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A MANUAL OF PHYSIOLOGY 



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

 stimulated 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 distance between the electrodes. 

 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 surrounding 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 narrow- 

 ing 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 

 correspond both to anode and 

 kathode (Fig. 255). 



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 con- 

 traction only appears in a dis- 

 guised form ; for since a kathode 

 and an anode exist at each pole, 

 there is, with a current of suffi- 

 cient strength (' strong cur- 

 rent '), excitation at each both 

 at make and break. The nega- 

 tive make contraction is, how- 

 ever, 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 nega- 

 tive electrode, and the current density at the latter is accordingly 

 much greater. The positive break-contraction is, for a similar 

 reason, stronger than the negative. 



With a ' weak ' current, the only contraction is a closing one at 

 the kathode ; with a ' medium ' current there are both opening and 

 closing contractions at the positive pole, and a closing but no opening 

 contraction at the negative (Practical Exercises, p. 743). 



The conductivity of the nerve, as we have seen in various 

 examples, is not necessarily altered in the same sense as the 



FIG. 255. 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 embedded, and secondary 

 kathodes ( - ) where the current 

 passes out of the nerve into the sur- 

 rounding tissues below the positive 

 electrode. 



