94 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



if quickly cooled ; if the temperature has reached 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 C. The inhibitory fibres of the vagus nerve of the dog show dimin- 

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

 fibres of the vagus of the rabbit become inactive at 15 C. 



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. 1 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 ; 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 on 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 and 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 it may be said that influences 

 which increase or decrease the one have a similar effect upon the other, but 

 there are important exceptions to the rule. Thus the direct application of 

 alcohol, ether, etc., may destroy the conductivity without greatly lessening the 

 irritability, while carbon dioxide may destroy the irritability, though leaving 

 the conductivity unimpaired. 



(e) Effect of a Constant Battery Current. A constant electric current, if al- 

 lowed to flow through a nerve or muscle, not only alters the irritability but also 

 the conductivity. The change in the conductivity affects both the strength 

 and rate of the conduction process. Von Bezold 2 found that weak and 

 medium currents have little effect on the conductivity, but that strong currents 

 completely destroy the power of the nerve to transmit the nerve-impulse. As 

 the strength of the current is increased the first effect is observed at the anode, 

 and shows itself in a slowing of the passage of the exciting impulse. This 

 action is the greater the more of the nerve exposed to the current, the stronger 



1 Howell, Budgett, and Leonard : Journal of Physiology, vol. xvi., Nos. 3 and 4, 1894. 



2 Untermchungen iiber die dektrische Erregung den Nerven und Muskeln, Leipzig, 1861. 



