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.
It may be broadly stated, that in every case locomotion is the result of the opening and closing of opposite sides of muscular cycles. By the closing or shortening, say of the flexor halves of the cycles, and the opening or elongation of the extensor halves, the angles formed by the osseous levers are diminished; by the closing or shortening of the extensor halves of the cycles, and the opening or elongation of the flexor halves, the angles formed by the osseous levers are increased. This alternate diminution and increase of the angles formed by the osseous levers produce the movements of walking, swimming, and flying. The muscular cycles of the trunk and extremities are so disposed with regard to the bones or osseous levers, that they in every case produce a maximum result with a minimum of power. The origins and insertions of the muscles, the direction of the muscles and the distribution of the muscular fibres insure, that if power is lost in moving a lever, speed is gained, there being an apparent but never a real loss. The variety and extent of movement is secured by the obliquity of the muscular fibres to their tendons; by the obliquity of the tendons to the bones they are to move; and by the proximity of the attachment of the muscles to the several joints. As muscles are capable of shortening and elongating nearly a fourth of their length, they readily produce the precise kind and degree of motion required in any particular case.[16]
Fig. 6.—Wing of bird. Shows how the bones of the arm (a), forearm (b), and hand (c), are twisted, and form a conical screw. Compare with Figs. 7 and 8.—Original.
Fig. 7. Fig. 8.
Fig. 7.—Anterior extremity of elephant. Shows how the bones of the arm (q), forearm (q´x), and foot (o), are twisted to form an osseous screw. Compare with Figs. 6 and 8.—Original.
Fig. 8.—Cast or mould of the interior of the left ventricle of the heart of a deer. Shows that the left ventricular cavity is conical and spiral in its nature. a Portion of right ventricular cavity; b, base of left ventricular cavity; x, y, spiral grooves occupied by the spiral musculi papillares; j, q, spiral ridges projecting between the musculi papillares. Compare with Figs. 6 and 7.—Original.