24JS A Manual of Veterinary Physiology. 



The electric changes in muscles are practically the same 

 as those occurring in nerves, and as this question has to be 

 dealt with more fully under the nervous system, we shall 

 postpone any further consideration of the subject until 

 then (see p. 255). 



A Muscle Curve. — If a muscle preparation be arranged so 

 as to record its movements on a revolving drum, and a 

 single shock from an induced current be passed into it, a 

 single contraction follows which traces an upstroke on the 

 drum, and then the muscle relaxing makes a downstroke : 

 in this way is obtained a muscle-curve. But it is found on 

 examining it that the muscle does not contract immediately 

 the stimulus is applied, but that a perceptible period elapses 



Fig. 21. — A Muscle Curve from the Gastrocnemius of the 

 Frog. 



a indicates the moment at which the induction shock is sent into the 



nerve ; b, the commencement ; c, the maximum ; and d, the close 



of the contraction (Foster). 

 The curve is read from left to right ; below the muscle curve is a 



curve made by a time recorder, each complete curve representing 



one-hundredth of a second. 



before contraction occurs; this is called the latent period 

 (a, b, Fig. 21). During this latent period, although the 

 muscle is not moving, yet it is possible to determine that 

 the natural muscle current is diminished and that negative 

 variation occurs — viz., a momentary reversal of the natural 

 current. The latent period differs in various muscles, and 

 depends also on their condition ; in the frog it is estimated 

 at ,,',„- of a second. Succeeding the latent period is the 

 stage of contraction, at first slow, then rapid, then again 

 slow till extreme contraction lias been produced, followed 

 by the third stage — viz., that of relaxation or elongation of 

 the muscle — which is slow at first and then rapid : while 



