GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 113 



(see Figs. 46 and 49), 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 uot 

 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 



Fig. 45.— Staircase contractions of gastrocnemius muscle of a frog, excited once every two seconds by 

 strong breaking induction shocks. 



katabolism, or that the waste products which result from the work are col- 

 lecting faster than they can be removed or neutralized and are exerting a 

 paralyzing influence on the muscle protoplasm (see p. 70). 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 connect the summits 

 of the declining contractions, the "curve of fatigue," as it is called, may 

 be a straight line. In the experiment, parts of the record of which are 

 reproduced in Figure 46, an isolated gastrocnemius muscle of a frog was 

 excited with maximal breaking induction shocks at the rate of 25 times 

 a minute for about one and one-half hours; the contractions were isotonic, and 

 the total weight of lever and load did not exceed 20 grains ; the records of 

 the succeeding contractions were recorded on a slowly moving cylinder. The 

 experiment consisted of two parts — in the first (i(J contractions, in the second 

 over 1700 contractions were made; an interval of rest of five minutes was 

 permitted between the two series. 



In the first part of the experiment there was a decline in the height of the 

 contractions tor tin; first five contractions, the "introductory contractions," 

 then during the next sixty-one contractions a gradual rise in the height of the 



Vol. II.— 8 



