INTERFERENCE OF EXCITATIONS 



217 



course of excitation, are favorable to the summation of excitation, 

 provided their influence does not exceed certain Hmits. 



Although the nerve as an isobolic system can only be rendered 

 capable of exhibiting summation when artificially influenced, 

 there are other forms of living substance which normally are 

 systems with a slow course of excitation, in which excitation 

 may be summated, for this type possesses at the same time a 

 heterobolic character. For example, a single mechanical exci- 

 tation elicits a hardly perceptible response in Amoeba, Actino- 

 spharium, Orhitolites. When it is perceptible at all, there occurs 

 a short interruption of the centrifugal movement of the proto- 

 plasm. After a pause the movement of the protoplasm and the 

 stretching out of the pseudopods again return. But if the 

 organism is agitated one or more minutes by rhythmically shak- 

 ing the edge of the slide by a special device, as a result of the 

 summation of weak excitations there occurs a complete drawing 

 in of the pseudopods and the amoebae become bell-shaped.^ The 

 ganglion cells also possess a great capability for summation. We 

 have already alluded to the fact that single induction shocks 

 below that of the threshold produce no evident eflfect, whereas 

 when rapidly repeated, summation occurs with reflex reaction. 



Fig. 54. 



Development of tonus by interference of sub-threshold stimuli. 



S— Level of the threshold of perceptible effect 



The summation of sub-threshold excitation to a certain height 

 oflfers very favorable conditions for the development of tonus. 

 (Figure 54.) This fact has been established for many kinds of 

 centers (cardio-inhibitory center, vasomotor center, etc.). During 

 the continuance of a series of stimuli, as we have already seen, 

 an equilibrium between disintegration and replacement soon takes 



1 Max Verworn: Psychophysiologische Protistenstudien. Experimentelle Unter- 

 suchvingen." Jena 1889. 



The same: "Die physiologische Bedeutung des Zellkerns." Pflugers Arch. Bd. 51, 

 1892. 



