ELECTRICAL RESPONSE TO SINGLE STIMULUS. 531 



(b) The duration of the response. — The electrical change (negativity) 

 under the surface contact reaches its maximum in the frog's sciatic 

 very rapidly, and then subsides ; the duration of the whole effect, at 15° C, 

 was found by Bernstein to be as little as '0007 sec. It is probable that 

 this time is too short ; Hermann estimated it as "005 sec. at 5° C, and 

 Head as - 02 sec. 1 These results, obtained with the repeating rheotome, 

 may be compared with those calculated from the records of the 

 capillary electrometer excursions. From these it appears that at 15° C. 

 the change under the surface contact reaches its maximum in - 001 sec. 

 after its first appearance, and subsides in -01 sec. to a small amount, 

 which, however, is followed by a second increase of low electromotive 

 force, lasting several hundredths of a second, this being the so-called after 

 effect. The duration is much prolonged by low temperature ( - 003 sec. 

 at 5° C), and diminished by rise up to 30° C. It is probably of different 

 value in different nerves. In non-medullated nerves, such as that of 

 the cephalopod, it has been estimated by Fuchs 2 as varying between 

 •01 sec. and "008 sec, a result which has been in the main confirmed 

 by Boruttau, though in his observations, carried out upon summer 

 animals, the duration was found to be a little shorter. 



(c) The electromotive force of the response. — This cannot be accurately 

 determined by the rheotome method; the only reliable estimation is 

 that afforded by analysing the curve of the capillary electrometer 

 variation, as photographed upon a rapidly moving sensitive plate. It 

 is roughly determined by comparing the total deflection obtained in 

 rheotome observations with that produced by a known electromotive 

 force, the currents of which are led through the repeating rheotome con- 

 tacts. In this way Hermann showed that the excitatory response may 

 in all probability involve an electromotive force, equal to or surpassing 

 that of the cross sectional difference. In some instances in the frog's 

 sciatic, it was nearly twice as great. The analysis of electrometer records 

 appears to show that the maximal electromotive force of the initial 

 change may in the frog's sciatic, at 5° C, amount to "03 volt, whilst that 

 of the more prolonged after effect is only T V of this, i.e. -003 volt. 3 



(d) The character of the response with two surface contacts. — The 

 rapidity with which the excitatory electrical change is propagated from 

 one part to another of a nerve, renders it difficult to determine the 

 character of the change with two surface contacts. In this case, first 

 the proximal and then the distal contact will be the seat of the same 

 electromotive alteration. The proximal electrode thus becomes negative 

 to the distal, until the change in the latter has developed sufficiently, 

 when it becomes positive to this. Since, however, the distal change in 

 nerve would occur only -001 sec. after the proximal, if 30 mm. of nerve 

 intervened between the two contacts, the experimental demonstration of 

 this diphasic effect is only rendered possible when the rate of propagation 

 is very much slowed. By taking a bundle of six sciatic nerves from the 

 frog and cooling these to 5° G, Hermann has been able to demonstrate 

 the diphasic effects with the rheotome. The proximal surface contact was 

 found to become first negative and then positive to the distal one. As 

 the negative effect under the proximal contact has not subsided when 

 that under the distal contact has commenced, the total result is still 



1 Hermann, Arch. f. d. ges. Physiol., Bonn, Bd. xxiv. S. 246 ; Head, ibid., 1887, 

 Bd. xi. S. 207. 



2 Loc. cit. s Gotch and Burch, Proc. Boy. Soc. London, 1898. 



