NOTES AND ADDITIONS. 303 



passes through those which are not cut off, those the excita- 

 bility of which is tested, and alters their excitability. This 

 influence changes in the whole mass, as the cut nerves die, 

 thus giving rise to irregularities the further nature of which 

 we need not trace. 



5. INFLUENCE OP THE LENGTH OF THE PORTION OF THE 

 NERVE EXCITED (p. 138). 



If the irritant remains the same, the longer is the portion 

 of the nerve irritated, the stronger is the action on the 

 muscle. If the excitability of a portion of the nerve is found 

 by the method of minimum irritants, that is, if the weakest 

 irritant capable of effecting an observable pulsation is looked 

 for, and if various degrees of excitability prevail in the por- 

 tions of the nerve simultaneously exposed to the irritant, 

 action may result, even if only a part of the portion of nerve 

 is really excited ; in reality, therefore, it is but the excita- 

 bility of the most excitable part of the whole nerve-portion 

 which is tested. In a fresh nerve this is generally the upper 

 part of the nerve-portion. But when there is no great dif- 

 ference in excitability within the nerve-portion, then every 

 part of the portion will be excited by an irritant of a certain 

 strength in an approximately like manner, and the action 

 observed in the muscle will therefore be the combined effect 

 of the excitement of the separate parts of the nerve-portion. 

 But if, as we have assumed, the loss of excitability in each 

 part follows the highest excitability very suddenly, the effect 

 must be that the portion actually irritated continually be- 

 comes shorter ; the parts which are irritated are however 

 still in the highest state of excitability, and therefore exhibit 

 the third stage of pulsation (the testing current having been 

 so chosen that, in the fresh nerve, it originally produced the 

 first stage). The form in which the third stage exhibits itself 

 pulsation on the closing of a descending current and on the 

 opening of an ascending current must therefore remain 



