THE REFRACTORY PERIOD AND FATIGUE 163 



these are self-evident facts which are in accordance with the con- 

 ception we have here developed of the course of the process of 

 excitation and its physical nature. But another important point 

 is evolved from the observations we have made of the nature of 

 the process of self-regulation. The process of self-regulation is 

 founded on the same principle as that which governs the taking 

 place of all chemical equilibrium, for metabolic equilibrium is 

 merely a special kind of a chemical equilibrium. The develop- 

 ment of a chemical equilibrium between reacting substances and 

 reaction products has, as known, a characteristic course in regard 

 to its duration. If the rapidity with which the equilibrium is 

 reached is expressed by a curve in which, the abscissa represents 

 the time, while the ordinates signify the number of contacts of 

 the interacting molecules, the rapidity of reaction is altered with 

 the approach to the equilibrium in the form of a logarithmic 

 curve; that is, the approach to the state of equilibrium, which is 

 represented by ordinate value zero, takes place at first very rap- 

 idly, then with more and more decreasing speed, for with the 

 decrease of the number of reacting molecules and the increase 

 of the amount of products of reaction, the contact of the inter- 

 acting molecules and with this the opportunity for the reaction 

 occurs less and less frequently. Although the self-regulation 

 of metabolic equilibrium is by no means such a simple pro- 

 cess as, for instance, that of the well-known example of the 

 forming of ethylester from acetic acid and aethyl alcohol, we have 

 still in every case to deal with the taking place of a chemical 

 mass equilibrium. Hence the progress to the metabolic equilib- 

 rium must likewise correspond with a logarithmic curve, i.e., resti- 

 tution after a disturbance of the equilibrium must take place at 

 first rapidly, then at a constantly decreasing rate. For reasons 

 readily to be understood the special form of this restitution curve 

 has so far not been accurately ascertained for any kind of living 

 substance. Even in those cases where the restitution occurs 

 very slowly we meet with the difificulty that, when the tests are 

 applied which are necessary to determine the restitution at dif- 

 ferent intervals, with each testing stimulus irritability is each 

 time reduced. Hence the construction of the restitution curve 



