130 ELECTRO-PHYSIOLOGY CHAP. 



continuous, or summated, contraction (tetanus) ; there can only 

 be a series of contractions interrupted by marked pauses. Bow- 

 ditch was the first to determine by excitation of the frog's heart 

 that even where the single induction shocks are separated by 

 intervals of several seconds, the number of the contractions is 

 often less than that of the stimuli. This disproportion between 

 stimulus and contraction is even more striking where the former 

 are working in quick succession, when the muscle of the heart will 

 often fail to respond to a whole series of excitations (Basch, 5). 

 Under these conditions a new cardiac rhythm, dependent on iii- 



FIG. 60. Bulbus aortie (Frog), tetanising excitation with induction currents. Stimulation- 

 frequency, 80 per sec. Tuning-fork tracing, sec. The ciphers under the figures give in- 

 tensities of tetanising currents. Intensity of coil pushed home = 1000. (Engelmann.) 



tensity and frequency of the stimulus, is always developed, since, as 

 Engelmann (6) found on tetanising the bulb of the frog's heart 

 with alternating currents, a very low excitation-strength will, 

 after some time, produce a systole by " latent summation," 

 followed perhaps by another, or several. The latent period of 

 the first, and the intervals of the subsequent, contractions, are 

 longer in proportion as the single stimuli are weaker. With 

 growing density of the exciting current the duration of the 

 latent period soon becomes minimal, as also the internal between 

 each systole (Fig. 60). Even with the strongest currents 

 Engelmann found no complete relaxation of the bulb after the 

 first contraction ; it remained in tetanus at a certain height. 



