180 



THE MUSCLE CELLS 



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 (Fig. 43 (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 (Fig. 4-3 {h) ). That is, A has first been 



(pi RESULTAMT 



"ACTION CURREMT" 



ACTIVE. R^IOH 



ACTIVE I?Ei:^ION 



Fio. 43. — Diagram to show the diphasic character of the current of action in muscle. 

 (See text for explanation.) Fig. c (in centre) is a representation of the photographic 

 trace obtained liy protecting the shadow of the mercury in the capillary tube tluough a 

 IciLs on to a rotating strip of sensitised paper. See Fig. 92. 



electropositive and then electronegative to B. Such a current is 

 termed diphasic and is an indication of a propagated change 

 (Fig. 43 (c) ). 



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 

 the latter. The essential point about this preparation which is 

 called the rheoscopic frog is that it actually proves the occurrence 

 of a diphasic current in muscle in consequence of its activity. 

 If the free nerve is stimulated by a tetanising current both muscles 

 go into tetanus. This secondary tetanus demonstrates that 

 although the stinuili are being applied so rapidly that the con- 

 tractions of the " battery " muscle are fused, the diphasic nature 

 of the excitatory process is still quite distinct and is indicated by 

 the contraction of the " galvanometer " muscle. 



