NERVE FIBRES. 35 



application to it of electrical stimuli, will be followed by contraction 

 of the muscle with which it is connected. The most convenient form 

 of stimulus for experimental purposes is the electric current. 



Induced Current. Induced electricity is generally used for purposes 

 of stimulation, because its strength can be easily controlled and also 

 because of the brief duration -of the current. When a single induction 

 shock of sufficient strength is applied to the nerve of a muscle-nerve 

 preparation, (1) an excitatory process is set up in the nerve at the point 

 stimulated, (2) a disturbance arising at that point is propagated along 

 the nerve to the muscle, and (3) the muscle contracts. 



Constant Current. If a length of nerve is introduced into the 

 circuit of a constant current, an excitatory process is only set up in the 

 nerve, as in muscles, when the circuit is made or broken, and not 

 during the passage of the current. The passage of the current is 

 nevertheless accompanied by changes in the excitability, conductivity, 

 and electromotivity of the nerve, which together constitute the condi- 

 tion known as electrotonus. The changes in excitability may be 

 shown in the nerve of a muscle-nerve preparation from the frog. The 

 nerve is laid upon a pair of non-polar i sable electrodes connected with 

 a galvanic cell, a reverser being introduced into the circuit. The 

 electrodes from an induction coil are applied to the nerve between the 

 non-polarrsable electrodes and the muscle. The position of the induc- 

 tion coil which gives a minimal effective stimulus is found before the 

 constant circuit is made, then the key of the latter is closed, and 

 the stimulus from the induction coil is repeated. If the positive pole 

 or anode is the nearer to the stimulating electrodes, the stimulus will 

 now be ineffective, because the stimulated area of the nerve is in a 

 condition of anelectrotonus or reduced excitability. If the direction of 

 the constant current be reversed, repetition of the stimulus will give 

 a maximal contraction of the muscle, because the stimulated area is in 

 a condition of kathelectrotonus or increased excitability. Associated 

 with this electrotonic variation in excitability is the fact that, as already 

 pointed out in connection with the stimulation of muscle, the excitatory 

 process set up by the closing of a constant circuit occurs at the kathode, 

 and that set up by breaking the circuit at the anode. In other words, 

 the excitatory process is produced by the setting up of kathelectrotonus 

 and by the resolution of anelectrotonus. 



The Propagated Disturbance. The propagation of an impulse along 

 a nerve may be demonstrated by the effect produced in an organ, such 

 as muscle, or by recording the electric response by means of a capillary 

 electrometer or string galvanometer. For the latter purpose the nerve 

 is included in the galvanometer circuit, non-polarisable electrodes 



