154 THE BODY AT WORK 



involved are somewhat complicated. If it were possible to 

 live on a single food, it would be as easy to ascertain the irre- 

 ducible minimum as it is to find out with how much coal or 

 with how much petrol an engine can be made to turn a wheel. 

 But -to support the body several different kinds of food are in- 

 dispensable. It is therefore necessary to determine, not only 

 the minimum quantity of the combined foods, but also the 

 minimum amount of each kind of food, and the effect upon the 

 total of variations in the relative amount of each of its several 

 factors. The problem is complicated, but certain limits are 

 impassably defined. In the first place, with regard to the 

 total amount, the work which the body does cannot under 

 any circumstances be reduced below a certain level. The 

 food consumed must provide a supply of energy equal, at the 

 least, to the performance of the minimum of work. The 

 body must receive each day food of due caloric value. Then 

 with regard to the amount of each several constituent. Many 

 considerations lead us to wish to increase one of them or to 

 diminish another. Some food is cheap, and other food is dear. 

 Economic reasons are in favour of the cheaper food. Even 

 ethical considerations are not without weight. We have, 

 perhaps, a prejudice against sacrificing life to supply the pot. 

 We have doubts as to whether our system can properly digest, 

 metabolize, and excrete meat. We need an unambiguous 

 answer to the question, To what extent can nitrogen-foods be 

 replaced by carbon-foods, and vice versa ? A cell, as already 

 said, consists of a framework of bioplasm bathed in cell- juice 

 which contains nutrient substances and manufactured pro- 

 ducts. The bioplasm is alive ; the proteins, carbohydrates, 

 and fats of the cell- juice are the materials with which it is 

 nourished, and upon which it works. Some physiologists 

 incline to the view that non-living substances must enter into 

 the bioplasm before they undergo metabolism. They consider 

 that the molecules of the non-living substance must at the 

 time when they undergo a chemical change be physically and 

 chemically a part of the living substance. Others take the 

 opposite view : that the living substance does not undergo 

 change, but brings about changes in the non-living substance 

 which is in contact with it, enclosed within its meshes. This 

 is a problem which is not likely to be solved, nor is its solution 



