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1. Systolic contractions of the isolated heart were usually composed of a first and second systolic contraction (FSC and SSC) which corresponded to the first and second large potentials (FLP and SLP), respectively, recorded in the cardiac muscle cells and represented excitatory junction potentials produced by impulse bursts of the large cells of the cardiac ganglion. 2. Under constant pressure, the magnitudes of the FSC and the SSC appeared to change according to the amplitudes and durations of the FLP and the SLP. Further, the total amplitude of systole was often affected by the time of occurrence of the SSC (or the SLP). 3. Internal perfusion pressure and heart tonus were linearly related over a considerable range. With an increase in heart tonus (0–30 mg), the magnitude of the FSC was enhanced markedly in the absence of equivalent increases in the amplitude and duration of the FLP. The elevation of heart tonus was also related to an increase in beat frequency. 4. The SSC decreased in magnitude and disappeared when the beat frequency exceeded approx. 1 Hz under high perfusion pressure. Further, the abolition of the SSC resulted in a steeper slope in the curve relating beat frequency to tonus. The SSC was absent during the decline of beating caused by rapid reduction of the pressure. 5. The SSC was abolished by transverse cuts of the ganglionic trunk between the 4th and 5th large cells or between the 5th large and the 6th small cells, but the SSC often remained after the trunk was severed at the region between the 6th and 7th small cells. After severing the trunk, the heart still had the ability to respond to pressure as detected by a change of the beat frequency. 6. Spontaneous slow contractions of the cardioarterial valves were often observed.

Effects of Perfusion Pressure on the Isolated Heart of the Lobster, Panulirus Japonicus