THE NATURE OF FATIGUE 



185 



oxygen. On the other hand, recent work suggests that one of the 

 means of increasing working power or temporarily, at least, delaying 

 its loss, is by artificially supplying oxygen to the body. It has 

 been known for some time that with the usual conditions under 

 which we live, the main source of the energy of muscles and prob- 

 ably of other organs is carbohydrate material, glycogen or its near 



Fig. 3. Series of 550 contractions of a frog*s 

 gastrocnemius muscle, excised and stimulated at 

 intervals of two seconds. Every contraction is 

 recorded, except at the places indicated by the 

 black bands, at each of which the records of 

 fourteen contractions are omitted. The record 

 of the first contraction is at the bottom of the 

 figure : that of the last one at the top. Fatigue 

 is shown in the progressive decrease in height 

 and the increase in length of the curves. 



Fig. 4. Series of con- 

 tractions of a rat's gas- 

 trocnemius muscle, excised 

 and stimulated at intervals 

 of two and one half seconds. 

 Fatigue is shown in the pro- 

 gressive decrease in height 

 of the curves. 



relative, sugar. In the burning of carbohydrate in the tissues its 

 potential energy becomes the actual energy of heat and muscle work. 

 This fact would suggest the loss of carbohydrate as one of the factors in 

 the oncoming of fatigue, especially in its later stages. Exact laboratory 

 investigation, moreover, shows that if most of the carbohydrate be 

 removed from an animal's body, he presents the symptoms of pro- 



