THE MUSCULAR SYSTEM. 



413 



The muscle curve obtained from the heart resembles that of unstriped 

 muscles in the long duration of the effect of stimulation; the descending 

 curve also is very much prolonged. 



The greater part of the latent period is taken up by changes in the 

 muscle itself, and the remainder occupied in the propagation of the 

 shock along the nerve. 



Tetanus. 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. 290) will occur, which will commence at the highest 

 point of the first and will rise nearly as high, so that the sum of the 



Fia. 289. Simple form of pendulum myograph and accessory parts. A, pivot upon which pen- 

 dulum swings; 'B, catch on lower end of myograph opening the key, C, in its swing; Z>, a spring- 

 catch which retains myograph, as indicated by dotted lines, and on pressing down the handle of 

 which the pendulum swings along the arc to D on the left of figure, and is caught by its spring. 



height of the two curves almost exactly equals twice the height of the 

 first. If a third and a fourth shock be passed, a similar effect will ensue, 

 and curves one above the othe* will be traced, the third being slightly 

 less than the second, and the fourth than the third. If a more numerous 

 series of shocks occur, hov/ever, the lever after a time ceases to rise any 

 further, and the contraction, which has reached its maximum, is main- 

 tained. The condition which ensues is called Tetanus. A tetanus is 

 really a summation of contractions, and unless the stimuli become very 

 rapid indeed, the muscle will be then in a condition of vibratory con- 

 traction and not of unvarying contraction. 



