CONDUCTIVITY 137 



ducting excitation into a long series, starting with those possess- 

 ing the least irritability, as we found in the rhizopods, then those 

 having greater irritability, as the smooth muscle and ganglion 

 cells, then those with still greater irritability, as the striped muscle, 

 and finally those having the greatest degree of irritability, as the 

 medullated nerves of the warm-blooded animal. Should the 

 processes of excitation, as we saw, result from the energy pro- 

 duction following the disintegration of the labile molecules of 

 the living substance, then the degree of irritability is determined 

 by the chemical constitution of the disintegrating molecules, by 

 the number of molecules which are broken down in a definite 

 space and a given time, and by the nature of the disintegration 

 itself. All of these individual components, if we observe the 

 problem from the physical standpoint, are manifested by the 

 quantity of energy production. The higher the irritability of a 

 living system, the greater is the amount of energy production in 

 a given time and space which the stimulus produces. This has 

 particular interest from the standpoint of the extreme cases of 

 medullated nerves of the vertebrates with their most highly devel- 

 oped conductivity, and which will be analyzed in somewhat greater 

 detail. How are we to explain their decrementless conductivity? 

 When we study the decrement of the excitation wave in the 

 series of living substances, before alluded to, we see that this 

 reduces with a progressive increase of irritability. Consequently 

 the extreme irritability of the nerve is a manifestation of its 

 decrementless conductivity. If we study the course of a process 

 of excitation and its conduction in its molecular details, the fact 

 of the decrementless conduction indicates that in excitation, pro- 

 duced by a stimulus, the same number of specific molecules 

 capable of disintegration are broken down in the same manner 

 at every following cross section, as at the point of stimulation ; 

 or in other words : an equal amount of energy is set free at every 

 cross section, which, in its turn, acts as stimulus to the next, 

 etc. Such a condition presupposes, however, in an elementary 

 fiber of the nerv-e, that by the conduction of the wave of excita- 

 tion from cross section to cross section, all those molecules 

 capable of disintegration are broken down. If it is assumed that 



