ELECTRO- PH YSIOL OGY 63 1 



be succeeded by a period during which B is more negative 

 than 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 B is long, for in this case A will have a chance of 

 becoming strongly negative while B is still normal. Simi- 

 larly, if A has again become normal, or nearly normal, 

 before the maximum negativity has passed over B, a strong 

 second phase will be favoured. The heart-muscle, accord- 

 ingly, where the wave of contraction, and its accompanying 

 electrical change, move with comparative slowness, is better 



FIG. 198. DIAGRAM TO ILLUSTRATE PROPAGATION OF THE NEGATIVE 

 CHANGE ALONG AN ACTIVE MUSCLE OR NERVE. 



Suppose A B 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 bring 

 filled with water. Suppose the rims of the cylinders originally to touch A B 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 k^vt 

 always in contact with its curved margin, will, after describing the curve of uie 

 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. This will corre- 

 spond to the time during which the point of the tissue represented by A would be 

 negative to a point repiesented 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 negative variation. 



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 



