NERVE 



71 



depressing protoplasmic activity diminishing it. Under 

 normal conditions in the fresh nerve of the froff, the velocity 

 is about 33 metres per second. In man it is about 100 to 

 150 metres per second, and in the octopus only about 2 

 metres, 



(b) Factors modifying Conduction. — Conduction is modified 

 by temperature. Cooling a nerve lowers its power of con- 

 duction ; gently heating it increases it. Various drugs 

 which diminish protoplasmic activity — e.g. chloroform, 

 ether, carbon dioxide, etc. — diminish conduction. But 

 while the excitability of the part of the nerve under the 



7^ 



Fig. 31. — N, a piece of a Nerve connected by non-polarisable electrodes to the 

 galvanometer, G. By an induction coil it may be stimulated at A. 

 When the nerve impulse reaches a, a deflection of the galvanometer 

 needle takes place. 



influence of these agents undergoes a steady decrement, i.e. 

 the contractions of the muscles supplied steadily decrease, 

 the conduction, of the impulse is either completely blocked, 

 or, if it is transmitted, it regains its full strength in the part 

 of the nerve peripheral to the block. Very important 

 conclusions as to the nature of the nerve impulse have been 

 based upon this fact. The electric current, too, acts differently 

 on conduction and on excitability (p. 63). 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 and to 

 block the passage of impulses (fig. 32). 



