206 IRRITABILITY 



height. The cessation of the depressing stimulus has, therefore, 

 the effect that the exciting stimulus again brings about its original 

 response. 



A second type of interference is produced when both stimuli 

 bring about depression. As an example, we may select the inter- 

 ference of cold and deficiency of oxygen. If we assume, for 

 instance, that each of these stimuli of itself brings about only a 

 partial reduction of living processes and not a complete suppres- 

 sion, then it would be possible to think of a summation of both 

 depressions. Nevertheless, the conditions for the summation of 

 depression have never been carefully analyzed. Quantitative 

 investigations upon the interference of depressing stimuli are 

 entirely lacking. One should not, however, in physiology pre- 

 suppose what may happen under certain given conditions with- 

 out first making the necessary experiments. The strength of 

 scientific investigation depends upon the fact that every deduc- 

 tion, no matter how small, must be substantiated by experience 

 before further progress can be made. So, likewise, we must 

 await the results of thorough experimentation upon the inter- 

 ference of depressing stimuli before we can establish a law. The 

 conditions are not as simple as they appear on first observation, 

 for the point of attack of the various kinds of the depressing 

 stimuli upon the chain of metabolic processes may be very diflEer- 

 ent. In such a case it is not at once possible to understand the 

 results of the interference. 



There is a third type in which two dissimilatory excitations 

 interfere with each other. Fortunately there is a great amount 

 of experimental data at our command so that today we have a 

 clear understanding of the essential points of the conditions 

 necessary for the development of summation of excitation on the 

 one hand, and inhibition on the other. If we take an instance 

 of a momentary dissimilatory excitation operating upon an 

 aerobic system in metabolic equilibrium, it is necessary to recall 

 the two effects thereby produced. The stimulus brings about an 

 oxydative decomposition of the living substance. Likewise there 

 is a reduction of irritability. Both of these alterations are the 

 foundation of interference. Both processes have a specific time 



