Mental work does not increase bodily waste. In this it differs radically from physical work. But it cannot be well done unless nutrition, and especially the nutrition of nervous tissues, is good. There is no kind of food especially adapted to create nervous energy or to restore worn nervous tissue to a condition of vigor. Agassiz at one time suggested that, as the brain was rich in phosphorus, a food containing it, such as fish, was best adapted to the needs of brain-workers. Experience has demonstrated that this is not true. Intense mental work checks digestion just as all kinds of strong emotions do. It is evident, therefore, that health will not be preserved if the habit is formed of doing hard mental work immediately after eating heartily. It is quite as harmful to accomplish hard mental work immediately after a large meal as it is to do hard physical work. Indigestion and slow digestion always make mental work difficult; therefore the most important dietary regulation for those who must do hard mental work constantly is that the food eaten shall be easy to digest. In other words, the regimen must be determined by the digestibility of food rather than by its composition. Brain-workers need a comparatively small amount of food. As proteins of animal origin are, as a rule, digested easily, and quickly and readily utilized by the living tissues, they are well adapted to the needs of such persons. There is a dependence of nervous activity upon muscular activity. While one is working hard, exhaustively, with his muscles, he is unable to do much mental work. One who does prolonged and intense mental work will find his mind grow dull and his temper become quick, irritable, and peevish unless he maintains a balance between muscular and nervous work by some gentle exercise. It is probable that under these circumstances, exercise does good chiefly by stimulating a better lymph circulation through the brain as well as other viscera, and therefore a more rapid and perfect elimination of the products of tissue waste.