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, no consumption of proteins as fuel when cheaper fats and sugar would answer equally well. In the case of Succi, who fasted for thirty days, the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3 grammes on the twentieth, and to 3·2 grammes on the last day. Clearly, we have to make a distinction, when all food is cut off, between the oxidation of the protein which, failing all other material, is withdrawn from the tissues for the purpose of supplying the force absolutely necessary to maintain respiration and such other movements as are inevitable, and to keep up the temperature of the body—force which under other circumstances might be supplied by non-nitrogenous food—and the oxidation to which bioplasm is inevitably subject, so long as it is alive. The oxidation of bioplasm under ordinary circumstances of course supplies force; but it does not follow that this is sufficient to maintain the respiratory movements and the contraction of the heart. When a herbivorous animal is starved, it not infrequently excretes more urea at the commencement of the starvation period than it was excreting when well fed. Its activities did not come to a standstill when carbohydrate food was cut off. For a time they were maintained at the expense of its own tissues. On the other hand, the results obtained from the observation of the man who went without food for thirty days show that Nature is able to economize force by reducing the metabolism of living substance below the normal. It might be supposed that the irreducible metabolism could be ascertained by giving a nitrogen-starved animal non-nitrogenous food, but it is found that this scarcely affects the tissue-waste. Becoming more active, the tissues, while saved from the necessity of supplying fuel for the production of heat and motion, suffer more waste. Again, it might be expected that if to an animal which had been starved for a few days, until its urea had fallen to the starvation limit, exactly sufficient