DIETETICS 155 



of great importance in relation to the question which we are 

 discussing. Whichever of the two views be justified, we have 

 to distinguish between the bioplasm of the cell the machine 

 and its raw materials and manufactured products. The 

 question to which we want an answer is the following : Must 

 the bioplasm undergo change ? There seems to be no reason 

 in the nature of things why it should. It is not, as we have 

 already pointed out, subject to wear and tear. A perfect 

 machine would in the absence of friction, which rubs down its 

 steel and brass, continue to turn out its products so long as it 

 was supplied with raw materials and the energy needed to 

 manufacture them. We could imagine the bioplasm as in- 

 destructible, receiving energy from a portion of the foods, and 

 expending this energy in the production of chemical change 

 in the remainder. We could imagine that when once the 

 tissues had attained their full growth they would require no 

 more protein for their own nutrition ; they would be occupied 

 in producing heat and motion from the non-nitrogenous foods. 

 But observation shows clearly that this is not the case. The 

 force which energizes the bioplasm, enabling it to evoke meta- 

 bolism in non-living substance, is obtained at the cost of its 

 own destruction. The bioplasm wastes unless constantly 

 supplied with proteid food. 



Under ordinary circumstances the amount of urea excreted 

 varies directly as the quantity of nitrogen contained in the 

 food. Since urea contains 45 per cent, of nitrogen, and 

 protein 15 per cent., every gramme of urea excreted represents 

 3 grammes of dry protein consumed ; or, in terms of nitrogen, 

 every gramme of nitrogen excreted represents 6-25 grammes 

 of protein consumed. If all food is withheld, the excretion of 

 nitrogen falls, but it never reaches zero. Many observations 

 have been made on fasting men. On the second day of 

 fasting the nitrogen excreted falls to about 13 grammes, 

 representing 80 grammes of protein used up. It is generally 

 thought that by the second day all " floating proteins " are 

 exhausted, and that therefore nitrogenous metabolism is 

 reduced, as it were, to a business basis. So long as the supply 

 of food is abundant, the body has a luxurious habit of using 

 proteins in preference to non-nitrogenous food. But after a 

 day's starvation there is no longer any fancy metabolism, 



