ELECTRICAL CHANGES IN MUSCLE 233 



THE DEMARCATION CURRENT OR CURRENT OF INJURY 



Muscle or nerve may become negative under two conditions : (1) During 

 activity ; (2) when dying as the result of injury. It is doubtful however 

 whether these two conditions are really distinct. Section or injury of a muscle 

 causes a prolonged stimulation of the adjacent parts of the muscle fibres. 

 These parts therefore being excited, must be negative to the unexcited parts 

 which are further away from the seat of injury so that a demarcation current 

 is really an excitatory current. We thus come to the conclusion, para- 

 doxical only in terms, that the so-called currents of rest are really currents of 

 action and are due to excitation around the injured spot.* 





SECONDARY CONTRACTION. RHEOSCOPIC FROG 



The negative variation of one muscle may be used to make another 

 contract. 



If the nerve of the preparation a (in Fig. 90) be laid so as to touch at two 

 points the cut end and surface of the i tiiscle 6, and the 

 nerve of b then stimulated with single induction 

 shocks, every contraction of b will be attended by a 

 contraction of a, excited by the negative variation 

 of the current passing through its nerve from the 

 point touching the cut end to that in contact with 

 the equator of 6. 



If the nerve of b is tetanised, a as well as b enters mfc * frog . 

 into a continued contraction. This 'secondary tetanus ' 

 is of interest as showing that, although the contractions of b are fused, the 

 excitatory process and negative variations are still quite distinct. 



* If the demarcation current is really due only to excitation, we should expect 

 to find it weaker than the action current obtained by exciting the whole muscle to 

 contract. And this is the case. The E.M.F. of the demarcation current of a sar- 

 torius equals about 0'05 of a Daniell cell. The action current of the same muscle may 

 attain to an E.M.F. = O08 of a DanieU cell (Gotch). 



