CONDUCTIVITY 145 



ative brings about disintegration of all the material involved 

 in excitation, and consequently the property of conductivity 

 in the nerve reaches the state of highest development of all 

 living structures, in that the medullated nerve conducts with 

 the greatest rapidity on the one hand, and on the other, there is 

 no decrement of the velocity and intensity of excitation. All 

 these characteristics : the existence of the "all or none law," the 

 rapidity of the conduction of excitation, the absence of a decre- 

 ment in the velocity, the absence of a decrement of the intensity 

 of the excitation wave, the want of the capability of summation 

 of excitation, are all dependent upon one another, for they are 

 the combined expression of one and the same factor, that of the 

 high state of irritability. When the irritability is artificially re- 

 duced, then the nerve approaches more and more, depending upon 

 the amount of reduction, to the series of living substances in 

 which we found the protoplasm of the rhizopoda to occupy the 

 other extreme. Between the normal medullary nerve with its 

 maximal, and the pseudopods of the rhizopods with their minimal 

 capability of reaction, we find innumerable gradations in groups 

 of living substances. Whether or not other forms of living sub- 

 stances follow the type of the nerve, whether for example the 

 "all or none law" can be applied to the skeletal muscle as the 

 investigations of Keith Lucas* seem to show, requires further 

 investigation. 



Finally, there arises the important question as to the finer 

 mechanism of conductivity. The progression of excitation from 

 cross section to cross section in a living system is brought about 

 by the decomposition of the molecules in one region acting as a 

 stimulus and producing a disintegration of the molecules in 

 another region, etc. We have already seen that the intensity is 

 dependent upon the amount of energy produced by the disinte- 

 gration of the molecules following the stimulus, that is, upon the 

 amount liberated in a definite space in a definite time. The ques- 

 tion which now arises is this : What form of energy is produced 



1 Keith Lucas: "On the gradation of activity in a skeletal muscle fiber." Journal 

 of Physiology, Vol. IX, 1888. The same: The "all or none" contractions of the 

 amphibian skeletal muscle-fiber. Journ. of Physiology, Vol. XXXVIII, 1909. 



