268 



PHYSIOLOGY 



excitation the greater the length of nerve that it flows through. It must 

 be remembered however that the nerve offers considerable resistance to the 

 passage of the current and so, to keep the current constant while increasing 

 the length of intrapolar nerve, we must largely increase the electromotive 

 force employed. 



A -very convenient method of showing the effect of the length of intrapolar nerve 

 on excitation has been suggested by Gotch. The two sciatic nerves of a frog are dissected 



out, one of them being in connection with 

 the gastrocnemius. These are first arranged 

 as in Fig. 117. a, 6, and c are three non- 

 polarisable electrodes, the terminals of a 

 constant battery being connected to a and 

 c. The position of the rider on the rheo- 

 chord is then ascertained at which make of 

 the current just excites contraction in the 

 muscle of nerve 2, the current in this case 

 passing from a to 6 along nerve 1, and 

 from b to c along a small piece of nerve 2. 

 We will suppose that eleven units of 

 current are necessary to produce excita- 

 tion, b is then withdrawn and the nerve 

 2 laid on a (Fig. 117, B), so that thfc 

 current can now pass from a to c entirely 

 through a long stretch of nerve 2. On 

 again seeking the minimal stimulus, it 

 will be found that a smaller current is 

 Fia. 117. sufficient to excite, contraction being 



obtained with seven units. Since the 



length of nerve traversed, and therefore the resistance to the current, are the same 

 in both cases, it is evident that a current is more effective the greater the length of 

 excited nerve that it traverses. 



A nerve cannot be excited by currents passed transversely across it, since 

 in such cases the anode and cathode lie so close to one another in a nerve- 

 fibril, as it is traversed by a current, that their effects counteract one another. 



ELECTRICAL STIMULI AS APPLIED TO HUMAN NERVES 

 When we attempt to apply the results gained on frog's nerves to man, we 

 are met at once by the difficulty that we cannot dissect out the nerves and 

 apply stimuli to them directly. So usually unipolar excitation is used, one 

 electrode, either anode or cathode, being applied to the skin over the nerve to 

 be stimulated, and the other to some indifferent point, such as the back. It 

 is evident under these circumstances that the current is concentrated as it 

 leaves the anode and reaches the cathode, and diffuses widely in the body, 

 seeking the lines of least resistance. Thus it is impossible to get pure anodic 

 or cathodic effects. If the anode be applied over the nerve, the current 

 enters by a series of points (the polar zone), and leaves by a second series 

 (the peripolar zone). The polar zone will thus be in the condition of anelec- 

 trotonus, and the peripolar zone in that of catelectrotonus. The current 

 however will be more concentrated at the polar than at the peripolar zone, 

 and so the former effect will predominate. These restrictions in the applica- 



