CHAP, ii.] THE CONTRACTILE TISSUES. 141 



A stimulus, in order that it may be effective, must have an 

 action of a certain duration, the time necessary to produce an effect 

 varying according to the strength of the stimulus and being different 

 in the case of a nerve from what it is in the case of a muscle. It 

 would appear that an electric current applied to a nerve must have 

 a duration of at least about "0015 sec. to cause any contraction at 

 all, and needs a longer time than this to produce its full effect. 

 A muscle fibre apart from its nerve fibre requires a still longer 

 duration of the stimulus, and hence, as we have already stated, 

 a muscle poisoned by urari, or which has otherwise lost the action 

 of its nerves, will not respond as readily to induction-shocks as to 

 the more slowly acting, breaking and making of a constant current. 



In the case of electric stimuli, the same current will produce a 

 stronger contraction when it is sent along the nerve than when it 

 is sent across the nerve ; indeed it is maintained that a current 

 which passes through a nerve in an absolutely transverse direction 

 is powerless to generate impulses. 



It would also appear, at all events up to certain limits, that the 

 longer the piece of nerve through which the current passes, the 

 greater is the effect of the stimulus. 



When two pairs of electrodes are placed on the nerve of a long 

 and perfectly fresh and successful nerve-preparation, one near to 

 the cut end, and the other nearer the muscle, it is found that the 

 same stimulus produces a greater contraction when applied through 

 the former pair of electrodes than through the latter. This has 

 been interpreted as meaning that the impulse started at the 

 farther electrodes gathers strength, like an avalanche, in its 

 progress to the muscle. It is more probable, however, that the larger 

 contraction produced by stimulation of the part of the nerve near 

 the cut end is due to the stimulus setting free a larger impulse, 

 i.e. to this part of the nerve being more irritable. The mere 

 section, possibly by developing nerve currents, increases for a time 

 the irritability at the cut end. A similar greater irritability may 

 however also be observed in the part of the nerve nearer the 

 spinal cord while it is still in connection with the spinal cord; and 

 it is possible that the irritability of a nerve may vary considerably 

 at different points of its course. 



80. We have seen that when single stimuli are repeated 

 with sufficient frequency, the individual contractions are fused 

 into tetanus ; as the frequency of the repetition is increased, the 

 individual contractions are less obvious on the curve, until at last 

 we get a curve on which they seem to be entirely lost and which 

 we may speak of as a complete tetanus. By such a tetanus a much 

 greater contraction, a much greater shortening of the muscle is of 

 course obtained than by single contractions. 



The exact frequency of repetition required to produce complete 

 tetanus will depend chiefly on the length of the individual 

 contractions, and this varies in different animals, in different 



