Fig. 6. Changes in pH modulate lobster cardiac activity. The middle trace shows a continuous 10 min tension record of the spontaneous beating of the neurogenic lobster heart in vitro at 16°C. The pH of the saline perfusing the heart is indicated below the trace. Upper traces show three 8 s samples of tension recordings selected from the indicated portions of the 10 min recording to illustrate changes in the amplitude and frequency of the heartbeat. All traces are shown with the same vertical amplification. Scale bar, 2 min.