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] [16] The extensor muscles preponderate in mass and weight over the flexors, but this is readily accounted for by the fact, that the extensors, when limbs are to be straightened, always work at a mechanical disadvantage. This is owing to the shape of the bones, the conformation of the joints, and the position occupied by the extensors. [Illustration: 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._] [Illustration: 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._]