LAW OF CONTRACTION. 509 



becomes, by a kind of negative modification, very much lowered 

 in its irritability ; while on the other hand, the anelectrotonic 

 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. 



FIG. 204. 



Diagram showing the meaning of the terms ascending and descending cur- 

 rents, 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 ascending 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. 



LAW OF CONTRACTION. 



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

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

 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. 



