PROTOPLASMIC MOVEMENT. 399 



the eflfect remained at the side of the animal which was in con- 

 tact with the positive pole so long as the circuit remained 

 closed. As a rule, however, the protoplasm soon regains its 

 original condition so long as the current continues to flow 

 with a constant intensity. BecquereP has observed this in 

 Chara. 



Making a constant current is a specifically stronger stimulus 

 than breaking it, but the latter requires a longer duration of 

 the current, or that the current should be more intense. 

 Often, more especially when the object is very sensitive to the 

 making shock, breaking a very strong current is quite in- 

 active. 2 



The efi'ect called forth increases very markedly within certain 

 limits with the suddenness and the extent of the variation of 

 the intensity of the current. Induction shocks are therefore as 

 as a rule more efi'ective than making constant currents. 



The effect of stimuli following one another at short intervals 

 may be cumulative, and in this way singly ineffective stimuli 

 may together produce a visible and powerful eflFect. The 

 intervals between the stimuli, in order that the summation of 

 effects may take place, must be as a rule of considerable amount 

 (e.g. with many Amoebae and vegetable cells four seconds and 

 more), and it appears that the more sluggish the spontaneous 

 movements of an object the longer are the intervals ne- 

 cessary.' 



After powerful excitations exhaustion sets in, stronger ex- 

 citations are then necessary to produce the same effect, or, the 

 excitations remaining the same, longer intervals of time for 

 recovery. Very strong excitations will kill protoplasm ; it 

 becomes opaque, enters into rigor, and shrinks up or breaks to 

 pieces : or they call forth secondary effects which completely 

 upset the course of physiological events. 



Behaviour of various types of protoplasm towards 

 electrical stimulus. — The action of single induction shocks 



1 Becquerel, * Compt. rend.,' 1837, ii, p. 787. 

 ' 'Arch. f. d. ges. Physiol.,' iii, p. 311, 1870. 

 * Author's observations. 



VOL. XXIV. NEW SER. E E 



