THE ELECTRICAL RESPONSE. 



533 



duration of each phase is altered by various nerve conditions, and 

 particularly by low temperature, which prolongs the time relations, and 

 thus increases the effect. The diphasic character of the effect, when the 

 tissue under each contact is susceptible of undergoing the change, is 

 beautifully shown in capillary records. 1 With two surface contacts 

 on an uninjured nerve, the record is of the character shown in Fig. 270, 

 curve /, the rapid upward displacement being succeeded by one of 

 opposite direction. If the sciatic nerve is excised and left for over twelve 

 hours in 0'6 jSTaCl, the subsidence of the injury at the cross section places 

 the whole nerve in the same physiological state, and a diphasic effect 

 may now be obtained wherever the two contacts are placed on the tissue. 

 Propagation being slowed, the effect is more evident than in the fresh 

 nerve. An example is given in Fig. 270, curve 77/. On warming 

 locally the tissues at the distal contact, the record resembles that of 

 curve II, the distal tissue being now incapable of responding to its full 

 extent, whilst on making a cross section the records are those of IV. 2 



The influence of various conditions upon the negative varia- 

 tion. — The intensity of the stimulus. — The electromotive change in- 

 creases in magnitude with the increasing intensity of stimulus. The 

 character of the increase is well shown in the photographic re- 

 cords of the galvanometric deflections due to a brief series of suc- 

 cessive excitatory changes. 3 With one contact on the surface, and the 



I /Z /•* /* /tf Z 22 2-4 26 2-9 3 -32 3* 3-6 3 S « 



Fig. 272. — Abscissae indicate increasing intensity of stimulus. Ordinates 

 indicate magnitude of response.— After Waller. 



other on the cross section of the nerve, a short series of about ten 

 induced currents, occurring at a rate of fifty per second, is allowed to 

 excite the excised nerve every fifteen minutes. The intensity of this 

 stimulus is regularly increased, and the resultant deflections are 

 recorded on a slowly moving photographic plate. Within limits the 

 excitatory negative variations, as estimated by the extent of the 

 galvanometric deflections, are augmented in strict proportion to the 

 increase of the stimulus intensity. 



1 Gotch and Burch, Proc. Roy. Soc. London, 1898. 



2 See analysis on jjrevious page (footnote). 



3 Waller, Brain, London, 1895, vol. xviii. p. 210; Croonian Lecture, Phil. Trans., 

 London, 1896. 



