EXCITATION AND TRANSMISSION 81 



we may say, the degree of excitation, is certainly under 

 some if not all conditions a function of the energy, the 

 intensity, or the effectiveness of the exciting factor, and 

 the same protoplasm is capable of different degrees of 

 excitation. But with the specialization of mechanisms 

 of excitation and transmission, the relation apparently 

 becomes more and more like that of the spark and the 

 powder, and such specialization reaches its final terms 

 in the more highly differentiated muscle and nerve 

 which apparently react according to the ; 'all-or-none' : 

 law, i.e., maximal excitation results from any stimulus 

 above the threshold. Under these conditions trans- 

 mission of the excitation as a wave to an indefinite 

 distance without decrement is possible, provided the 

 maximal excitation at successive points of the path 

 does not decrease, in other words, provided the path is 

 not already a physiological gradient. 



In conditions intermediate between the two extremes 

 transmission may occur to a greater or less distance 

 beyond the limit of the gradient resulting from the 

 original excitation, but with a decrement. If, for 

 example, the excitation induced at a point b (Fig. 6) by 

 the electrical changes resulting from excitation at a 

 gives rise to sufficient electric current to increase the 

 strength of the current produced at a the limit of effect- 

 iveness will be not x but a more distant point y. In 

 this case then the excitation at b will become a factor in 

 determining excitation at a point beyond the range of 

 the current produced at a and the length of the excitation- 

 transmission gradient will be increased. Similarly, exci- 

 tation at c may be sufficient to determine excitation at z 

 and so on. In such a case transmission is not limited by 



