472 NERVE. 



generally. In many such tissues, involuntary muscle for instance, 

 there is a limit beyond which increased rapidity of current change 

 ceases to augment its excitatory value, and there are instances of such 

 results even in nerve. Thus the motor nerves of the toad are more 

 effectually excited by the make than by the break induced current, 1 and 

 even the motor nerve of the frog, when locally cooled, will respond to 

 slowly developed better than to very rapidly developed variations in 

 current intensity. 2 Finally, the increased effectiveness for cooled nerve 

 of condenser discharges, when with constant electromotive force the 

 condenser capacity is augmented, shows that the duration of such dis- 

 charge is a potent factor. 3 



All that can be said is that, as a rule, effectiveness of stimulation in 

 the case of electrical currents is increased in proportion as the rate of 

 variation in current flow is accelerated. 



3. The influence of the direction of current upon its exciting" 

 value. The above considerations relate to the intensity and time rela- 

 tionship of a current, a third important factor is its direction. It will be 

 seen in a subsequent section that, owing to a blend of excitability and 

 conductivity changes, the muscular response is affected to a very con- 

 siderable degree by such alterations in direction as are implied by the 

 terms " ascending " and " descending." There is, however, one feature 

 in regard to direction which must be referred to at this stage, since it 

 brings forward an important point in reference to the electrical 

 stimulation of nerves. This is the great decrease in exciting value, 

 when the current flow is so directed as to be in a plane which 

 cuts the nerve fibres at right angles to their length. The attainment 

 of such direction is by no means easy ; it can, however, be approxi- 

 mately achieved by stretching the immersed nerve in a bath of physio- 

 logical saline, as in Bernstein's trough experiment with the sartorius 

 muscle. 4 The stretched nerve is arranged so as to be strictly parallel 

 to the facing sides of the trough which form the contacts through which 

 the current is led into the liquid. Under these conditions the nerve 

 appears to be almost inexcitable. In this connection the electrical re- 

 sistance of nerve must be taken into account ; this resistance for any 

 given bulk of the tissue is five times as great in the transverse as it is in 

 the longitudinal direction ; but the increased resistance, although large, 

 is insufficient to account for the failure in exciting efficiency. The most 

 plausible explanation of the failure is the close proximity of the changes 

 caused at the poles, which in this case are on either side of each single 

 nerve fibre. 



It may be remarked, in conclusion, that although the various factors 

 which determine the exciting value of an external agent have been 

 ascertained in the case of electrical currents, in their fundamental 

 aspects they are true of other stimuli. Thus, mechanical pressure is 

 most effectual at the moment of onset ; and the exciting value of 

 such onset is increased in proportion as this is more rapidly effected. 

 By an ingenious arrangement, it can be shown that mechanical press- 



1 Schott, Arch.f. d. ges. Physiol., Bonn, 1891, Bd. xlviii. S. 354. 



2 v. Kries, Arch.f. Physiol., Leipzig, 1884; Gotch and Macdonald, Journ. Physiol., 

 Cambridge and London, 1896, vol. xx. p. 247. 



3 Hoorvveg, Arch.f. d. ges. Physiol., Bonn, 1892, Bd. Hi. S. 87 ; ibid., Bd. liii. S. 587 ; 

 Waller, Proc. Roy. Soc. London, 1899. 



4 Albrecht and Mayer, Arch. f. d. ges. Physiol., Bonn, 1880, Bd. xxi. ; see also Leicher, 

 ffntersuch. a. d. physiol. Inst. d. Univ. Halle, 1888, S. 1, where previous literature is given. 



