68 THE ORIGIN OF THE NERVOUS SYSTEM 



disappears or becomes ineffective. There is a large 

 body of evidence for the existence of such a decrement 

 in plant tissues and in the simpler animals. 1 It is a 

 familiar fact of observation in the simpler organisms 

 that the effects of slight stimuli are effective only over 

 short distances, while those of more intense stimuli are 

 effective over greater distances, perhaps throughout the 

 whole organism. If the energy concerned in the excita- 

 tion process is derived from the original excitation, 

 and transmission is merely the transmission of this 

 energy without any further energy liberation at the 

 various points of the transmission path, a decrement is 

 readily accounted for as due to the work done in the 

 course of transmission. If, on the other hand, energy 

 is liberated at each point affected by the transmitted 

 change, a decrement and limited range of effectiveness 

 is possible only in protoplasm in which a relation 

 between the energy or the intensity of stimulus and 

 the energy or the intensity of excitation exists, in other 

 words only when the same protoplasm is capable of 

 different degrees of excitation and when the energy or 

 the intensity of stimulus decreases in the course of 

 transmission. In the transmission of excitation in the 

 less highly specialized protoplasms such a relation evi- 

 dently does exist. It is only in tissues for which the 

 :< all-or-none" law holds good, that is, in which maximal 

 excitation occurs with any stimulus above the threshold^ 

 that transmission of excitation without decrement is 



1 As regards plants see Bose (1902, 1906, 1907, 1913), Fitting (1907 

 and references given), Kretzschmar (1904), Pringsheim (1912). For a 

 general discussion of transmission in animals, see Verworn (1913), par- 

 ticularly chap, vi and references there given. 



