THE ELECTRICAL RESPONSE. 411 



of potential between different parts are, as has been said, scarcely 

 measurable, for which reason these are best adapted for the study of the 

 natural electromotive phenomena of muscle, and their modifications. 



"Current" of injury. — When the tendinous expansion of the tendo 

 Achillis is injured by heat or otherwise, a very much larger difference 

 of potential comes into existence between it and the proximal 

 contact at /. Heat injury may be effected either by dipping the end 

 of the muscle into salt solution at any temperature above 55° C, or by 

 applying a camel-hair pencil, which has been dipped in hot salt solution, 

 to the surface of the muscle at the tendon end. Whichever plan is 

 used, the ends of the fibres are made to shorten and swell. In the 

 gastrocnemius the difference may amount to more than T ± v of a volt. 

 When the sartorius is similarly treated, the difference seldom exceeds 

 T 2_ a muscle injured at one end in the way above described 

 is referred to by physiological writers as having a " thermal section." 

 As this expression is founded on du Bois-Eeymond's " doctrine of the 

 muscle cylinder," it will now be convenient to discuss this subject. 



It may be assumed that the essential physiological properties of a 

 muscle are possessed by each of its constituent fibres. If it were 

 possible to divide a muscle fibre into parts by cross-sections without 

 injuring it, each part might be expected to exhibit the endowments of 

 the whole muscle. Proceeding from this consideration, the theory in 

 question further supposes that each such cylindrical part of a fibre is 

 built up of structural elements which have the same properties ; these 

 in their turn consist of electromotive molecules, to the endowments 

 of which those of the whole organ are attributable. It is obvious that 

 cylinders cut from single fibres could not be subjected to direct 

 investigation. The experimental basis for the theory of electromotive 

 molecules had therefore to be sought for in the data to be obtained by 

 the examination of a truncated, cylindrical, and parallel-fibred muscle. 

 A cylinder so prepared possesses a natural and two cut surfaces. These 

 are called by du Bois-Eeymond respectively " longitudinal " and 

 " transverse," designations which are open to the objection that they 

 conceal the all-important fact that each transverse sectional surface is 

 that of a wound. At the centres of the opposite cut surfaces there are 

 found to be spots which are mutually equipotential, but relatively 

 negative to all other parts ; and on the natural surface there is an 

 equatorial zone which divides the cylinder into two, and is positive to 

 all other parts. If the other parts of the surface are explored galvano- 

 metrically, by placing one electrode on the equator, the other first on 

 the centre of an end surface, and then successively on spots nearer and 

 nearer to the equator, it is found that the difference of potential in each 

 successive observation is less than in the preceding one. The exact 

 relation between these differences has been minutely investigated both 

 theoretically and empirically by du Bois-Eeymond, but has little 

 physiological interest. The essential point is that the distribution of 

 tension on either side of the equator is symmetrical. If the cylinder is 

 cut from a muscle of irregular construction like the gastrocnemius, no 

 such symmetry is observed ; for the central spot of the cut surface 

 which faces the tendon is negative to the corresponding spot in the 

 surface which looks in the opposite direction. The reason of this must 

 now be considered. 



The muscle rhombus. — Long ago, du Bois-Eeymond discovered that 



