“Experience indicates that appetite does not lead to a diet fixed in amount or constituents, but moves in undulating waves or cycles. The men who took part in the experiment were encouraged, after any of the symptoms which seemed to be associated with high proteid (such as heaviness, sleepiness, stiffness, or soreness after exercise, or catching cold), to cut down on their proteid and substitute fat to restrain the gastric juice. This advice was intended to make application of the theories of Folin that we usually carry a reservoir of proteid, enough to supply our needs for body-building for a fortnight. If this reservoir is exhausted, proteid starvation occurs and the body feeds on itself; if it is filled too far it overflows and causes the evils of excessive proteid. If this theory is correct, the art of eating may consist largely in maintaining a golden mean, such that the proteid reservoir is neither empty nor overflowing much. Many persons fear to reduce their proteid to the Chittenden minimum for fear of proteid starvation; but the experience of those who have tried it would seem to show that this fear is groundless, _provided_ no violence is done to natural appetite. This may be trusted, so it would appear, to raise a warning in the form of ‘nitrogen hunger,’ before the danger point is reached.” In other words, the body will ask in the language of hunger for proteid food, if you are not eating as much as you should. Professor Fisher considers that an amount of meat equivalent to about one small chop will supply all the proteid necessary in the daily ration, since proteid is also consumed in bread, potatoes and nearly all other foods. It might be added that one of the writers has found the remedy for continual bilious headaches in the rigid exclusion from his diet of all foods that are rich in proteids, including meat, fish, eggs, milk, cheese, peas and beans; and maintains weight and working efficiency upon such amount of proteid as he derives from ordinary breadstuffs. He has found that the craving for high proteid foods soon disappears if it is not gratified; and that the quantity of bread, potatoes, etc., which the average person would eat at dinner and supper supplies all the nitrogen which his system needs, without leaving any to cause autointoxication. IV HOW DIGESTION IS ACCOMPLISHED In order not to interrupt the narrative of the Yale experiments, we have foregone defining certain of the technical terms which it was necessary to use. It will be well, before going further, to give a simple description of the manner in which the food we eat is transformed in the body into tissue building material and energy: a process the many parts of which are grouped by physiologists under the name of Metabolism.