PART IV. 



THE CHEMICAL PROCESSES WHICH UNDER 

 LIE AND ACCOMPANY LIFE-PHENOMENA. 



CHAPTER XVII. 

 PROCESSES INFERRED FROM DIRECT OBSERVATION. 



THE INTERMEDIATE METABOLISM OF THE CARBOHYDRATES: 

 MUSCULAR CONTRACTION. 



We have seen, in considering the chemical regulation of the respira- 

 tory movements, that the energy-expenditure in muscular exertion is 

 derived from oxidations. This follows immediately from the low heat- 

 value of the hydrolyses which occur in the body, and which render 

 them insufficient sources of energy, and from the greatly increased 

 consumption of oxygen and output of carbon dioxide which accompanies 

 the performance of muscular work. 



It remains to consider, however, what class of foodstuffs undergoes 

 the oxidations which release muscular energy. That Carbohydrates 

 afford a proportion of the necessary heat-units is evident from the fact 

 that during the performance of muscular work the Glycogen which is 

 normally stored up in muscular tissues, is greatly diminished in quan- 

 tity, and even the further reserve which is stored up in the liver becomes 

 much reduced by the performance of severe and long-sustained muscu- 

 lar work. The potential energy contained in these reserves of glycogen 

 is very considerable. Thus the liver of a man, when fully stocked 

 with glycogen, contains about 150 grams of this polysaccharide, 

 while the muscles, at rest and after feeding, contain a like amount. 

 The total available reserve of carbohydrate material in the body is 

 therefore about 300 grams, having a heat- value of 4.1 calories per 

 gram or 1230 in all. If only one-fifth of this potential energy were 

 converted into -mechanical work, its remainder being dissipated as 

 heat, it would lift a weight of one hundred tons to a height of over three 

 feet. 1 



The exhaustion of glycogen by the performance of Muscular Work 



1 The equivalent of one calorie in mechanical work 'is 426.5 kilogram-meters. 



