ELECTRICAL CHANGES IN MUSCLE 



well as from the mammalian heart is due to the twisting and alteration in 

 direction of the fibres and of the course of the contraction wave which hav- 

 occurred in the evolution of the heart^from a simple tube. The question will 

 be discussed more fully in chapter xiii. 



THE DEMARCATION CURRENT OR CURRENT OF INJURY 

 According to Hermann, 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, only paradoxical 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 muscle 6, and the 

 nerve of b then stimulated with single induction 

 shocks, every contraction of 6 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. Flo ^ 



If the nerve of 6 is tetanised, a as well as 6 Rheoscdpic frog, 

 enters into a continued contraction. This ' secondary 

 tetanus ' is of interest as showing that, although the contractions of 6 are 

 fused, the excitatory process and negative variations are still quite distinct. 



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

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

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

 torius equals about 0-05 of a Daniel] cell. The action current of the same muscl 

 attain to an E.M.F. = 0-08 of a Daniell cell (Gotch). 



8* 



