NEGATIVE VARIATION 



825 



diphasic current of action. If the wave has not passed over A before 

 it reaches B, as would in general be the case in an actual experiment, 

 there will be first a period during which A is relatively negative to 

 B (first phase) ; this will end as soon as B has become iso-electric 

 with A, and will be succeeded by a period during which B is rela- 

 tively negative to A (second phase). Since the wave takes time to 

 reach its maximum, it is evident that a well-marked first phase will 

 be favoured when the interval between its arrival at A and at B is 

 long, for in this case A will have a chance of becoming strongly 

 negative while B is still normal. Similarly, if A has again become 

 normal, or nearly normal, before the maximum negative change has 

 passed over B, a strong second phase will be favoured. The heart- 

 muscle, accordingly, where the wave of contraction, and its accom- 

 panying electrical change, move with comparative slowness, te 

 better suited for showing a well-marked diphasic variation than 

 skeletal muscle, and still better suited than nerve. In the gastroc- 



T*^* r 



Fig. 295. Photographic Electrometer Curves from Sartorius Muscle (Sanderson). 

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

 muscle; the lightly-shaded curve the monop basic 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. 



nemius muscle of the frog, when excited through its nerve, the elec- 

 trical response begins about T <y 4 oo second, and the change of form of 

 the muscle about ^QQ second, after the stimulation. It is believed 

 that in a muscle directly excited the electrical change begins in less 

 than yJ^ second, and the mechanical change in T ^^ second (Burden 

 Sanderson, Figs. 295-300). 



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 an ordinary galvan- 

 ometer, which in general is not 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 action 

 current appears, while stimulation is kept up, as a permanent 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 



