LAW OF CONTRACTION. 



511 



part, by a positive rebound, becomes more excitable than in its 

 normal state. The rebound over the line of normal irritability 

 lasts but a very short time; but still, as we shall see presently, it 

 is of greater duration than the passage of the negative variation 

 along the nerve. 



LAW OF CONTRACTION. 



Upon the foregoing facts (-e), and others already mentioned, 

 viz., that the impulse starts in the nerve from different poles 



FIG. 204. 



Diagram showing the meaning of the terms ascending and descending currents, used in 

 speaking of the law of contraction. The end of the vertebral column, sciatic nerves, 

 and calf muscles of the frog are shown, while the arrows indicate the direction of the 

 asaending current, A, on the left, and the descending current, D, on the right, according 

 as the positive pole, c, of the battery is below or above. 



and with different force, with a making and a breaking shock 

 depends the law of contraction, which would be difficult to under- 

 stand without bearing in mind all these interesting points. 



It was found that, with the same strength of stimulation, not 

 only were different degrees of contraction produced with making 

 and breaking shocks, but also that, other things being similar, a 



