298 PHYSIOLOGY 



stimulated at various points with a single break induction shock, and 

 the contractions recorded. The heights of these contractions serve 

 to indicate the irritability of the nerve at the point stimulated. We 

 now throw the polarising current into the nerve. At the make of this 

 current the muscle will probably respond with a twitch which is not 

 recorded. We then test once more the irritability of different points 

 of the nerve, and we find that, when the stimulus is applied near a, 

 the point where the current enters the nerve (anode), the stimulus, 

 which before gave a moderately large contraction of the muscle, now 

 has either no effect or else produces a very weak contraction. On 

 the other hand, in the region of the cathode the stimulus, which before 

 was submaximal, has now become maximal, as is shown by the 

 increase in the height of the contraction evoked by the induction 

 shock. 



We now reverse the direction of the polarising current, so that 

 the current of the nerve runs from k to a. With this reversal of 

 current there is also a reversal of the changes in the nerve ; that is 

 to say, the normally submaximal stimulus is maximal when applied 

 near a, and minimal when applied near k. On break of the polaris- 

 ing current the condition of the nerve returns to normal, and the sub- 

 maximal stimulus is once more submaximal throughout. 



This return to normal conditions, however, is not immediate, since the first 

 effect of breaking the current is a swing-back, so to speak, past the normal, 

 the diminished irritability at the anode giving place to an increased irritability, 

 which only gradually subsides. In the same way, immediately after the 

 polarising current has ceased to flow, the neighbourhood of the cathode 

 acquires a condition of diminished irritability, and this only gradually gives 

 place to a normal condition. 



This experiment teaches us that, when a constant current is 

 passed through a nerve, there is increase in the irritability in the 

 nerve near the cathode, and a diminution in irritability near the anode. 

 These conditions of increased and diminished irritability are spoken of 

 as catelectrotonus and anelectronus respectively. In muscle we have 

 seen that a make contraction always starts from the cathode, and a 

 break contraction from the anode. Now the event that takes place at 

 the cathode on make and at the anode on break of a constant current 

 is, as the last experiment shows us, a rise in irritability, in the former 

 case from normal to above normal, in the latter from subnormal to 

 normal. Hence we may say that the excitation is caused by a sudden 

 rise of irritability, which may be due either to a sudden appearance of 

 catelectrotonus, or a sudden disappearance of anelectrotonus. I have 

 said suddan because the steepness of the rise of irritability is a neces- 

 sary factor in causing excitation. If the polarising current passing 

 through a nerve be slowly and gradually increased to considerable 



