CAUSES AND PHENOMENA OF MOTION. 33 



will be traced, the third being slightly less than the second, and the 

 fourth than the third. If the shocks be repeated at short intervals, how- 

 ever, the lever after a time ceases to rise any further, and the contraction 

 which has reached its maximum is maintained (Fig. 285), and the lever 

 marks a straight line on the recording cylinder. This condition is called 

 tetanus of muscle. The condition of "an ordinary tetanic muscular 

 movement is essentially a vibratory movement, the apparently rigid and 

 firm muscular mass is really the subject of a whole series of vibrations, a se- 

 ries namely of simple spasms; it will be readily understood why a tetanized 

 muscle, like all other vibrating bodies, gives out a sound" (M. Foster). 



If the stimuli are not quite so rapidly sent in the line of maximum 

 contraction it becomes somewhat wavy, indicating a slight tendency of the 

 muscles to relax during the intervals between the stimuli (Fig. 284). 



Muscular Work. We have seen (p. 124, Vol. I. ) that work is esti- 

 mated by multiplying the weight raised, by the height through which it 

 has been lifted. It has been found that in order to obtain the maximum 

 of work, a muscle must be moderately loaded: if the weight be increased 

 beyond a certain point, the muscle becomes strained and raises the weight 

 through so small a distance that less work is accomplished. If the load is 

 still further increased the muscle is completely overtaxed, cannot raise the 

 weight, and consequently does no work at all. Practical illustrations of 

 these facts must be familiar to every one. 



The power of a muscle is usually measured by the maximum weight 

 which it will support without stretching. In man this is readily deter- 

 mined by weighting the body to such an extent that it can no longer be 

 raised on tiptoe: thus the power of the calf -muscles is determined 

 (Weber). 



The power of a muscle thus estimated depends of course upon its cross- 

 section. The power of a human muscle is from two to three times as 

 great as a frog's muscle of the same sectional area. 



Fatigue of Muscle. A muscle becomes rapidly exhausted from 

 repeated stimulation, and the more rapidly, the more quickly the induc- 

 tion-shocks succeed each other. 



This is indicated by the diminished height of contraction in the ac- 

 companying diagrams (Fig. 286). It will be seen that the vertical lines,, 

 which indicate the extent of the muscular contraction, decrease in length 

 from left to right. The line A B drawn along the tops of these lines is; 

 termed the "fatigue curve." It is usually a straight line. 



In the first diagram the effects of a short rest are shown : there is a 

 pause of three minutes, and when the muscle is again stimulated it con- 

 tracts up to A', but the recovery is only temporary, and i\& fatigue curve? 

 after a few more contractions, becomes continuous with that before the: 

 rest. 



In the second diagram is represented the effect of a stream of oxygenated 

 VOL. II. 3. 



