If a body expends energy, that energy must have a source. The law of conservation of energy teaches us that nothing is created and nothing destroyed, but things do change. So if the body needs 3000 calories, these have their source in the food taken. But where does the hungry man - the starving man - get his energy from? The answer to that is that no matter how reckless a man may be in his spending habits, the body is never quite so reckless; it always stores up a little food for "the rainy day." But this little capital does not last for many days. When the stored food has disappeared, the energy requirements continue to be met by the "oxidation" of the tissues - the muscle, the fat, the skin, the liver, the blood are used up bit by bit until the human machine snaps. Most remarkable is the fact that to the very last the brain and heart continue to function with presumably little impairment. They are the last to be attacked.

The question is often asked, to what extent does mental work influence calorific needs? No very decided answer to this all-absorbing question can be given. Many experiments have been tried, but the results have been uniformly negative. In one of these experiments Dr. Benedict, of the Carnegie Institute, measured the heat evolved by a number of college students during examination periods. These poor fellows were penned up in calorimeters and their examination questions then set before them. They "sweated blood." When the exams, were over, the same men were put in calorimeters and allowed to rest for a period equivalent to their examination period - three hours - and during this time the heat was measured. No material difference between the two measurements was noticed.

However, it may be noticed in passing that Tashiro, a talented Japanese instructor at the University of Cincinnati, has shown that when a nerve fiber is stimulated, the carbon dioxide output is increased; which is another way of saying that when the nerves are active, "oxidation" is increased, and therefore the calorific needs are greater. Tashiro had devised an apparatus for detecting the minutest traces of carbon dioxide, and it was only with so delicate an instrument that the increased output of carbon dioxide could be determined. The amount measured was so small, that it can hardly make any appreciable difference in ordinary metabolic studies. The calorie is a true guide to muscular activity; it seems to be no guide to the activity of the brain.