1. An inward movement which carries materials from the outside of the body toward the cells.
2. An outward movement which carries materials from the cells to the outside of the body.
Passing inward are the oxygen and food materials in a condition to unite with each other and thereby change their potential into kinetic energy. Passing outward are the oxygen and the elements that formed the food materials after having united at the cells and liberated their energy.
As a final and all-important result, there is kept up a continuous series of chemical changes in the cells. These liberate the energy, provide special substances needed by the cells, and preserve the life of the body (Fig. 92).
In the chapters which follow, we are to consider the problem of adjusting the body to and of bringing it into proper relations with its surroundings.