630 A MANUAL OF PHYSIOLOGY 



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 symmetrical 

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



A particular case of this symmetrical or ' streamless ' arrangement 

 is where the middle points of the two cross-sections are led off to the 

 galvanometer ; here, if the sections are similar, their potential is the 

 same, and the needle remains at zero. Between two points of the 

 longitudinal surface at unequal distances from the equator there is a 

 current in the galvanometer from the nearer to the more distant point, 

 the potential of a longitudinal point nearer a cross-section being lower 

 ( than that of one more remote. Between two points on the same 

 , cross-section there is a current if they are not symmetrically placed 

 with reference to its centre, the direction in the muscle being from 

 more central to more peripheral point. 



The above may be taken as applying to nerve also, with the proviso* 

 that less is known as to electrical differences between points on the 

 same cross-section, since ordinary cold-blooded nerves are too small 

 for such experiments, and that two parallel cross-sections of a nerve 

 are not iso-electric, so that a current (the so-called axial stream) passes 

 between them when they are connected with the galvanometer. 



Current of Action, or Negative Variation. When a muscle 

 or nerve is excited, a wave of diminished potential (nega- 

 tivity) sweeps over it. Suppose two points, A and B 

 (Fig. 198), on the longitudinal surface of a muscle to be 

 connected with a capillary electrometer (p. 538), the move- 

 ments 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 more negative than B (first phase) ; this will end 

 as soon as B has become equally negative with A, and will 



