GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. Ill 



four contractions is the rule when a normal resting muscle is called into action 

 (see Figs. 43 and 46), and these contractions at the beginning of a series have 

 received the name of the " introductory contractions." The introductory con- 

 tractions appear to indicate that the first effect of action is to lessen irritability, 

 or that anabolic changes are too slow to compensate for katabolic changes, and 

 each of the first few contractions leaves behind it a fatigue effect. It is not 

 long, however, before the influence of activity to heighten anabolism and 

 increase irritability shows itself in the growth of the height of the succeeding 

 contractions, and the " staircase contractions" are observed. This growth of the 

 height of contractions must necessarily reach a limit, and the amount of 

 increase is found to gradually lessen until the succeeding contractions have the 

 same height. Sometimes the full height of the staircase is not reached before 

 more than a hundred contractions have been made. These maximal contractions 

 may be repeated many times ; sooner or later, however, an antagonistic effect of 

 the work manifests itself and the height of the contractions begins to lessen. 



Effect of Fatigue. A decline in the height of the contractions is an 

 evidence of fatigue, and indicates that anabolism is failing to keep pace with 



1000 



1100 



1200 



1300 



1400 



1500 



1600 



1700 



FIG. 43. Effect of fatigue on the height of muscular contractions. The figure is a reproduction of 

 parts of a record of over 1700 contractions made by an isolated gastrocnemius muscle of a frog. The con- 

 tractions were isotonic, the weight being about 20 grams. The stimuli were maximal breaking induction 

 shocks, and were applied directly to the muscle, at the rate of 25 per minute. Between the first group of 

 66 contractions and the following groups a rest of five minutes was given ; after this rest the work was 

 continued without interruption for about one and a half hours. The second group of contractions, that 

 immediately following the period of rest, contains the first twenty contractions of the new series ; the 

 next group the 100th to the 110th ; the next the 200th to the 210th, and so on. 



katabolism. From this time on, the height of the succeeding contractions 

 continually lessens, and often with great regularity, so that a line drawn so as to 



