82 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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with which it may be varied as to rate and as to intensity. Each 

 time that the battery current in the primary coil is made or broken 

 there is an induction current established in the secondary coil, 

 and if the nerve is on the electrode the current passes through it 

 and stimulates it. This induced current is, however, extremely 

 short, and alternates in direction, passing in one direction when the 

 primary current is made and in the opposite direction when it is 

 broken. The induced current set up by the making of the battery 

 current in the primary coil we designate as the making shock, that 

 set up by the breaking of the current in the primary as the breaking 

 shock. On account of the very brief duration of the induced cur- 

 rent it is difficult to distinguish between the effects of its opening 

 and closing. 



The Stimulation of the Nerve by the Galvanic Current. When 



however, we employ the galvanic 

 current, taken directly from a bat- 

 tery, as a stimulus, we can, of 

 course, allow the current to pass 

 through the nerve as long as we 

 please and can thus study the effect 

 of the closing of the current as 

 distinguished from that of the open- 

 ing, or the effect of duration or 

 direction of the current, etc. 



Du Bois-Reymond's Law of Stim- 

 ulation. When a galvanic current 

 is led into a motor nerve it is 

 found, as a rule, that with all 

 moderate strengths of currents there 

 is a stimulus to the nerve at the 

 moment it is closed, the making or 

 closing stimulus, and another when 

 the current is broken, the breaking 

 or opening stimulus, while during 

 the passage of the current through the nerve no stimulation takes 

 place: the muscle remains relaxed. We may express this fact 

 by saying that the motor nerve fibers are stimulated by the mak- 

 ing and the breaking of the current or by any sudden change 

 in its intensity, but remain unstimulated during the passage of cur- 

 rents whose intensity does not vary. 



The Anodal and Cathodal Stimuli. It has been shown quite con- 

 clusively that the nerve impulse started by the making of the current 

 arises at the cathode, while that at the breaking of the current 

 begins at the anode, or, in other words, the making shock or 

 stimulus is cathodal, while the breaking stimulus is anodal. This 



Fig. 30. Schema of the arrange- 

 ment of apparatus for stimulating the 

 nerve by a galvanic current: b, The 

 battery; k, the key for opening and 

 closing the circuit ; c, the commutator 

 for reversing the direction of the cur- 

 rent; + the anode or positive pole; 

 the cathode or negative pole. 



