66 PHYSIOLOGY 



through the body; this also is estimated by sample in 

 the calorimeter. 



This subtraction gives the total energy an animal is 

 extracting from its food. From the law proved by 

 Lavoisier for the inanimate world, namely that in it 

 matter is neither added to nor consumed, life offers no 

 departure. The doctrine which states that energy, though 

 it pass from form to form, can, like matter, neither be 

 added to or consumed in any of its permutations, was 

 likewise drawn in the first instance from study of the 

 inanimate world. Now there is a great deficit of energy 

 obvious when the energy of the waste matter leaving the 

 body is compared with that of the food supplied. If 

 the conservation of energy holds in vital reactions, this 

 deficit will be exactly accounted for and balanced by 

 energy expended by the body to which the food is 

 furnished. The energy expended by the body can be 

 measured. A calorimeter is used, more complex than the 

 one for food values. In such a calorimeter a man may 

 live days at a time and divide his energy between novels 

 and a dummy bicycle. Investigation shows that the 

 quantity of energy coming out of the body exactly 

 equals the deficit of the energy of the material that 

 passes through it. The conservation of energy obtains 

 therefore in our own frame as in the tide, the waterfall, 

 the furnace. It is a law in the living world as well as 

 the inanimate. This it is which encourages the physio- 

 logist to feel that in studying the living machine he has 

 assured at least the source of energy whence all its 

 powers are drawn. 



But the engineer regarding his machinery knows also 

 ' how ' those powers are drawn. Of this in the body we 

 have far less knowledge. Food is not fuel in the ordinary 

 sense. The fuel of the gas-engine becomes no part of the 



