The Respiratory Process in Muscle. 



459 



formed, and to the new surface tension or elastic tension imposed on the 

 fibril by the acid ions. If the lactic acid be allowed by repetitions of the 

 process to accumulate, fatigue phenomena are produced, and on this view 

 fatigue is the expression, not of an exhaustion of energy supply, but only 

 of a clogging of the machine. With a normal oxygen supply, however, the 

 lactic acid is promptly removed after each contraction, and each successive 

 stimulus, with its associated breakdown, is followed by a normal contraction. 

 The removal of the lactic acid might be thought of as a direct oxidation — in 

 the presence of oxidases — and it might be supposed to be burnt, so to 

 speak, to waste, when the energy liberated by its combustion would supply 

 nothing to the mechanical energy of contraction. 



But this simplest view we are driven at once to forgo, and perhaps not 

 unwillingly, since it would be unwelcome to believe that a body of such 

 high energy value as lactic acid can be only a waste product yielding 

 nothing in its discharge except the indirect benefits of heat production 

 unconnected with the muscle machinery. Direct observation assures us, 

 however, that of the energy of combustion of the lactic acid part at least 

 remains in the muscle, for the work at Cambridge, both of Hill and more 

 lately of Parnas (20), though they differ in other vital respects, concurs in this, 

 that the energy leaving the muscle as heat in the oxidative removal of lactic 

 acid is less than that calculated for the combustion in the oxygen used up. 

 Some of the energy of that combustion is restored in some shape and to 

 some degree at least to the muscle system. 



On this account the general trend of opinion based upon the recent 

 work we have described has returned to an old conception tentatively 

 offered by Hermann, who suggested that in the building up of the inogen 

 molecule there might enter again, with oxygen, part of the lactic acid from 

 the previous contraction, there to be arranged in the explosive complex from 

 which at the next contraction lactic acid and carbon dioxide should emerge. 



There are grave theoretical difficulties associated with the conception of an 

 " inogen ,: capable of rapid breakdown and rebuilding, if it is to be formed, 

 as we have shown it must be, without the inclusion of oxygen. Yet the idea 

 that the lactic acid, instead of being burnt away, may be actually restored 

 into its former position in the molecule of its precursor, by the energy of a 

 combustion of some other material in the oxidative recovery, has gained 

 ground lately, and partly on account of an observation of our own. In our 

 studies of the lactic acid of frog's muscle we made out the following facts. 

 Supp'ose for a given set of similar muscles the ' maximum " lactic acid pro- 

 duction, as induced by heat rigor, be determined. Suppose, further, that 

 another comparable set of muscles be stimulated to fatigue, then allowed to 



