THE BEGINNINGS OF LIFE. 29 



we learn, from the experiments of Matteucci *, that the 

 activity of the processes of combustion within the 

 muscle increase during its contraction-. 



Many separate sets of investigations do indeed tend 

 to show that an excess of heat is developed during mus- 

 cular activity, though, on the other hand, there is evi- 

 dence to prove, from the highly interesting experiments 

 of M. Beclard 3 , whose results have been confirmed and 

 extended by M. Heidenhain, the increase in the acti- 

 vity of the chemical changes which undoubtedly exists 

 during muscular exercise, is much greater than can be 

 accounted for by the actual increase of sensible heat in 

 the body. After alluding to these various investigations, 

 M. Gavarret generalizes their results as follows 4 : 



1 ' Letture sul 1' elettro-physiologia,' Milano, 1867, p. 36. 



2 During a state of rest, or moderate exercise, combustion and eli- 

 mination of its products are duly regulated in the muscle. So long as 

 this balance is maintained the muscle preserves its physiological pro- 

 perties, and the chemical reaction of its juice remains neutral or alkaline. 

 But when excessive activity of the muscle is maintained, then the pro- 

 cesses of elimination can no longer keep pace with those of combustion : 

 lactic acid accumulates within the muscle, and the reaction of its juice 

 becomes decidedly acid. The contractility is gradually enfeebled by 

 the increasing accumulation of these effete products within the muscle, 

 and the feeling of fatigue is induced. There is good reason for believing 

 that this feeling of fatigue is rather dependent upon the accumulation of 

 these products of combustion within the muscle than upon an actual 

 molecular wasting of the muscle-substance. There is, however, a more 

 general feeling of fatigue which is dependent rather upon state of nerv- 

 ous system than state of muscle. 



5 ' De la Contraction musculaire dans ses rapports avec la temperature 

 animale,' Paris, 1861. 

 4 Loc. cit. p. 135. 



