Internal environment and overt behavior present a reciprocal relation, for the constancy of the body's composition and function depends on behavior for its maintenance, and behavior in turn depends on the body's contents and physiological states. With respect to water, elimination of excesses and replacement of shortages are symmetrical aspects of recoveries and adjustments of water content. Each is quantitatively related to the other in a kinetic pattern that is but rarely incomplete, even in diseased states.Bernard said (1878, p. 113): "The fixity of the internal medium presupposes an arrangement in the organism such that the variations are at each instant compensated and equilibrated." This arrangement I have tried to describe.Abstract Teaser