86 THE MUSCLE-NERVE MACHINE. [Boom. 



a contraction, when a gradually increasing pressure will fail to do 

 If so ; and in general the efficiency of a stimulus of any kind will 

 II depend in part on the suddenness or abruptness of its action. 



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 its strength and being different in nerve from 

 what it is in 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 longer than this to 

 produce its full effect. When the current is applied directly to a 

 muscle, whose nervous elements are placed hors de combat by 

 the action of urari, or by degeneration of the nerve-fibres this 

 period of necessary duration seems to be still longer, and to be 

 especially increased by deficient nutrition. And this may be 

 offered as an explanation of the well-known clinical fact that in 

 various cases of paralysis, muscles which have by degeneration 

 of their nerves, lost their nervous supply, more readily respond 

 to the break and make of the constant current than to induction 

 shocks, the duration of the former as stimuli being much greater 

 than that of the latter. 



In the case of electric stimuli, the strength of the contrac- 

 tion, and by inference of the nervous impulse, depends on the 

 manner in which the current flows into the nerve. Though the 

 matter has been disputed, it appears that the current must 

 pass along some appreciable length of nerve-fibre in order to 

 produce an effect: a current which passes through a nerve in 

 an absolutely transverse direction being powerless to generate 

 impulses ; and further there is a connection between the efficiency 

 of the current and the angle at which it falls into the nerve. 



It would also appear, at all events up to certain limits, and as 

 a general rule, 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. Two inter- 

 pretations of this result are possible. Either the nerve at the part 

 farther away from the muscle is more irritable, i.e. that the 

 stimulus gives rise at the spot stimulated to a larger nervous 

 impulse ; or the impulse started at the farther electrodes gathers 

 strength, like an avalanche, in its progress to the muscle. The 

 latter view has been strongly urged by Pfluger, and is generally 

 known under the name of the 'avalanche theory'. Against it 

 may be urged that as far as we know, the progress of the current 

 of action along a nerve is marked by no such increase. It is 

 probable that the larger contraction produced by stimulation of 

 the portions of the nerve near the spinal cord is due to the 



