632 A MANUAL OF PHYSIOLOGY 



nerve, the electrical response begins about Tinnr second, and 

 the change of form of the muscle about T TyW second after 

 the stimulation. It is believed that in a muscle directly 

 excited the electrical change begins in less than i oVcr second, 

 and the mechanical change in y^Vrr second (Burdon Sander- 

 son). (Figs. 199-204.) 



When one electrode is placed on an injured part, the wave 

 of action and of electrical change diminishes as it reaches 

 the injured tissue ; and if the tissue is killed at this part, it 

 diminishes to zero ; so that here the second phase may be 

 greatly weakened or may disappear altogether, and we then 

 have what is. called a monophasic variation. 



In this case the current of action can be demonstrated, even for a 

 single excitation, but still better for a tetanus, with the galvano- 



FIG. 199. PHOTOGRAPHIC ELECTROMETER CURVES FROM SARTORIUS 

 MUSCLE (SANDERSON). 



The darkly-shaded curve represents the diphasic variation of the uninjured muscle ; 

 the lightly-shaded curve the monophasic variation of the muscle after injury of one end. 

 The toothed curve at the top is the time-tracing registered by photographing the prong 

 of a tuning fork vibrating five hundred times a second. 



meter, which in general is no f quick enough to analyze a diphasic 

 variation with equal phases, and gives, therefore, only their algebraic 

 sum that is, zero. When the muscle or nerve is tetanized, the 

 negative variation appears, while stimulation is kept up, as a per- 

 manent deflection representing the ' sum ' of the separate effects. 

 It is in the opposite direction to the current of rest, since the injured 

 tissue, being less affected by the excitation, and therefore undergoing 

 a smaller negative change than the uninjured, becomes relatively to 

 the latter less negative. 



When the current of rest is compensated by a branch of an external 

 current just sufficient to balance it and bring the galvanometer image 

 back to zero, the action current appears alone in undiminished 

 strength. This shows that the latter is not due to a change of 

 electrical resistance during excitation, since such a change would 

 equally affect current of rest and compensating current, and they 

 would still balance each other. The action current is really due to 



