ELECTRO-PHYSIOLOGY 719 



metrically negative. 5 It is in this sense that we shall employ 

 the term. 



The best object for experiments on the demarcation current is a 

 straight-fibred muscle like the frog's sartorius. If this muscle be 

 taken, and the ends cut off perpendicularly to the surface, a muscle- 

 prism or muscle-cylinder is obtained (Fig. 267). The strongest 

 current is got when one electrode is placed on the middle of either 

 cross-section and the other on the ' equator ' that is, on a line 

 passing round the longitudinal surface midway between the ends. 

 The direction of this current is from the cross-section towards the 

 equator in the muscle. If the electrodes are placed on b - ymmetrica_ 

 points on each side of the equator, there is no current. 



FIG. 268. 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. 



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. 268), on the 



