They are divided into two groups: the chemical process of building up complex substances from simple ones is known as _anabolism_; the chemical process of oxidizing and breaking down the complex substances into simple ones, so that they are in a state to be excreted, is called _catabolism_. While the process of oxidation in catabolism is going on, heat and energy are set free. Many of the chemical changes in the body are catabolic in character. This work never ceases—even in sleep. It is not enough that the proper foods be furnished the body in kind and quantity. The essential thing is that the system be kept in condition to _assimilate_ the foods to its needs and to promptly eliminate the waste. Few people assimilate all of the foods eaten. By _assimilation_ is meant the process by which foodstuffs are made soluble and diffusible, so that they can pass into the blood; also, the metabolic activity by which the food is converted into cells and tissues. Truly the body is a busy workshop. Think of the billions on billions of cells being formed and destroyed every instant in the liberation of heat and force! Think, also, of the necessity of perfect circulation to bring sufficient blood to the lungs, that it may gather the oxygen and carry it, without pausing for rest, to every tissue of the body! Even in sleep this stream continues incessantly. There is also a great lesson here in the law of supply and demand. When the body is at mental or muscular work, the potential energy liberated leaves through muscle or brain, as energy, and is expressed in the result of the work. When the body is at rest, energy leaves it as heat (excepting such part as is necessary to carry on metabolism, circulation, etc.). If much muscular energy is called for, a deep, full breath is instinctively drawn to supply the oxygen necessary for the added force. If strong mental work is required, attention should be given to exercise and deep breathing, that the blood may carry off the waste liberated by brain activity. The difficulty is that in doing close mental work, the body is too frequently bent over a desk in such a manner as to restrict the action of the lungs; thus, the brain worker, in order to continue strong mental work, must often go into the open air, as he says, “to rest his brain,” but in reality to obtain the oxygen needed to put the waste, liberated by brain energy, in condition to be carried away. The supply of blood has been called on for the brain work; the poor circulation through the body has allowed an excess of carbon dioxid to accumulate and the condition of the body designated as “tired” has resulted. Until the necessary oxygen has been supplied, the brain and body are not balanced, not “rested.” In its conversion into tissue, heat, energy, and waste, the importance of the chemical exceeds that of the mechanical action of digestion, absorption, assimilation, and elimination; yet the chemical changes are aided by the mechanical.