WORK THE MUSCLES 95 



when in its natural situation, and is supplied with blood. It is 

 clear that its store of energy is made up again. This is found to 

 be by the oxidation of some material brought to it by the blood. 

 It is also found, experimentally, that the supply of energy obtained 

 in this way follows the act of contraction itself. While there is no 

 consumption of oxygen in the act of contraction itself, nor any 

 carbon dioxide given off, both of these take place in the period 

 following the contraction. That oxygen is not used in the act of 

 contraction itself is readily proved by the fact that a muscle 

 can execute a long series of contractions in an atmosphere of 

 nitrogen. 



Some food material is burned, therefore, to supply the potential 

 energy which a muscle has lost in doing work and to prepare it 

 for more work. It appears that glucose is used preferably when 

 available, but that fat or the non-nitrogenous part of protein can 

 be used. The same amount of food energy is used for a given 

 supply of muscle energy in each case. We may note here that 

 the fact that either carbohydrate or fat can be utilised, places a 

 difficulty in interpreting the muscle system as being a chemical 

 one, in the strict sense. 



The method by which it is discovered whether carbohydrate or 

 jat is being used in muscular work in any particular case is of interest. 

 Since the former may be looked upon as having all its hydrogen 

 already completely oxidised, all the oxygen used is taken up in 

 oxidising the carbon to carbon dioxide, and the volume of carbon 

 dioxide produced is equal to that of the oxygen taken in. If, then, 

 we determine, during a period of muscular work, how much oxygen 

 is taken in and how much carbon dioxide is given off, and compare 

 the ratio with that before the work, we shall find this ratio increased 

 if a larger proportion of carbohydrate is being burned. If nothing 

 else but carbohydrate is burned the ratio, obviously, is unity. This 

 ratio is known as the " respiratory quotient." On the other hand, 

 fat requires oxygen to burn its hydrogen as well as its carbon, so 

 that the carbon dioxide given off in proportion to the oxygen used 

 is much less than unity, and the respiratory quotient would be low 

 when fat is being burned in the organism. 



The only chemical change definitely known to occur in the 

 contractile process itself is the production of lactic acid (E., p. 205). 

 It is clear that this must arise from some source in the muscle, but 

 what this is we do not exactly know. In the second stage, which 

 succeeds the contractile one, and that in which the muscle recovers 

 its energy by the aid of a combustion process, this lactic acid 

 disappears, and there Ts evidence that it js_burned in order to give 

 the energy. Glucose must be taken up in some way in order to 

 afford the lactic acid produced in a subsequent contraction, but it 



