784 THE AMERICAN JOURNAL OF SOCIOLOGY
ture of the ensemble of organic bodies, including the human body, depends first of all upon their internal constitution and organization. The latter, from the point of view of accommo- dation to the environment, acquires an increasing importance. But whatever may be the stage of perfection of the internal organization and activity, the structure and function of the human species present constant and necessary relations of equili- bration existing between the external and internal environment. Let us see how this equilibrium of movement is produced. When the muscles are contracted they move the bones to which they are attached. The muscles represent the organs of move- ment the motor forces. The bones serve as levers. The point of support is furnished by the articulation. The resistance is the bone itself. The working of the bones in the ensemble of our structure serves to maintain the equilibrium, to overcome the great resistances, or to impart more or less extended move- ments, as, for example, the equilibrium given the head by the atlanto-occipitalis articulation. There is a center of gravity of the head. The weight of the latter acts in the sense of a vertical line drawn from the center of gravity. It is the resist- ance. The point of support is the articulation. The muscles of the neck represent the power. When there is action in order to overcome resistances, the resistance is between the power and the point of support ; for example, when one places the total weight of the body on the tiptoes. When one executes move- ments more or less extended, the power is .found between the point of support and the resistance. In all these cases there is no movement executed without a corresponding equilibrium. Photography, at least instantaneous, provides an excellent pro- cess for noting the different modes of locomotion, as walking, running, jumping, and flying. In all these movements there is always an equilibrium of the internal structure, that is to say, of all parts of the organism among themselves, and besides an equilibrium of the ensemble with the environment. The latter ought never to be lost from view. It is an essential element of the modifications of every equilibrium and its support. When one makes a false movement, the failure is accompanied at the
