THE CHARACTERISTICS OF STIMULI 57 



of the intensity from one moment to another. The more rapidly 

 these changes are produced, the greater is the excitation. His 

 arguments were based upon the fact that a contraction can only 

 take place on the "making" or "breaking," or by rapidly strength- 

 ening or weakening the constant current ; it is possible to subject 

 a nerve muscle preparation to a current of considerable strength 

 without a muscle contraction resulting, provided it is slowly 

 increased. One might be disposed to conclude from this that 

 the constant current, when showing no fluctuations, has no stimu- 

 lating effect whatsoever. Should this observation be carried 

 even further and the attempt made to extend it into a general law 

 of excitation by assuming that the effects of stimulation are only 

 produced by variations in the intensity, not by its continued 

 duration, one would commit the error of judging the occurrence 

 of a stimulus only by the unsatisfactory criterion of an abrupt 

 muscle contraction. Today we know with positiveness that a 

 continued effect also exists during the uninterrupted flowing of 

 a constant current in nerve or muscle, though much weaker, how- 

 ever, than in the case of the excitations produced by sudden 

 fluctuations of the intensity. This is shown in the nerve by an 

 altered excitability, which continues at the poles during the whole 

 duration of the current. In the region of the anode the excita- 

 bility is diminished, in that of the cathode it is increased. An 

 excitation can also be demonstrated which extends from the 

 cathode through the nerve, which can easily be detected by 

 sufficiently delicate methods. Among other effects of prolonged 

 stimulation is that of cathodal contracture, which remains local- 

 ized in the region of the cathode and which excitation persists as 

 long as the current continues. This permanent excitation can be 

 particularly well observed in the single cells of the rhizopods. 

 If a constant current is allowed to flow through an Actino- 

 sphwrium,^ the straight, smooth, ray-shaped pseudopods of the 

 cell body at the moment of "making," show evidence of con- 

 traction by being drawn' m, particularly those directed towards 



1 Kiihue: "Untersuchungen fiber das Frotoplasma uiid die Contractilitat." Leipzig 

 1864. Max Verworn: "Die polare Erregung der Protisten durch der galvanischen 

 Strom." Pflugers Arch. Bd. 35, 45, 1889. 



