NERVE 



599 



gradual weakening, and final extinction can be very well shown by 

 means of the action stream (p. 630). 



The above formula can only be verified upon isolated 

 nerves, and, even for these, exceptional results are apt to be 

 obtained as soon as the nerves begin to die. 



A formula similar to the law of contraction has been 

 shown to hold for the inhibitory fibres of the vagus (Donders), 

 * inhibition ' being substituted for ' contraction.' There is 

 also some evidence that a similar law obtains for sensory 

 nerves. 



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

 is stimulated without previous isolation in the human body, 

 for instance, through elec- 

 trodes 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 distance be- 

 tween the electrodes. 



FIG. 186. DIAGRAM OF LINES OF 

 FLOW OF A CURRENT PASSING 



THROUGH A NERVE. 



A, an isolated nerve ; B, a nerve in 

 situ. Secondary anodes (+) are formed 

 where the current re-enters the nerve 

 below the negative electrode after passing 

 through the tissues in which it is em- 

 bedded, and secondary kathodes ( ) 

 where the current passes out of the nerve 

 into the surrounding tissues below the 

 positive electrode. 



On the contrary, when, as is 

 usually the case in electro-thera- 

 peutical 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 sur- 

 rounding tissues ; so that both an anode (surface of entrance) and 

 a kathode (still larger surface of exit) will correspond to the single 

 positive pole. Similarly, the single negative 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 electrodes were on the course of the nerve, the stream- 

 lines would still cut it in such a way that each electrode would cor- 

 respond both to anode and kathode (Fig. 186). 



