5» THE PHYSIOLOGY OF MUSCLE AND NERVE. 



nerve impulses which then propagate themselves along the nerve 

 fibers. The induction current is convenient because of its intensity, 



Fig. 30.— Stimulating (catheter) electrodes for nerves: b. Binding posts for attachment 

 of wires from the secondary coU; s, insulating sheath of hard rubber; p, platinum points 

 laid upon the nerve. 



which overcomes the great resistance offered by the moist tissue ; be- 

 cause of its very brief duration, in consequence of which it acts as a 

 sharp, quick, single stimulus or shock, and because of the great ease 

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

 account of the very brief duration of the induced current it is dif- 

 ficult 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 Low 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 nerv^e at the 

 moment it is closed, the making or 

 closing stimulus, and another when 

 the current is broken, the breaking 

 or opening sthnulus, 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 Stimidi. — It has been shown quite con- 

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



Fig. 31. — 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. 



