136 IRRITABILITY 



wave of excitation coming from above. This, of course, only 

 occurs provided the length of the narcotized stretch is great 

 enough. The separation of conductivity and irritability is, there- 

 fore, only an apparent one. In reality, the facts obtained from 

 experimentation indicate that with the reduction of irritability the 

 decrement of the wave of excitation increases, whilst the shorter 

 the stretch, the smaller is the decrement. This shows that con- 

 ductivity is a manifestation of irritability. 



The facts just mentioned have, however, a much deeper mean- 

 ing. They show us that it is possible by means of narcosis to 

 convert an extreme type of a living system, with decrementless 

 conductivity, into another extreme type of living substance, in 

 which excitation in its progress meets with a strong decrement, 

 like that seen in the rhizopods. The same results may also be 

 obtained by asphyxiation and other forms of temporary and per- 

 manent injury of the nerve. We are, therefore, in the fortunate 

 position in the case of the medullated nerve of having a sub- 

 stance to study, which, depending upon conditions which are 

 under our control, may become a type in which conductivity 

 occurs with or without the presence of a decrement. We can 

 likewise reduce the irritability to various degrees, producing all 

 intermediate gradations between the two extremes. This latter 

 is particularly valuable in that it permits us to study the condi- 

 tions in one and the same substance necessary to bring about the 

 various peculiarities of conductivity. The great differences in 

 the conductivity of excitation are conditioned by variations in 

 the degree of irritability. If the irritability of the nerve is at the 

 normal level the wave of excitation progresses to the end of the 

 nerve without manifesting a decrement of its intensity or 

 rapidity. 



If the level of irritability of the intact nerve is artificially 

 reduced, the wave of excitation meets with a greater decrement 

 and reduces in velocity, and in fact disappears the more quickly 

 in the stretch of nerve, as the reduction in irritability is increased. 

 These three factors, irritability, intensity and velocity of the 

 progress of the wave of excitation, are inseparable. All living 

 substances may be grouped according to their capability of con- 



