INFLUENCE OF TEMPERATURE UPON NERVE. 485 



(A and B\ placed in the same circuit. A small glass tube through which water 

 at different temperatures can flow is placed under the traversed region of A, 

 and the intensity of the current is adjusted so that with both A and B at the 

 normal temperature of 15 C., it is just adequate to excite and evoke in B a 

 minimal contraction. On now warming the nerve of A a few degrees the 

 contraction of B becomes maximal, on cooling it the contraction fails. Since 

 the nerve of B is unaltered during the experiment, the apparent physiological 

 alteration in its excitability is in reality due to physical increase and decrease 

 in the current intensity through the change in the electrical resistance of A. 1 



The experiments of Hirschberg, Efron, Howells, 2 etc., were made 

 by stimulating with induction currents without providing against 

 sources of error ; hence the conclusion arrived at by them and others 

 that nerve excitability is augmented by rise of temperature up to 35 C. 

 cannot be legitimately deduced from their observations. The introduc- 

 tion of a sufficiently large external resistance, as in Fig. 254, renders 

 any change of resistance too small to cause any marked alteration in 

 current intensity. By using these and other precautions, it has been 

 ascertained that nerve excitability is raised by local cooling for the 

 following forms of external stimulus : galvanic currents, exciting either 

 at closure or at opening ; condenser discharges of large capacity ; 

 sinusoidal magneto - induction currents ; mechanical and chemical 

 stimuli. On the other 

 hand, the excitability 

 is definitely lowered by 

 local cooling from 35 

 C. to 2 C., for the 

 break induced currents 

 (see Fig. 255) or dis- 

 charges from conden- 

 Sers Of small capacity.' FlG 254 ._ r= Temperature tube under nerve, ^large 

 Galvanic and Sinusoidal resistance placed in exciting circuit, the nerve con- 



Currents of very short tacts of which are at a and c. The stimulating 



duration, '002 sec. or r ent is descendin s> and thus its closure excites 



under, appear to excite 



rather better when the tissue is locally warmed from 2 C. to 

 35 C., but this augmentation is not so marked as in the case of 

 the break induced current. It thus appears that whilst change of 

 temperature alters the susceptibility of the nerve to excitation, the 

 character of the alteration is one which is not the same for all methods 

 of excitation. The results are the same for the motor nerves of the 

 frog, rabbit, and cat, and for the afferent nerves. In the case of 

 mechanical stimulation the augmentation of the excitability through 

 cold is in harmony with the observation of Steinach, that in cooled 

 frogs a single mechanical excitation can evoke a prolonged electrical 

 response in the nerve. 4 



These facts can only be interpreted as indicating what various 

 observations suggest, that break induced currents excite by the produc- 

 tion of changes which are not of the same kind as those caused by 



1 Gotch and Macdonald, Journ. PhysioL. Cambridge arid London, 1896, vol. xx. p. 247. 



2 Arch. f. d. yes. PhysioL, Bonn, 1886, Bd. xxxix. S. 75; ibid., 1885, Bd. xxxvi. S. 

 488 ; Journ. PhysioL, Cambridge and London, 1894, vol. xvi. p. 298. 



3 Gotch and Macdonald, loc. cit.; Waller, " Proc. Physiol. Soc.," Journ. PhysioL, Cam- 

 bridge and London, Feb. 1899, vol. xxiv. 



4 Steinach, Arch. f. d. ges. PhysioL, Bonn, 1894, Bd. Iviii. S. 490. 



