MUSCLE 249 



to be considered is whether muscle develops the tension by 

 which it can shorten and do work by a process of direct 

 oxidation, or in some indirect way. 



Pfluger long ago deprived a frog of all free oxygen in an 

 air-pump and then kept it in an oxygen-free atmosphere and 

 found that it moved and gave otf COo. He concluded that 

 the process of oxidation is not a direct one. 



Subsequent experiments have confirmed this view. 



It has been found that — 



1. The muscle of a frog may be made to go on contract- 

 ing for some time in nitrogen in the absence of oxygen, 

 but that fatigue is soon manifested and is not removed by 



Fig. 130. — (To be read from right to left). Contractions of muscle. A, in 

 oxygen ; B, in nitrogen with no oxygen. In both, note onset of 

 fatigue ; in A recovery after brief rest, in B no recovery. (Fletcher.) 



rest. Sarcolactic acid is liberated but no carbon dioxide is 

 produced (fig. 180, B). 



2. In the presence of oxygen, sarcolactic acid does not 

 accumulate. It is oxidised to CO2 and H2O. The muscle 

 recovers from fatigue after a short rest (fig. ISO, J.). 



3. Hence contraction is not due to oxidation, but to the 

 throwing out of sarcolactic acid, i.e. to an increase of H ions 

 which produces changes in surface tension with the develop- 

 ment of " tension." The tension varies with the length of 

 the fibre, which indicates that it is a surface phenomenon 

 (Appendix), When shortening occurs the tension is decreased, 

 and the potential energy becomes kinetic. Muscle, therefore, 

 in contracting does not act as a heat engine by liberating 

 energy by combustion, but as a compression engine which 

 liberates energj^ already stored. 



