78 



PHYSIOLOGY 



CHAP. 



exceed the value of the demarcation current; for instance, the 

 demarcation current may equal 0*04 volt, the negative variation 

 0'08 volt. The negative variation also increases up to a certain 

 maximum with increase of the elastic tension or load of the 

 muscle, parallel with the development of work and of heat. 



In order to understand the nature of the negative variation 

 of the demarcation current in muscle when the nerve is tetanised, 



FIG. 53. Myogram of tetanic contraction of frog's gastrocnemius (white line on black ground) and 

 simultaneous photograph of negative variation (black line on white ground). (A. D. Waller.) 

 a, gradually diminishing demarcation current ; 6, its sudden decrease during tetanus (negative 

 variation) ; c, subsequent positive variation on cessation of tetanus ; d, return of slowly 

 declining demarcation current. 



it must be remembered that in consequence of stimulation the 

 whole mass of the muscle undergoes an explosive chemical change 

 associated with the passage from the state of rest to the state of 

 activity, which is greater in the normal than in the altered parts 

 of the muscle. This effect of excitation sets up a difference of 

 electrical potential and gives rise to the action current, which 

 neutralises the demarcation current, and may even exceed it 

 (Gotch and Sanderson). 



