THE TISSUES 83 



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 powers of 

 conduction, gently heating it increases it. Various drugs 

 which diminish 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 



FIG. 39. N, a piece of a Nerve connected by non-polarisable electrodes 

 to the galvanometer, G. By an induction coil, it may be stimu- 

 lated at A. And when the nerve impulse reaches a, a deflection 

 of the galvanometer needle takes place. 



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 excita- 

 bility and conductivity certain differences are to be observed 

 in the effects of stimulating an exposed nerve with cur- 

 rents of various strengths and directions. These have 

 been formulated as Pfltiger's Law, but since they have 

 no bearing upon the stimulation of nerve in the living 

 body they need not here be considered. (Practical Physi- 

 ology, Chap. VII.) 



By using the electric changes as an index of nerve 

 action, it has been found that when a neuron is stimulated 



