94 EXPERIMENTAL PHYSIOLOGY 



starts from the electrode nearest the muscle, and there is nothing 

 tending to block its transmission. On break the stimulus starts from 

 the upper electrode. There is nothing which prevents it reaching the 

 muscle, for the diminished conductivity round the kathode disappears 

 as soon as the current is broken. With an ascending current on 

 make the stimulus starts from the upper electrode, and as with this 

 strength of current the block at the anode is only slight, it reaches 

 the muscle which contracts. On break the stimulus starts from the 

 lower electrode, and there is nothing to prevent it reaching the 

 muscle. 



With a strong descending current on make the stimulus starts 

 from the lower electrode, and therefore leads to a contraction. On 

 break the stimulus starts from the upper electrode, and has to travel 

 through a piece of nerve whose conductivity is strongly depressed, 

 sufficiently so to block the impulse, and no contraction results. 

 With an ascending current the impulse at make starts from the upper 

 electrode, but is blocked by the strong depression of conductivity 

 around the anode, and therefore does not reach the muscle. On 

 break the stimulus starts from the lower electrode, and therefore 

 causes a contraction, as nothing prevents it travelling down to the 

 muscle. 



THE VELOCITY OF A NERVOUS IMPULSE 



Just as we found that if a muscle be excited at any point the con- 

 traction begins there, and then spreads in a wave-like manner over the 

 remainder of the muscle, so too it is found for nerve that an impulse 

 started at any point travels along the nerve as a wave of excitation. 

 This is brought out most clearly by a study of a motor nerve, in 

 which we determine the latent periods of two twitches in one of which 

 we stimulate the nerve close to the muscle, in the other at some 

 distance from the muscle. Any difference of time in these two 

 measurements must be due to the time occupied by the impulse in 

 travelling from the one point to the other. The method of carrying 

 out the experiment is as follows : — 



Experiment 12. — Arrange the apparatus for recording a simple twitch, 

 but with the drum arranged to rotate at as fast a rate as possible. Dissect 

 out the gastrocnemius and the whole of the sciatic nerve up to the vertebra;. 

 Place two pairs of electrodes under the nerve — one pair near to its 

 entrance into the muscle, the other at its further extremity. Connect these 

 electrodes e 1 and e' 2 to the terminals of a Pohl's commutator without cross 

 wires, as in fig. 80. Interpose a Du Bois key between the secondary coil 

 and the remaining terminals of the commutator. The secondary circuit can 

 in this way be connected up to either pair of electrodes, e 1 or e' 2 , by 



