EXCITATION AND TRANSMISSION 83 



in the rate of living, possess all the characteristics of 

 excitation gradients, and are determined by differential 

 exposure to the action of external factors. I believe 

 we are amply justified in the light of all the facts in 

 regarding them as the most general and primitive expres- 

 sion of the excitation- transmission relation in protoplasm. 



The fact that these gradients are usually more or less 

 persistent and not readily or rapidly reversible requires 

 some consideration. From our knowledge of momen- 

 tary excitations and the highly specialized processes in 

 nerve and muscle we are accustomed to think of excita- 

 tion as completely reversible. Excitation may be 

 completely reversible in the fully developed nerve fiber, 

 but certainly in many parts of the nervous system it is 

 not completely reversible, as memory and the possi- 

 bility of learning show clearly enough. Moreover, it is 

 evidently not completely reversible in muscle, since 

 frequent excitation, if not exhausting, leads to growth, 

 i.e., functional hypertrophy, and after a certain stage of 

 development excitation is necessary for the continued 

 existence of the muscle. Whether we invoke a mys- 

 terious " over-compensation " or some other hypothetical 

 process to account for these facts, they show that while 

 muscular contraction is completely reversible the pro- 

 cess of excitation as a whole is not. 



The fully developed organism represents an approach 

 to dynamic equilibrium. The protoplasmic record of 

 the events of development is about as complete as is 

 possible for the protoplasm concerned. The various 

 mechanisms are all developed and the further changes 

 in them are largely temporary and reversible except so 

 far as the gradual changes which constitute senescence 



