The Respiratory Process in Muscle. 451 



oxidative removal of some product of activity which is a basis of fatigue and 

 of stiffen in o\ giving at the same time a yield of carbon dioxide as the obvious 

 sign of a completed combustion. 



o 



Hou/ts 3 



F/fOM fJTC/S/O/V 



12 



15 



21 



Fig. 3. — Changes in length of a pair of excised gastrocnemii, after fatigue. The ordinates are 

 measured directly from the record upon the drum. The levers magnified 65 times. Load 

 3 grm. Temp. 23° C. A. Exposed to oxygen. B. Exposed to air. 



Fig. 4. — Changes in length of a pair of excised gastrocnemii, after fatigue. Ordinates and 

 magnification as in fig. 3. Load, 3 grm. Temp. 16° C. A. Exposed to oxygen. 

 B. Exposed to air. From the ' Journal of Physiology,' vol. 28, p. 479 (1902). 



Now lactic acid itself is the most obvious determining cause of the signs 

 both of fatigue and of the stiffening of rigor mortis. Its artificial application 

 to muscle can mimic the signs of both(l). And, indeed, it had often been 

 suggested from observations in the whole animal that lactic acid was a 

 product of activity whose expulsion was effected by burning to carbon 

 dioxide and water. One sign of the presence of lactic acid in fatigued 

 muscle is to be found in a characteristic change of the osmotic properties of 

 the muscle, and it was further shown at Cambridge that immersion of a 

 fatigued muscle in oxygen restored the osmotic properties to those of resting 

 muscle (4). 



All these results pointed irresistibly to the conclusion that lactic acid 



