X.] ALBUMINOID RATIO 205 



bustible — most by fat, less by proteins, less still by 

 carbohydrates and crude fibre as far as it is digestible — 

 and the amount of energy that must be supplied by the 

 food depends simply upon the amount of work done by 

 the animal. In animals at rest energy is only required 

 to perform the operations of mastication and digestion, 

 internal work like breathing, and to maintain the 

 temperature of the body, though the heat developed in 

 performing the former functions may be sufficient to 

 keep up the bodily heat. When an animal is at work 

 combustion is going on so rapidly that far more heat is 

 developed than is necessary to the maintaintence of the 

 normal temperature, and the excess is got rid of by an 

 increased evaporation of water from the lungs and the 

 skin, for the animal's temperature never rises sensibly 

 as long as it is healthy. In addition to food supplying 

 the necessary energy, the animal must also receive at 

 least a minimal amount of protein which it can utilise 

 for tissue formation ; any excess is rapidly reduced to 

 non-nitrogenous combinations and then burnt up for the 

 development of energy like any other food. 



From the point of view of the food, we must consider 

 that it contains first of all a certain store of energy 

 measured by its value as fuel, from which to begin with 

 we must deduct the energy of the indigestible portions, 

 and the energy still possessed by excreta like urea and 

 marsh-gas, in order to get the heat value of the food to 

 the animal. Further still, we must deduct from this 

 value the work spent by the animal in masticating and 

 digesting the food if we are to obtain the energy value 

 of the food to the animal — i.e. the surplus remaining out 

 of which it can do work. We now want an expression for 

 the protein value of the food, and to do this we are 

 accustomed to calculate the ratio between the non- 

 nitrogenous food constituents and the protein. To make 



