INTERFERENCE OF EXCITATIONS 199 



The locomotion of the animal and especially the direction is 

 in part a manifestation of heterotopic interference of response. 

 At the same time, however, especially in the coordinated move- 

 ments of nervous origin, the homotopic interference also plays 

 an important role and, not rarely, is combined with heterotopic 

 interference. 



Although the physical analysis of heterotopic interference is 

 extremely attractive, we must, however, temporarily set aside its 

 consideration, for at this point the question arises as to what 

 happens when there is interference of two stimuli at the same 

 point. In the heterotopic interference the effect of each stimulus 

 is the same as if it were applied singly. In the homotopic inter- 

 ference the interfering effects of stimulation influence each other. 



The above examples of homotopic interference introduce us to 

 the two principal types of these manifold kinds of interference 

 effects; the excitation brought about by galvanic stimulation is 

 summated by the excitation produced by temperature. The other 

 type consists of an inhibition of one effect of stimulation brought 

 about by another. The depression produced by alcohol on the 

 Paramecia weakens the excitation of the galvanic current. These 

 examples of the two principal types of interference effects are 

 quite simple ; nevertheless, in other cases, the conditions are very 

 complex. This is especially true in the field of nervous inhibi- 

 tion, so important in the functionation of the nervous system, 

 and which has presented the greatest difficulties to physiological 

 investigators until the last few years. That a stimulus bringing 

 about excitation in a ganglion cell can be inhibited by another 

 exciting stimulus, or that the development of excitation in a 

 ganglion cell may be prevented by another exciting stimulus can- 

 not be easily understood. The problem as to how two interfering 

 excitations can bring about inhibition is one that has received 

 many explanations. An interesting incident in the history of 

 physiology is that the first explanation of the principles of inhibi- 

 tory processes was close on the track of being a correct one, 

 but was subsequently abandoned by its originator. Schiff'^ 

 (1858) has endeavored to explain this inhibition as a manifes- 



1 M. Schiff: "Lehrbuch der Physiologie des Menschen." Bd. I, Lahr 1858. 



