556 



Comparative Animal Physiology 



trocardiograms probably correspond to discharge from the pacemaker gan- 

 ghon cells. The presence or absence of an oscillatory ECG under "normal" 

 conditions may be an indication of whether a heart is neurogenic or myo- 

 genic. The electrocardiogram of the polychaete Arenicola also shows oscilla- 

 tory waves in addition to slow potentials.^^** 



In Limulus the electrogram of the pacemaker ganglion separated from 

 the heart shows bursts of impulses corresponding to each heart beat.^*^- ^'^' 

 i5« Each burst of impulses corresponds to a heart beat, and a single neurone 



ww\./v 



D 



Fig. 205. Electrocardiogram from grasshopper: A, at room temperature; B, after treatment 

 with cold saline; C and D, recovery from cold effect. From Crescitelli and Jahn.^" 



Fig. 206. Electrogram from dorsal ganglion of Limulus heart. A, Beats recorded from 

 fifth and from sixth segments of isolated ganglion, showing slow potentials with super- 

 posed axon spikes. B, Discharge in single neurone of pacemaker ganglion during one 

 beat. From Prosser.^'* 



discharges several (2-15) times during a burst lasting approximately a half 

 second (Fig. 206). Each neurone usually discharges initially at a high rate 

 and later at decreasing frequency. In preparations which are no longer fresh, 

 or after treatment with abnormal concentrations of potassium or calcium, 

 asynchrony appears and the individual ganglion cells discharge at random. 

 Records from the midsegments where the large unipolar pacemaker cells 

 are located show slow waves lasting more than 0.1 second, with axon spikes 



