824 ELECTRO-PHYSIOLOGY 



Current of Action, or Negative Variation. When a muscle or 

 nerve is excited, an electrical change sweeps over it in the form of 

 a wave. Suppose two points, A and B (Fig. 294), on the longi- 

 tudinal surface of a muscle to be connected with a capillary electro- 

 meter (p. 729), 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 



Fig. 294. Diagram to illustrate Propagation of the Electrical Change along an 

 Active Muscle or Nerve. Suppose AB to be a horizontal bar representing the 

 muscle or nerve. Let C be a curved piece of wood representing the curve of the 

 electrical change at any point. Let W, W' be two glass cylinders connected by 

 a flexible tube, the whole being filled with water. Suppose the rims of the 

 cylinders originally to touch AB at the points A and B, and let them be movable 

 only in the vertical direction. The level of the water being the same in both, 

 there is no tendency for it to flow from one to the other. This represents the resting 

 state of the tissue when A and B are symmetrical points. Now let C be moved 

 along the bar at a uniform rate. The cylinder W, being free to move down, but 

 not horizontally, will be displaced by C, and, if it is kept always in contact with 

 its curved margin, will, after describing the curve of the electrical variation, 

 come again to rest in its old position at A. B will do the same when C reaches 

 it. But since C reaches A before B, the level of the water in B will at first be 

 higher than that in A, and water will flow from B to A as the current flows 

 through the galvanometer. This will correspond to the time during which the 

 point of the tissue represented by A would be galvanometrically negative to a 

 point represented by B. Later on, when C has reached the position shown by 

 the dotted lines, the level of the water in A will be higher than that in B, and a 

 flow will take place in the opposite direction to the first flow. This corresponds 

 to a second phase of the electrical variation. 



become negative to B. If there is a resting difference of potential 

 between A and B, this will be altered, the new and transitory differ- 

 ence adding itself algebraically to the old. When the wave reaches 

 B, it may already have passed over A altogether, and B now be- 

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

 of the electrometer in the opposite direction. This is called the 



