A preparation is described in which the peripheral inhibitor and accelerator nerves of the crayfish (Cambarus clarkii) heart can be separately or simultaneously stimulated. The effect of different frequencies of stimulation were investigated; complete stoppage of the heart was obtained only with rather high frequencies (45 per sec.); maximal acceleration, with lower frequencies (30 per sec.). No difference in effects between left and right nerves was found. Perfusion with acetylcholine and stimulation of the accelerator nerve produce identical effects. Perfusion with eserin does not influence the normal heart beat but enhances the effect of acceleration.

The nerve of Lemoine has no regulatory influence on the heart.

Special inhibitory and acceleratory tracts have been prepared in the suboesophageal commissures. Each of these tracts makes heterolateral as well as homolateral connexions. The influence of the frequency of stimulation on the central tracts has been studied.

An hypothesis with regard to the mechanism of the crayfish heart beat and its control is presented.

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