THE EFFECTS OF BATTERY CURRENTS ON NERVE FIBER 501 



a motor nerve is not fatigued by at least ten hours' continuous stimulation 

 with induction currents. 



One must hesitate to draw the conclusion, however, that the nerve fiber 

 conducts the nerve impulse without loss of energy. The fiber can be 

 anesthetized, it responds to temperature changes, and gives other evidences 

 of susceptibility to conditions which influence metabolism in other forms of 

 protoplasm. Perhaps the nerve fiber is capable of repairing its wastes as 

 rapidly as they occur. 



The Effects of Battery Currents on Nerve Fiber. Galvanic currents 

 influence nerves in ways that call for special discussion. A constant cur- 

 rent, say from a Daniell battery, can be introduced into the nerve of a muscle- 

 nerve preparation by means of a pair of non-polarizable electrodes, figure 

 323, and a convenient key arranged for turning the current on or off the 

 nerve. It will be found that with a current of moderate strength there will 

 be a contraction of the muscle, both at the closing and the opening of the key 

 (called, respectively, making and breaking contractions), but that during the 

 interval between these two events the muscle remains flaccid, provided the 

 battery current continues of constant intensity. If the current be a very 

 weak or a very strong one, the effect is not quite the same; one or the other 

 of the contractions may be absent. Which of these contractions is absent 

 depends upon another circumstance, viz., the direction of the current. The 

 direction of the current may be ascending or descending: if ascending, the 

 anode or positive pole is nearer the muscle than the cathode or negative pole, 

 and the current to return to the battery has to pass up the nerve; if descend- 

 ing, the position of the electrodes is reversed. It will be necessary before 

 considering this question further to return to the apparent want of effect of 

 the constant current during the interval between the make and the break 

 contraction. To all appearances no change is produced, but in reality a very 

 important alteration of the irritability and conductivity is brought about in 

 the nerve by the passage of this constant or polarizing current. 



A second way of showing the effect of the polarizing current is by stimu- 

 lating the nerve by a pair of electrodes from an induction coil, while the polar- 

 izing current from the battery is flowing through the nerve. If the strength 

 of stimulus required in order that a minimum contraction be obtained by the 

 induction shock before the polarizing current is applied and the secondary 

 coil be removed slightly further from the primary, the induction current 

 cannot now produce a contraction. If, now, the polarizing current be sent in 

 a descending direction, that is to say, with the cathode nearest the muscle, and 

 the induction current which was before insufficient be applied between the 

 cathode and the muscle, it will now prove sufficient to cause a contraction. 

 This indicates that with a descending current the irritability of the nerve is 

 increased at the cathode. If, instead of applying the induction electrodes 

 below the polarizing electrodes, they are applied above them, the irritability of 



