GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 93 



a second on heating. Both the sympathetic and the vagus nerve-fibres in the 

 frog have their influence on the heart-beat decreased by cold and increased by 

 heat. 1 The Favorable influence of heat on the conduction power seems common 

 to all nerves, but only within certain limits. The motor fibres of the sciatic of 

 the frog lose their power to conduct at 41° to 44° C, but may recover the power 

 if quickly cooled; if the temperature lias readied 50° C. conductivity is per- 

 manently lost. 



Nerves of like function in different animals may lose the power of conduc- 

 tion at different temperatures. Thus the motor fibres of the sciatic nerve of 

 the dog cease to conduct at 6° C, those of the cat at 5° to 3° C, of the frog at 

 about 0° C. The inhibitory fibres of the vagus nerve of the dog show dimin- 

 ished activity at 3° C, and become wholly inactive at 0° C. ; the inhibitory 

 fibres of the vagus of the rabbit become inactive at 15° ( '. 



Different kinds of fibres in the same nerve-trunk may be differently affected 

 by temperature, and this difference may be sufficiently marked in some cases to 

 be used as a means of distinguishing them. 2 For example, the temperature 

 limits at which the vaso-constrictor fibres of the sciatic of the cat can conduct 

 are 2°-3° C. to 47° C, while the limits for the dilator fibres are both lower 

 and higher than for the constrictors. If cold be applied to the sciatic nerve, 

 the fibres supplying the extensor muscles seem to fail before those which in- 

 nervate the flexors. 



Further, it has been observed that if cold be applied locally to a nerve, the 

 part affected loses its power to conduct, and acts as a block to the passage of 

 the nerve-impulse generated in another part of the nerve. Application of 

 extreme cold to the ulnar nerve of man at the elbow results in a complete 

 loss of feeling in the parts which the nerve supplies. 3 On the other hand, 

 the strength of an impulse is increased by passage through a region which has 

 been warmed. These facts remind us of the effect of heat and cold <>n the 

 activity of other forms of protoplasm and would find a comparatively easy 

 explanation were we content to look upon conduction as the result of chemical 

 change in the axis-cylinder. The fact that conduction does not cause fatigue 

 is opposed to such an explanation, and so we take refuge in the idea that heat 

 is favorable and cold unfavorable to molecular activity in general. 



(d) Effect of Chemicals <ni<l Drugs. — The conductivity, like the irritability, 

 of nerve and muscle is greatly influenced by anything which alters the chemical 

 constitution of active substance. In general, influences which increase or 

 decrease the one have a similar effecf upon the other. There are important 

 exceptions to the rule, however. The direct application of alcohol, ether, 

 ('{c., to the nerve may destroy the conductivity without greatly lessening the 

 irritability, while carbon dioxide 1 or cocain will destroy the irritability very 

 much sooner than the conductivity. Such observations suggest that con- 



1 Stewart: Journal of Physiology, L891, vol. zii. No. '■<>. p. 22. 



'Howell, Budgett, and Leonard i Journal <;/' Physiology, vol. xvi. Nob. •" ami t, L894. 

 3 Weir Mitchell : Injuries of Nerves and their Consequences, Philadelphia, 1S72, p. 59. 

 * Griinhagen : PflUger'a Archiv, 1872, vi. 8. 180. 



