CHAP. IL] THE CONTRACTILE TISSUES. 81 



(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 shewn in Fig. 17, where the primary current of an 



FIG. 17. TETANUS PRODUCED WITH THE ORDINARY MAGNETIC INTERRUPTOR OF AN 

 INDUCTION-MACHINE. (Kecording surface travelling slowly.) 



The interrupted current is thrown in at a. 



induction-machine was rapidly made and broken by the magnetic 

 interruptor, Fig. 5. 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 towards 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 shewn 

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

 form or other, are employed. They are seen, however, what- 

 ever 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 

 character, 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 movements, such as a sudden 

 jerk of a limb or a wink of the eyelid, are in reality examples of 

 tetanus of short duration. 



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. 



F. 6 



