484 NER VE. 



results, although of great importance as indicating specific change in 

 consequence of the interposition of the ganglion, cannot be taken as 

 clear evidence that this change is brought about by decreased rapidity of 

 transmission through the nerve cell. The delay, which is so marked a 

 feature in the central nervous system, is obviously one involving nerve- 

 endings and interposed breaches of continuity as well as nerve cells. 1 



Influence of temperature upon nerve excitability and conduc- 

 tivity. It has long been known that within certain limits the tempera- 

 ture of a nerve may be altered without causing any permanent impair- 

 ment of its functions. The lower limit does not appear to be reached, 

 even by a fall to C. ; the higher limit differs slightly in different 

 animals, but is reached in the case of the frog by a temperature of 

 40 C., and in most mammals by one of 50 C. 



In a remarkable work published in 1824, W. F. Edwards 2 dealt with 

 the influence of many physical agents upon life, and described a number of 

 interesting experiments as to the effects upon frogs, fish, etc., of prolonged 

 immersion in the Seine when at different temperatures. The results 

 showed that the low temperature of winter was far more favourable to 

 life than the higher temperature of summer. The differences between 

 winter and summer frogs are not improbably associated with the effect of 

 rise of temperature, which augments the general tissue metabolism. It is 

 well known that the nerves of summer frogs are less tolerant of exposure, 

 but in addition the nerve functional activity as a whole is lowered. 

 Thus the rate of propagation in the freshly prepared sciatic nerves of E. 

 esculenta may be only 14'5 metres per sec. in summer, at 15 C., i.e., half 

 the rate obtained in winter specimens. 3 These differences are caused 

 by temperature affecting the whole organism. As regards the direct 

 action of changes of temperature upon nerve trunks, permanent impair- 

 ment is obviously caused by temperature alterations which exceed certain 

 limits (45 C.), and temporary diminution or even abolition of function 

 is produced by smaller variations within the limits, C. to 40 C. One 

 function, that of conductivity, appears to be very generally lowered by 

 fall and augmented by rise of temperature up to 35 C. 



It is otherwise with the property of excitability. It was long held 

 that this was similarly augmented by a rise of temperature ; but in 

 opposition to this view was the well-known fact that the nerves of 

 cooled frogs were far more excitable than those of frogs kept at the 

 normal temperature. 



In order to ascertain changes of excitability, it is necessary to eliminate as 

 far as possible changes in the conductivity which may otherwise lead to 

 erroneous conclusions as to excitability ; this can be approximately affected by 

 localising the change of temperature to a small portion of the nerve, and 

 ascertaining the extent to which the minimal exciting intensity is changed in 

 this region only. A further fruitful source of error is present when electrical 

 currents are used as the exciting stimulus; this is due to the very marked 

 alteration in the electrical resistance of the moist conductor which is produced 

 by a change of temperature. Warming a moist electrolyte diminishes, cool- 

 ing increases the resistance. A simple proof of this is afforded by the follow- 

 ing experiment. An exciting induced current is allowed to traverse in 

 succession similar portions of nerve in each of two muscle-nerve preparations 



1 See on this subject Schafer's article " Nerve Cell " in this volume, p. 592. 



2 " De 1'influence des agents physiques sur la vie," Paris, 18'24. 



3 Popielski, CentralU.f. PhysioL, Leipzig u. Wien, 1896, No. 9, Bd. x. 



