MUSCLE-NEKVE PHYSIOLOGY. 517 



the duration of the contraction is much prolonged. Later a condition 

 is reached in which the muscle remains more or less contracted for a 

 considerable time. This condition is called contracture. 



Effect of Temperature. Heat up to a certain point increases the irri- 

 tability of muscle and favors rapidity in chemical activity, with the re- 

 sult that when it contracts the latent period is shortened, the height of 

 the wave is increased, and the duration of the contraction is lessened. 

 Cold produces contrary effects. 



Effect ofDrugs.VeY&trine does not alter the rapidity with which 

 contraction occurs, but enormously prolongs the stage of relaxation. 

 The salts of barium act similarly, and to a less extent those of calcium 

 and strontium. In this connection it is interesting to recall that supra- 

 renal extract acts likewise on voluntary muscles. 



Effect of Strength of Stimulus. A strength of current that is just 

 sufficient to give a contraction is called a minimal stimulus. As the 

 strength of the current is increased, the height of the contraction curve 

 increases until the maximal stimulus is reached, beyond which no in- 

 crease occurs. The latent period shortens with increased strength of 

 stimulus. 



Effect of Rate of Stimulation. If we stimulate the nerve-muscle 

 preparation with two induction shocks, one immediately after the other, 

 when the point of stimulation of the second one corresponds to the 

 maximum of the first, a second curve (fig. 330) will occur, which will 



to start from the first, as does the first from the base line. (M. F< 



commence at the highest point of the first and will rise nearly as high, 

 so that the sum of the height of the two curves almost exactly equals 

 twice the height of the first. If a third and fourth shock be passed, a 

 similar effect will ensue, and curves one above the other will be traced, 

 the third being slightly lower than the second, and the fourth than the 

 third. If a more numerous series of shocks occur, however, the lever 



