144 THE MUSCLE CELLS 



a passage of current from the uninjured to the injured parts of 

 the preparation through the galvanometer. Within the tissue, of 

 course, the circuit will be completed by the passage of the current 

 from injured to uninjured. This difference of electromotive 

 force may be demonstrated without a galvanometer. If the 

 nerve from an uninjured muscle be laid over an injured muscle 

 in such a way that at one point it touches a cut portion, then, the 

 undamaged muscle will contract every time the circuit is com- 

 pleted by laying a second point of the nerve on an uninjured 

 portion of the injured muscle. 



This difference in E.M.F. persists as long as the injury. In a 

 degenerating muscle its degenerating portion is electropositive, 

 galvanometrically negative or " zincative " to its normal portion. 

 Naturally, the difference ceases when degeneration is complete. 

 The whole mass is then isoelectric. The current is due, as has 

 previously been explained (p. 133), to physico-chemical differences 

 at the junction of living and dying tissue. Dead tissue gives no 

 current. 



Current of Action. 



Similar physico-chemical changes are answerable for the wave 

 of " negativity " which precedes the mechanical change in a 

 contracting muscle. The part which is just about to contract 

 is electropositive, or " zincative," to the rest. Consider for a 

 moment a muscle, say 5 cm. long. The preparation is supposed 

 to be perfect and, therefore, there will be no demarcation current. 

 If such a muscle be stimulated by a single induction shock at one 

 end and two points A 3 cm. and B 5 cm. from the point of stimu- 

 lation be led to an electrometer, then each stimulus will cause 

 a wave of contraction to pass along the muscle, preceded by a 

 wave of " negativity." That is, A will become " zincative " to 

 the rest of the muscle so that current would pass through a 

 galvanometer from B to A. A fraction of a second later, the 

 disturbance will have passed on to B which will now be " zincative " 

 to the rest, causing a current to pass through the galvanometer 

 from A to B. That is, A has first been electropositive and then 

 electronegative to B. Such a current is termed diphasic and is 

 an indication of a propagated change. 



A muscle nerve preparation may be used to demonstrate the 

 presence of the current of action. If the sciatic nerve of a frog's 

 gastrocnemius be placed on another gastrocnemius, the former 

 muscle may be made to contract by stimulating the nerve of 



