120 Inside the Living Cell 



stored. The excess of sodium ions which has reached the inside of the 

 fibre is pumped out again and some potassium ions also escape so 

 that the original condition of the charges on the membrane is restored. 

 The nervous impulse is thus a transitory disturbance in the electri- 

 cal condition of the nerve membrane which travels along the nerve. 

 A single disturbance of this kind takes as a rule a few thousandths of 

 a second to pass any point on the fibre. Its speed down the fibre is 



Na-" K + 



+• + -(- + + 



\ 



+ 4- + 



-I- ■+ -I- -I- -M 



DIRECTION OF IMPULSE 



-h-f-h-hH I-H--4- 



FiG. 23. How the electric impulse passes down the 

 nerve fibre 



comparatively low, depending on the diameter of the fibre (about 

 twenty metres a second in the giant squid nerves). This speed is of 

 course very much less than the normal speed of electric conduction 

 processes (which may approach the speed of Hght). It is determined 

 by the rates of the various processes by which the impulse is trans- 

 mitted. 



Much remains to be learnt about the nervous impulse, and about 

 how it is started and how it acts when it reaches its destination in the 

 muscle. It seems a rather unpromising mechanism for precise control 

 of muscle action. We could hardly have anticipated from what we 

 know of its mechanism that such a process should have been capable 

 of such an extraordinary degree of elaboration so that it eventually 

 became the basis of mentality. 



