GENERAL LAW OF NERVE EXCITEMENT. 145 



The only explanation of the fact that weak currents, 

 whatever their direction, act only on being closed, is 

 that the changes in the nerve probably begin more 

 quickly than they disappear on the closing of the cur- 

 rent. The differences are, however, very slight; and 

 a very slight strengthening of the current suffices to 

 elicit opening pulsations of the nerve also. This is 

 especially true of the descending current ; if the nerve 

 is not quite fresh, opening pulsations may occasionally 

 be observed even in the case of very weak currents 

 which do not as yet afford any closing pulsations. 

 This is connected with the circumstance that the ex- 

 citability is somewhat greater in the upper than in the 

 lower portions of the nerve. The natural superiority 

 of the closing pulsation is thus cancelled in the case of 

 the descending current, and opening pulsation is con- 

 sequently rendered more easy. 



7. From what has been said it seems very probable 

 that every excitement in the nerve is due to a change 

 in its condition, which might be directly shown in the 

 case of the electric current by the electrotonic change 

 in the excitability. The more quickly these changes 

 occur, the more easily are they able to excite the 

 nerve. This law is exhibited even in the case of 

 non-electric excitement. It is, for instance, possible 

 by gradually increasing pressure on the nerve entirely 

 to crush the latter without producing any excitement, 

 though every sudden pressure is, as we have seen, 

 inseparable from excitement. A similar fact may be 

 observed in the case of thermic and chemical irrita- 

 tion. From this it may be inferred that the excitement 

 in the nerve is due to a certain form of motion of its 

 smallest particles, and that a sudden blow is better 



