720 A MANUAL OF PHYSIOLOGY 



longitudinal surface of a muscle to be connected with a capillary 

 electrometer (p. 621), the movements of the mercury being 

 photographed on a travelling surface, for example, a pendulum 

 carrying a sensitive plate. Let the muscle be excited at the 

 end, so that the wave of excitation will be propagated in the 

 direction of the arrow. The wave will reach A first, and while 

 it has not yet reached B, A will become negative to B. If there 

 is a resting difference of potential between A and B, this will 

 be altered, the new and transitory difference adding itself 

 algebraically to the old. When the wave reaches B, it may 

 already have passed over A altogether, and B now becoming 

 negative to A, there will be a movement of the meniscus of 

 the electrometer in the opposite direction. This is called the 

 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 



FIG. 269. 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. 



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 relatively 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 accompanying 

 electrical change, move with comparative slowness, is better 

 suited for showing a well-marked diphasic variation than skeletal 

 muscle, and still better suited than nerve. In the gastrocnemius 

 muscle of the frog, when excited through its nerve, the electrical 



