THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 763 



at once, after a strong current, become passable. The break-excitation, 

 accordingly, cannot get through to the muscle. 



A formula similar to the law of contraction has been shown to hold 

 for the inhibitory fibres of the vagus (Bonders), ' inhibition ' being 

 substituted for ' contraction.' There is also some evidence that a 

 similar law obtains for sensory nerves. 



It is not difficult to see that with currents of brief duration the break 

 follows so quickly on the make that interference of their opposed effects 

 may occur. This is the reason or, at least, one reason why, above 

 a certain frequency, a muscle or nerve ceases to respond to all of a 

 series of rapidly recurring electrical stimuli (p. 732). It is also the 

 reason why, with single very brief stimuli, a greater current intensity 

 must be employed in order to cause excitation than when the duration 

 of the stimulating current is greater (Woodworth, Lusas). 



The Law of Contraction for Nerves ' in Situ.' When a nerve is stimu- 

 lated without previous isolation in the human body, for instance, 

 through electrodes laid on the skin the current will not enter and 

 leave it through definite small portions 

 of its sheath, nor will it be possible to 

 make the lines of flow nearly parallel to 

 each other and to the long axis of the 

 nerve, as is the case in a slender strip of 

 tissue when there is a considerable dis- 

 tance between the electrodes. 



On the contrary, when, as is usual in 

 electro -therapeutical treatment, a single 

 electrode say, the positive is placed 

 over the position of the nerve, and the 

 other at a distance on some convenient 

 part of the body, the current will enter 

 the nerve by a broad fan of stream-lines 

 cutting it more or less obliquely, and pass 

 out again into the surrounding tissues; 

 so that both an anode (surface of en- 

 trance) and a kathode (still larger surface 

 of exit) will correspond to the single 

 positive pole. Similarly, the single nega- 

 tive electrode will correspond to an 

 anodic surface where the now narrowing 

 sheaf of lines of flow enters the nerve, and 

 a smaller kathodic surface, where they emerge. . Even if the two elec- 

 trodes were on the course of the nerve, the stream-lines would still cut 

 it in such a way that each electrode would correspond both to anode and 

 kathode (Fig. 269). 



It is impossible under these circumstances to take account of the 

 direction of a current in a nerve, or to connect direction with any specific 

 effect. When we place one of the electrodes over the nerve and the 

 other at a distance, the law of contraction only appears in a disguised 

 form; for since a kathode and an anode exist at each pole, there is, with 

 a current of sufficient strength (' strong current '), excitation at each, 

 both at make and break. The negative make contraction is, however, 

 stronger than the positive, for the excitation corresponding to the latter 

 arises at the secondary kathodic surface, where the sheaf of current-lines 

 spreading from the positive electrode passes out of the nerve. Now, 

 this is much larger than the primary kathodic surface, through which 

 the narrow wedge of stream-lines passes to reach the negative electrod \ 

 and the current density at the latter is accordingly much greater. The 



Fig. 269. Diagram of Lines of 

 Flow of a Current passing 

 through a Nerve. A, an isolated 

 nerve ; B, a nerve in situ. Secon- 

 dary anodes ( + ) are formed 

 where the current re-enters the 

 nerve below the negative elec- 

 trode after passing through the 

 tissues in which it is embedded 

 and secondary kathodes ( - ) 

 where the current passes out of 

 the nerve into the surrounding 

 tissues below the positive elec- 

 trode. 



