibited, said triggering signal; first inhibit means, responsive to said arm signal, for inhibiting said logic means with respect to any actuation signal generated in the absence of said desired armed condition; and second inhibit means, responsive to said signal indicative of said second prescribed period, for inhibiting said logic means with respect to any actuation signal generated during said second prescribed time period after generation of a preceding sign change signal.