518 HANDBOOK OF PHYSIOLOGY. 



after a time ceases to rise any further, and the contraction, which has 

 reached its maximum, is maintained. The condition which ensues is 

 called Tetanus. A tetanus is really a summation of contractions, but 

 unless the stimuli become very rapid indeed, the muscle will still be in a 

 condition of vibratory contraction and not of unvarying contraction. 



Fig. 331. Curve of tetanus, obtained from the gastrocnemius of a frog;, where the shocks 

 were sent in from an induction coil, about sixteen times a second, by the interruption of the 

 primary current by means of a vibrating spring, which dipped into a cup of mercury, and broke 

 the primary current at each vibration. 



If the shocks, however, be repeated at very short intervals, being 15 

 per second for the frog's muscle, but varying in each animal, the muscle 

 contracts to its utmost suddenly and continues at its maximum contrac- 

 tion for some time and the lever rises almost perpendicularly, and then 

 describes a straight line (fig. 332). If the stimuli are not quite so rapid 

 the line of maximum contraction becomes somewhat wavy, indicating a 

 slight tendency of the muscle to relax during the intervals between the 

 stimuli (fig. 331). 



Muscular Work. We have seen that work is estimated by multi- 

 plying 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 mus- 

 cle must be moderately loaded: if the weight is increased beyond a cer- 

 tain point however, the muscle becomes strained and raises it through 



Fig. 338. Curve ofr cknus, from a series of very rapid shocks from a magnetic interrupter. 



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

 further increased, the muscle is completely overtaxed and cannot raise 

 the weight. No work is then done at all. Practical illustrations ot 

 these facts must be familiar to every one. 



