112 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 43, 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 grams ; the records of 

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

 experiment consisted of two parts in the first 66 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 for the first five contractions, the " introductory contractions," 

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

 contractions, the " staircase contractions." These phenomena repeat themselves 

 in the second part of the experiment, that following the interval of rest. The 

 contractions at the beginning of the second series were not so high as those at 

 the end of the first series, though somewhat higher than those seen at the 

 beginning of the first series; the rest of five minutes was not sufficient to 

 entirely do away with the stimulating influence of the preceding work. The 

 contractions of the second series took the following course : The first four 

 introductory contractions gradually declined, then came the staircase contrac- 

 tions, which continued to rise until the 100th contraction, when a gradual 

 lessening of the height of the contractions began. This decline continued 

 throughout the long series of more than 1 700 contractions given in the record, 

 and, had the experiment been continued, would have undoubtedly gone on 

 until the power was completely lost. The curve of fatigue was not a straight 

 line, but fell somewhat more rapidly during the early part of the work than 

 toward the end. 



That the peculiar changes in the height of the contractions which occur in 

 the early part of an experiment such as that which we have described are not 

 abnormal, and the result of the artificial conditions under which the work is 

 done, is shown not only by the fact that they are observed when a muscle 

 which has its normal blood-supply is rhythmically excited to a large number 

 of contractions, but by the personal experience of every one accustomed to 

 violent muscular exercise. Everyone is conscious that he cannot put out the 

 greatest muscular effort until he has " warmed up to the work." The runner 

 precedes the race by a short run ; the oarsman takes a short pull before going 

 to the line ; in all the sports one sees the contestants making movements to 

 " limber up " before they enter upon the work of the game. These prelim- 

 inary movements are performed not only to put the muscles in better condition 

 for action, but to ensure more accurate co-ordination that is to say, the facts 

 ascertained for the muscle can be carried over to the central nervous system. 

 The finely adjusted activities of the nerve-cells which control the muscles reach 

 their perfection only after repeated action. 



In such experiments as that recorded in Figure 43 the record shows to 



