262 PHYSIOLOGY 



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 con- 

 clusion, 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.* 



We shall see later, in dealing with the electrical changes which 

 accompany the excitatory state, that the two conditions of injury 

 and of excitation are really attended with similar molecular changes 

 in the muscle or the nerve. 



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. 91) 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 b will be attended by a con ; 

 traction of a, excited by the negative varia- 

 tion of the current passing through its nerve 



from the point touching the cut end to that 

 FIG. 91. r 7 



Rheoscopic frog. m contact with the equator of b. 



If the nerve of b is tetanised, a as well 



as b enters into a continued contraction. JThis ' 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 only due 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 demar- 

 cation current of a sartorius equals about 0-05 of a Daniell cell. The action 

 current of the same muscle may attain to an E.M.F. = 0-08 of a Daniell cell 

 (Gotch). 



