There are in animals very few simple movements, _i.e._ movements occurring in one plane and produced by the action of two muscles. Locomotion is for the most part produced by the consentaneous action of a great number of muscles; these or their fibres pursuing a variety of directions. This is particularly true of the movements of the extremities in walking, swimming, and flying. Muscles are divided into the voluntary, the involuntary, and the mixed, according as the will of the animal can wholly, partly, or in no way control their movements. The voluntary muscles are principally concerned in the locomotion of animals. They are the power which moves the several orders of levers into which the skeleton of an animal resolves itself. The movements of the voluntary and involuntary muscles are essentially wave-like in character, _i.e._ they spread from certain centres, according to a fixed order, and in given directions. In the extremities of animals the centripetal or converging muscular wave on one side of the bone to be moved, is accompanied by a corresponding centrifugal or diverging wave on the other side; the bone or bones by this arrangement being perfectly under control and moved to a hair’s-breadth. The centripetal or converging, and the centrifugal or diverging waves of force are, as already indicated, correlated.[15] Similar remarks may be made regarding the different parts of the body of the serpent when creeping, of the body of the fish when swimming, of the wing of the bird when flying, and of our own extremities when walking. In all those cases the moving parts are thrown into curves or waves definitely correlated. [15] Muscles virtually possess a pulling and pushing power; the pushing power being feeble and obscured by the flaccidity of the muscular mass. In order to push effectually, the pushing substance must be more or less rigid.