THE TISSUES 81 



The rate of conduction varies considerably; everything 

 stimulating protoplasmic activity accelerating, and every- 

 thing depressing protoplasmic activity diminishing it. Under 

 normal conditions in the fresh nerve of the frog, the nerve 

 change travels about 33 metres per second. 



Factors modifying Conduction. Conduction is modified 

 by the temperature. Cooling a nerve lowers its power of 

 conduction, gently heating it increases it. Various druys which 

 dimmish protoplasmic activity e.g. chloroform diminish 

 conduction. The electric current acts differently on conduction 

 and on excitability. While a weak current has little or no 

 effect, a strong current markedly decreases conductivity round 

 the positive pole, and to a less extent decreases it at the 

 negative pole, so that the general effect of a strong current is 

 to decrease conductivity. 



From this influence of the electric current upon excitability 

 and conductivity certain differences are to be observed in the 

 effects of stimulating an exposed nerve with currents of various 

 strengths and different directions downwards to the muscle or 

 upwards from the muscle. These have been formulated as 

 Ptiiiger's Law ; but since they have no bearing upon the stimula- 

 tion of unexposed nerves in the living body they need not 

 here be considered. 



By using the electric changes as an index of nerve action, 

 it has been found that when a neuron is stimulated in the 

 middle, the change travels in both directions, although its 

 result is made manifest only by the action of the structure at 

 one end ou which it normally acts. This two-way conduction 

 may also be demonstrated by the experiment of paradoxical 

 contraction, in which, by stimulating the branch of the sciatic 

 nerve of the frog going to the muscles of the thigh, the nerve 

 fibres to the gastrocneinius lying alongside of them are also 

 stimulated, and cause that muscle to contract. That this does 

 not occur when impulses from the central nervous system pass 

 along the nerve is because the strength of these impulses, as 

 indicated by the electrical change, are very much weaker than 

 those caused by direct stimulation. (Practical Physiology.) 



Classification of Neurons by the direction of Conduction. 

 Since a nerve is normally stimulated from one or other end, 

 and hence conducts in one direction, and since the passage of 



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