[Sidenote: Life and death in changes of seasons] We see the same thing in the changes of the seasons upon the face of the earth. Throughout autumn and winter there is a process of decay, death, and disintegration; leaves fall; plants and vegetables die; fruits ripen, fall and decay. This process continues until former beautiful and symmetrical bodies of matter are thoroughly disintegrated, and the particles once composing them are separated into their original elements, to be appropriated in new manifestations of life in springtime and summer. [Sidenote: Human body compared to a machine] We are inclined to think of the human body as a machine--a marvelous, intricate, and complex mechanism which serves our will and our desires; as a tool with which we work out our earthly destiny. But unlike man-made machines, it is self-repairing, self-adjusting, and contains within itself the forces of construction, which are constantly tending toward perfection, while our industrial machines are constantly tending toward their own disintegration and destruction. [Sidenote: Constant changes in body-tissue] Every movement of the body, conscious or unconscious, even thought and emotion, use up some part of the body-tissue which must be replaced by new material. This constant change in the texture and the make-up of the body we call "metabolism," involving the functions of digestion, absorption, assimilation, and elimination. While we may regard the body as a machine, there are many points in which the favorite comparison to a steam-engine is not exact. [Sidenote: Favorite comparison of the body with the steam-engine] The inert metal composing the steam-engine has no power in itself, nor does power act through the different particles of metal, but it is controlled by the external application of force, which is the result of chemical changes caused by combustion in the fire-box. The metal of the engine has no part in the production of this energy. It does not need to take periods for rest, and if it were possible to supply it continually with water and fuel, it could run steadily from the time it was started until one or more of its essential parts were destroyed through friction. [Sidenote: Necessity for rest] But the engineer and the fireman who drive the engine find it necessary to rest from their labors at certain intervals, not merely for fuel and water, but to prevent serious destruction of body-tissue. This is true because man is compelled by hitherto unrecognized laws to give his body an opportunity, not only for readjustment in its composition, but also for the actual renewal of that power which animates him and makes him an intelligent, self-adjusting, and self-controlled being. THE OLD PHYSIOLOGY [Sidenote: The stomach as a fire-box] [Sidenote: Phenomenon of rest and sleep]