THE PHENOMENA OF MUSCLE AND NERVE. 75 



When the shocks succeed each other still more rapidly than in Fig. 26, 

 the individual contractions, visible at first, may become fused together and 

 wholly lost to view in the latter part of the curve. When the shocks succeed 

 each other still more rapidly (the second contraction beginning in the 

 ascending portion of the first) it becomes difficult or impossible to trace 

 out any of the single contractions. 1 The curve then described by the lever 

 is of the kind shown in Fig. 27, where the primary current of an induction- 





Tetanus Produced with the Ordinary Magnetic Interrupter of an Induction-machine. (Recording 

 surface travelling slowly.) The interrupted current is thrown in at a. 



machine was rapidly made and broken by the magnetic interruptor, Fig. 15. 

 The lever, it will be observed, rises at a (the recording surface is travelling 

 too slowly to allow the latent period to be distinguished), at first very 

 rapidly, in fact in an unbroken and almost a vertical line, and so very 

 speedily reaches the maximum, which is maintained so long as the shocks 

 continue to be given ; when these cease to be given, the curve descends at 

 first very rapidly and then more and more gradually toward the base-line, 

 which it reaches just at the end of the figure. 



This condition of muscle, brought about by rapidly repeated shocks, this 

 fusion of a number of simple twitches into an apparently smooth continuous 

 effort, is known as tetanus or tetanic contraction. The above facts are most 

 clearly shown when induction-shocks, or at least galvanic currents in some 

 form or other, are employed. They are seen, however, whatever be the form 

 of stimulus employed. Thus, in the case of mechanical stimuli, while a 

 single quick blow may cause a single twitch, a pronounced tetanus may be 

 obtained by rapidly striking successively fresh portions of a nerve. With 

 chemical stimulation, as when a nerve is dipped in acid, it is impossible to 

 secure a momentary application ; hence tetanus, generally irregular in cha- 

 racter, is the normal result of this mode of stimulation. In the living body, 

 the contractions of the skeletal muscles, brought about either by the will or 

 otherwise, are generally tetanic in character. Even very short sharp move- 

 ments, such as a sudden jerk of a limb or a wink of the eyelid, are in reality 

 examples of tetanus of short duration. 



If the lever, instead of being fastened to the tendon of a muscle hung 

 vertically, be laid across the belly of a muscle placed in a horizontal position 

 and the muscle be thrown into tetanus by a repetition of induction-shocks, it 

 will be seen that each shortening of the muscle is accompanied by a corre- 

 sponding thickening, and that the total shortening of the tetanus is accom- 

 panied by a corresponding total thickening. And, indeed, in tetanus we can 

 observe more easily than in a single contraction that the muscle in contract- 



1 The ease with which the individual contractions can be made out depends in part, it 

 need hardly be said, on the rapidity with which the recording surface travels. 



