414 ELECTRO-PHYSIOLOGY CHAP. 



the wave of electrical variation set up in the latter, directly, or 

 by excitation from the nerve, there can only be two alternatives 

 as regards positive change of sign in secondary action ; either the 

 conditions for neutralisation of the existing P.D. by the super- 

 posed nerve become more favourable, or the magnitude, form, and 

 velocity of the wave alter in a direction more favourable to 

 excitation of the former. That the first of these possibilities 

 does not come into the present consideration may be concluded 

 from the fact that the experiment comes off as well with regu- 

 larly constructed muscles as with the usual nerve-muscle pre- 

 paration. Moreover it is possible, by altering the position of the 

 secondary nerve on the surface of the primary muscle, to render 

 the external conditions of the discharge of secondary twitches 

 during extension as unfavourable as possible, either by only 

 allowing it to come into contact with a very short strip of the 

 extended muscle, or by placing it across, or round, the muscle, 

 which, however, in the majority of cases has no effect on the 

 result. Only the other possibility, therefore, need be considered, 

 and the magnitude, form, and velocity of the wave of electrical 

 variation have therefore been investigated comparatively in 

 stretched and unstretched muscle. Heidenhain, e.g., was the 

 first to show that the proportion of vital energy developed as 

 heat in contraction depends essentially upon muscular ten- 

 sion, since, up to a certain point, the evolution of heat increases 

 with the loading. This suggests the idea that the other factor in 

 the sum of energy, which appears as electricity in the action 

 current concomitant with excitation, may be influenced in the 

 same degree by tension. The experiments of Lamansky 

 (Pfliigers Arch. iii. p. 193), who observed an increase of the 

 negative variation in the gastrocnemius with increase of loading, 

 would be in favour of this assumption if the exclusive use of the 

 irregularly constructed gastrocnemius did not suggest objections 

 already pointed out by du Bois-Eeymond. 



If this theory is correct it might be expected that other data, 

 which are experimentally found to augment the capacity of work in 

 the muscle, would also increase its secondary activity. The favour- 

 able influence exerted under some conditions by repeated excitation, 

 at uniform intensity, upon the mechanical capacity for work in 

 cardiac and skeletal muscle, where a " staircase " is formed at the 

 beginning of a series of contractions, has already been referred to. 



