STIMULATION OF NERVES. 501 



easily produced when the positive pole or anode is next the muscle. 

 Sometimes in particular conditions of the nerve, and with certain 

 strength of stimulation, a making tetanus also occurs, but more 

 rarely and only when the negative pole is next the muscle. 



When a constant current, such as we get directly from a Daniell 

 cell, is used, that part of the nerve between the stimulating points 

 through which the current passes is found not to be equally af- 

 fected throughout its entire length, but one single point is stimu- 

 lated whence the impulse spreads. This may be the point where 

 either of the poles is in contact with the nerve ; and further, a dif- 

 ferent pole starts from the stimulus, according as the circuit is made 

 or broken. With a making shock the stimulation takes place at 

 the negative pole or cathode, and with a breaking shock at the 

 positive pole or anode. That is to say, the point where the current 

 leaves the nerve is affected at the make, and the point where the 

 current enters the nerve is affected at the break of the current. 



It has also been found that, other things being equal, the making 

 shock is a more powerful stimulus than the breaking shock ; i.e., 

 a weak current will sooner cause a contraction when the circuit 

 is made than when it is broken. 



This remarkable fact that the impulse starts from the anode in 

 a breaking shock is proved by means of the breaking tetanus just 

 alluded to. It has been found that when the positive pole of 

 anode is next to the muscle the breaking tetanus lasts longer and 

 is stronger than when the anode is placed at a greater distance 

 from the muscle than the cathode ; and further, when the anode 

 is beyond the cathode section of the nerve in the intrapolar region, 

 during stimulation the tetanus stops it at once, because the point 

 from which the stimulus comes is thereby cut off from the muscle. 

 Section has no effect if the anode be next the muscle, the tetanus 

 proceeding in a normal way, only the inactive pole being cut away 

 from the muscle. That the stimulus occurs at the cathode in a 

 making current may also be demonstrated by the fact that it takes 

 a certain measurable time for the impulse to travel along the nerve. 

 If then the cathode be placed far from the muscle and the anode 

 near it, the contraction after a breaking shock, when the stimulus 

 starts from the anode, will occur sooner than that which follows 



