THE EXCITATION OF NERVE FIBRES 265 



We first find the position of the secondary coil, at which the break induction shock 

 is a submaximal stimulus, and we employ this strength of stimulus throughout the 

 experiment. The make induction shock is prevented from acting on the nerve by ol. 

 a shortcircuiting key in the circuit of the secondary coil. The nerve is now 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 mak 

 of this current the muscle will prob- 

 ably 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 con- 

 traction 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, 



^ ji 

 ^-o'^p-' 



which Before was submaximal, has 



now become maximal, as is shown by 



the increase in the height of the con- FIG. 113. Arrangement of apparatus for showing 



traction evoked by the induction electrotonic changes in irritability. 



shock. e, exciting current ; p, polarising current ; 



We now reverse the direction of r > Poh1 ' 8 terser. 



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 polarising current the condition of the nerve returns to normal, and 

 the submaximal 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 cafnode 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 lease 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 appear- 

 ance of catelectrotonus, or a sudden disappearance of anelectrotonus. I 

 have said sudden because the steepness of the rise of irritability is a necessary 

 factor in causing excitation. If the polarising current passing through 

 a nerve be slowly and gradually increased to considerable strength, it will 



