246 ORIGIN AND NATURE OF LIFE 



compared with inorganic nature, they vary 

 amongst themselves from a period of several 

 years, as in the growth to maturity of the 

 human individual, to a minute fraction of a 

 second, as in the time of passage of a nervous 

 impulse which occupies not more than the 

 one ten-thousandth part of a second. 



Some of the cyclic periods of living cells are 

 so short that their truly cyclical character, 

 and the nature of the alternations, have been 

 missed or mis-stated by physiologists in the 

 past. Conspicuous examples of this are to be 

 found in regard to nervous activity and the 

 rhythmic contractions of cardiac muscle. 

 In nearly every current textbook of physiology 

 the statement will be found, followed by 

 elaborate proofs, that nerve and heart- 

 muscle cannot be fatigued, or are indefatigable, 

 as it is called. The true statement which 

 ought to replace this is, that the cycle of 

 fatigue and recuperation is so rapid that 

 fatigue and recovery occur between each 

 heart-beat, or each nervous impulse. 



It has been shown by Schafer that the 

 nervous impulse sent out to the voluntary 

 muscles is not continuous, but consists of a 

 series of impulses at the rate of ten to twelve 

 per second. Now the period of passage of each 

 of these over any given portion of nerve is 



