80 VARIATIONS OF IRRITABILITY. [BOOK i. 



coil, and thus make use of extremely feeble induction-shocks, we 

 shall probably find that these shocks, applied even to a quite fresh 

 muscle-nerve preparation, produce no contraction. If we then 

 gradually slide the secondary coil nearer and nearer the primary 

 coil, and keep on trying the effects of the shocks, we shall find 

 that, after a while, in a certain position of the coils, a very feeble 

 contraction makes its appearance. As the secondary coil comes 

 still nearer to the primary coil, the contractions grow greater and 

 greater. After a while, however, and that, indeed, in ordinary 

 circumstances very speedily, increasing the strength of the shock 

 no longer increases the height of the contraction; the maximum 

 contraction of which the muscle is capable with such shocks 

 however strong has been reached. 



If we use a tetanizing or interrupted current, we shall obtain 

 the same general results ; we may, according to the strength of the 

 current, get no contraction at all, or contractions of various extent 

 up to a maximum, which cannot be exceeded. Under favourable 

 conditions the maximum contraction may be very considerable : 

 the shortening in tetanus may amount to three-fifths of the total 

 length of the muscle. 



The amount of contraction then depends on the strength of 

 the stimulus, whatever be the stimulus; but this holds good 

 within certain limits only ; to this point however we shall return 

 later on. 



51. If, having ascertained in a perfectly fresh muscle-nerve 

 preparation the amount of contraction produced by this and that 

 strength of stimulus, we leave the preparation by itself for some 

 time, say for a few hours, and then repeat the observations, we 

 shall find that stronger stimuli, stronger shocks, for instance, are 

 required to produce the same amount of contraction as before ; that 

 is to say, the irritability of the preparation, the power to respond 

 to stimuli, has in the meanwhile diminished. After a further 

 interval, we should find the irritability still further diminished : 

 even very strong shocks would be unable to evoke contractions 

 as large as those previously caused by weak shocks. At last we 

 should find that no shocks, no stimuli, however strong, were able 

 to produce any visible contraction whatever. The amount of 

 contraction, in fact, evoked by a stimulus depends not only on the 

 strength of the stimulus but also on the degree of irritability of 

 the muscle-nerve preparation. 



Immediately upon removal from the body, the preparation 

 possesses a certain amount of irritability, not differing very 

 materially from that which the muscle and nerve possess while 

 within, and forming an integral part of the body ; but after re- 

 moval from the body the preparation loses irritability, the rate of 

 loss being dependent on a variety of circumstances ; and this goes 

 on until, since no stimulus which we can apply will give rise to 

 a contraction, we say the irritability has wholly disappeared. 



