[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.
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.--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._]
The force of muscles, according to the experiments of Schwann, increases with their length, and _vice versa_. It is a curious circumstance, and worthy the attention of those interested in homologies, that the voluntary muscles of the superior and inferior extremities, and more especially of the trunk, are arranged in longitudinal, transverse, and oblique spiral lines, and in layers or strata precisely as in the ventricles of the heart and hollow muscles generally.[17] If, consequently, I eliminate the element of bone from these several regions, I reproduce a typical hollow muscle; and what is still more remarkable, if I compare the bones removed (say the bones of the anterior extremity of a quadruped or bird) with the cast obtained from the cavity of a hollow muscle (say the left ventricle of the heart of the mammal), I find that the bones and the cast are twisted upon themselves, and form elegant screws, the threads or ridges of which run in the same direction. This affords a proof that the involuntary hollow muscles supply the type or pattern on which the voluntary muscles are formed. Fig. 6 represents the bones of the wing of the bird; fig. 7 the bones of the anterior extremity of the elephant; and fig. 8 the cast or mould of the cavity of the left ventricle of the heart of the deer.