144 IRRITABILITY 



tion of the centrally situated normal stretch, the wave of excita- 

 tion, which enters from here into the influenced stretch, is oblit- 

 erated at the same point simultaneously for the weakest as well as 

 for the strongest stimulus ; stimulation of the affected stretch, the 

 wave of excitation which is set free at one point by a weak stimu- 

 lus, is obliterated sooner and after passing through a shorter 

 stretch than that which is produced by a stronger stimulus. It is 

 self-evident that in the first instance, in which the stimulus acts on 

 the centrally situated normal stretch, the wave of excitation, 

 thereby set free, must in passing through the affected stretch 

 undergo a decrement of its intensity. If, therefore, the wave 

 of excitation, coming from above, is obliterated exactly at the 

 same point, whether brought about by weak or strong stimuli, the 

 inevitable conclusion must be drawn that, whether either a weak 

 or a strong stimulus is operative, the wave of excitation must 

 have entered into the influenced stretch from the normal stretch 

 with exactly the same intensity. In other words: the weakest 

 as well as the strongest stimuli produce excitations of equal 

 intensity in the normal nerve, that is, the "all or none law" is 

 valid for the nerve. 



This information can no longer be doubted in the presence of 

 such evidence as was presented above. This indeed is a fact of 

 far-reaching importance in the understanding of the functional 

 activity of our nervous system, for it is evident that the differ- 

 ence of intensity in the conduction of excitation is not, as has 

 been assumed until now, the result of the conduction of vary- 

 ing strengths of a single excitation in the same elementary 

 fibers, but rather the involvement of a various number of fibers, 

 and that a series of processes which we have to the present 

 attributed to the varying intensities are now to be explained 

 by difference in the duration and form of excitation. This gives 

 us an entirely different but nevertheless a more definite picture 

 of the physiological character of the processes in the nervous 

 system. Still, this question belongs to another chapter of phys- 

 iology. Here we are interested in the fact that we have in 

 the nerve a form of living substance, in which irritability has 

 reached a high degree, and every stimulus which is at all oper- 



