INTERFERENCE OF EXCITATIONS 



221 



ference, we may have a summation of the exciting effect of each 

 series of stimuli or an inhibition of one series by the exciting 

 effects of the other series. If the frequency of both series is 

 essentially different, we may have here the conditions for periodi- 

 cally increasing and decreasing excitations. Nevertheless these 

 conditions have not been systematically analyzed and experimen- 

 tally studied. 



Fig. 55. 



Interference of two series of stimuli. A Effect of the one series alone. Development of tonus 

 by summation. The dots below the curve indicate the points of time at which the stimuli 

 of the second series will operate. B Effect resulting from the interference of both series. 

 By the addition of the second series the frequency has been doubled. The result consists 

 in an inhibition. 



The greatest number of instances of the interference of two 

 series of stimuli have been given to us by investigation of the 

 physiology of the nervous system. In the functionation of the 

 nervous system the fact that two series of stimuli from different 

 tracks affect the same ganglia plays a very important role. It 

 is this to which S herring ton 1 has alluded as "the principle of 

 the common path." Where two nervous excitations involve the 

 same paths, there arises an interference of the effect of the two 

 series of stimuli, for the impulses in the nervous system, as 

 already stated, possess a rhythmic character. This principle has 

 a broad application in the phenomena of association in the cere- 

 bral cortex. The simpler and, therefore, the most easily under- 

 stood cases are, however, in the spinal cord. The motor neurons 

 of the anterior horns of the spinal cord are the junction of a 



1 Sherrington : "Ueber das Zusammenwirken der Riickenmarksreflexe und das 

 Princip der gemeinsamen Strecke." Ergebnisse der Physiologic. Jahr. IV, 1905. 



