910 CENTRAL NERVOUS SYSTEM 



contraction several times in rapid succession each twitch is somewhat 

 higher than its predecessor. This phenomenon is known as treppe and 

 is explained by the fact that chemical changes arising from one con- 

 traction make the muscle better able to contract the next time. If 

 the successive stimuli are continued for some time the height of the 

 contractions soon reaches a maximum and then begins to fall off as the 

 muscle becomes fatigued. Fatigue is due, in part at least, to the fact 

 that after prolonged activity each fiber in the muscle is able to develop 

 less tension when it contracts. 



The Chemistry of Tetanic Contraction. In order to understand the 

 nature of fatigue, the chemical changes which occur during the activity 

 of muscle must be considered. When a muscle is excited, energy is lib- 

 erated which sets up a state of tension. The tension may result in doing 

 external work, as in lifting a weight, or it may dissipate its energy 

 as heat if the muscle is not allowed to shorten. In either case a supply 

 of potential energy stored in the muscle has been drawn upon. It has 

 been shown by Roaf that the hydrogen-ion concentration of muscle 

 substance becomes increased at the moment of contraction, and chemical 

 analysis shows that lactic acid appears in muscle as the result of pro- 

 longed excitation. It is consequently believed that the liberation of 

 lactic acid in the muscle substance is connected with setting free the 

 energy for contraction. 



In the process of recovery which follows the activity of muscle when 

 it is allowed to rest, it is found that the lactic acid disappears provided 

 a supply of oxygen is available. Since at the same time carbon dioxide 

 is set free from the muscle, one might suspect that the disappearance 

 of the lactic acid is due to its being oxidized to carbon dioxide and water. 

 This, however, cannot be the case, for it is known that after the repeated 

 fatigue and recovery of an isolated muscle the total quantity of lactic 

 acid which may be extracted from it is undiminished. In other words 

 lactic acid does not disappear from the muscle during rest, but is re- 

 stored to the condition in which it occurred before contraction took 

 place. Further evidence that lactic acid is not oxidized is afforded by the 

 fact that the disappearance of 1 gram of lactic acid from fatigued mus- 

 cle is accompanied by the production of 450 calories of heat, whereas 

 the oxidation of 1 gram of lactic acid would set free 3700 calories. 

 Apparently the oxidation of some other substance is necessary in order 

 to restore lactic acid to the percursor condition, and to replace the po- 

 tential energy lost in the contractile process, and in the course of the oxi- 

 dation of this substance carbon dioxide is liberated and heat is given 

 off. The nature of the substance oxidized is not definitely known, but 

 it is presumed from the high respiratory quotient of muscular work that 

 it is chiefly carbohydrate. (Bayliss, 24 Fletcher and Hopkins. 25 ) 



