CYCLIC ACTIVITIES OF LIFE 247 



only one ten-thousandth of a second, and on 

 adding up the number of impulses in a second, 

 it is obvious that at most the tissue is only 

 active for twelve ten-thousandths of a second 

 in each second, or for about 0.12 per cent, of 

 the total time. There is thus sufficient inter- 

 val for recuperation between each period of 

 activity. Even, when an electric current of 

 1000 shocks per second is used as a stimulus, 

 there is ample time between each stimulation 

 for recuperation. At a much higher rate of 

 stimulation than this the nerve tissue solves 

 the problem by ceasing to respond. A strong 

 enough current to light up powerful electric 

 lamps can be sent through the human body 

 if the electric oscillations exceed 30,000 to 

 40,000 per second, without causing any 

 effect upon the muscles or nerves. 



The rate is much slower for the contractions 

 of heart muscle, although still very rapid 

 amongst biological phases. As a result, 

 the sequence of events becomes more obvious. 

 Just after each contraction, so far from not 

 being fatigued, the heart muscle is so com- 

 pletely fatigued that it has what is called a 

 refractory period. It relaxes and cannot be 

 caused to contract even by a strong external 

 stimulus. It rapidly, however, loses the 

 products of its fatigue, its colloids re-arrange 



