CALORIMETRY 147 



regulation of the chemical changes occurring in the cell. 

 It controls the dynamic changes, but the energy which it 

 itself possesses is not thereby utilised. Any changes which 

 it undergoes consist in an adaptation to the part which 

 it has to play. Now certain of the substances which form 

 essential parts of the cell-structure cannot be synthesised 

 in the body. Some are minerals, others can only be manu- 

 factured by plants. It follows that a quantitative con- 

 sideration of the food, as a source of energy, is only valid 

 when the adequacy of the food for the maintenance of the 

 machine is guaranteed. To take an example. Supposing 

 we wish to determine whether fat is necessary as a source 

 of energy : were this merely an energy question it could 

 easily be settled by feeding an animal on a fat-free diet. 

 But it is known that on such a diet the animal will fail to 

 hve, not because the energy-supply is inadequate, but 

 because of the loss of certain substances present in fat, 

 which are constantly required by the body for effecting 

 chemical changes within it. 



The chemical changes occurring from the time of absorp- 

 tion to the time of excretion, and the transformation 

 of energy involved therein constitute what is known as 

 metabohsm. 



THE EXPENDITURE OF ENERGY 



In this chapter we shall consider the body as a machine, 

 and proceed to investigate quantitatively the transformation 

 of energy involved in the processes of life. For estimating 

 the amount of energy hberated two methods are employed — 

 Direct and Indirect Calorimetry . 



Direct Calorimetry 



In this method the subject is put into a specially con- 

 structed calorimeter and the amount of energy estimated 

 as heat is recorded. The most modern apparatus for 

 experimenting upon man is that invented by Benedict. 

 It consists of a chamber of the size of a small room in 

 which the subject can hve for a prolonged period. The 



