in ELECTRICAL EXCITATION OF MUSCLE 297 



late period after the closure of fairly strong currents. The cause 

 of this deviation is presumably to be sought in the different 

 mode of exit of the current in either case. For while in 

 the first case this occurs at the ends of the fibres, in the 

 other the " physiological kathode " lies at the centre of the 

 muscle, where current density must be less on account of tke- 

 larger section ; while, on the other hand, the oblique course of the 

 isolated lines of current directed towards a small zone of the 

 muscle surface causes the innermost fibres to be less strongly 

 excited than those at the periphery, since the current leaves the 

 former with a less density than the latter. Accordingly we 

 always find that when the exit of the current occurs, as described 

 above, at the centre of the muscle, much stronger currents 

 will, as a rule, be required to produce a make twitch than 

 under the opposite conditions. So long as the total section of 

 the muscle at the exit of the current is not fatigued by pro- 

 longed passage of current, each wave of excitation arriving at the 

 kathodic section will endeavour to pass beyond it, since it can- 

 so to speak glide under the most strongly excited peripheral 

 zone, and is first completely inhibited when the muscle is, if we 

 may so express it, functionally separated, i.e. divided by a small 

 unexcitable zone into two excitable sections, from the continuance 

 of the state of local excitation. This separation makes it con- 

 ceivable that, as a rule, tolerably protracted polarisation by 

 fairly strong currents is required in order to depress conductivity 

 at any point in the continuity of the muscle to such a degree 

 that an approaching wave of excitation will be hindered in its 

 progress. If the upper half of the muscle, as we have been 

 assuming, is polarised in a descending direction, and the current 

 then suddenly reversed, the vigorous make excitation discharged 

 at the previously anodic end of the muscle, fails to pass beyond 

 the section which has been rendered incapable of conducting by 

 the sustained excitation at the kathode, and at closure only the 

 directly excited and formerly polarised half of the muscle will 

 twitch, while the other half beyond the fixed part remains 

 passive. 



Conductivity is recovered, under certain conditions, when the 

 current is broken ; but if it has been too strong, or if polarisation 

 is prolonged unduly, the kathodic section may remain per- 

 manently incapable of conducting. 



