LAW OF PULSATIONS. 137 



rents must already act as would stronger, and that, when 

 the excitability has risen yet further, that they will act as 

 very strong currents. The expressions weak, strong, and 

 medium currents bear no absolute meaning, the same 

 in the case of all nerves, but must always be under- 

 stood relatively to the excitability of the nerve. That 

 which in the case of one nerve is a weak current may 

 evidently act as much stronger in the case of another 

 nerve the excitability of which is much greater; and, 

 moreover, one single nerve, at different times, may be 

 conditioned in this respect as though it were two diffe- 

 rent nerves, if its excitability has in the interval under- 

 gone considerable changes. There can, therefore, be 

 no difficulty in understanding how, as the excitability 

 gradually rises, the action of weak currents gradually 

 becomes equal to that of medium and strong currents. 

 One striking fact must, however, be observed. As the 

 excitability after it has reached its highest point begins 

 to fall again before it entirely disappears, it might be 

 supposed that the same currents which at the extreme 

 height of the excitability acted as strong currents, 

 would now act again as currents of medium strength, 

 and then as weak currents, before they entirely lose 

 their power. According to this, the third stage of 

 excitability, in which a closing pulsation is observable 

 in the case of the descending current, an opening pul- 

 sation in the case of the ascending current, should 

 be succeeded by a fourth and a fifth stage, of which 

 the fourth should resemble the second, and the fifth 

 the first. This has indeed been said to occur by some 

 observers, but it does not appear as a rule. In explana- 

 tion of this, it has been assumed that no real, but only 

 an apparent decrease of the excitability takes place after 



