On the Movement of Animals, Part Two

PROPOSITION II.

A muscle is not contracted by a simple traction of its fibers in the same way that weights are lifted by cords.

a of this prop. 1.

Let the muscle ACB be tied at X to a fixed terminal of bone XT, and at point Z to another bone composing the joint T. I say that it cannot be contracted like a cord pulled upwards. Let us imagine that the fibers are all pulled upwards towards A by equal forces C, E, F, D, so that point B is transferred to D, and point D to F, and F to E, and E to C: in the end, it is necessary that the extreme part of the muscle CA remains slack and curved; because the intermediate parts are not pulled toward one another, but by hypothesis tend toward the same parts A, nor are they moved by contrary motions, without which corrugations and contractions cannot be brought about: but this is impossible, and repugnant to sensible experiments; because in the anatomy of the living, the last part of any muscle AC remains hard, tense, and direct. Furthermore, we see that the terminal X is transferred downwards towards B just as well, with the bone Z remaining fixed in the same position; therefore all parts C, E, F, D would move downwards with equal velocity towards B, and thus the extreme part DB would be rendered slack and curved, which is impossible; for previously they were all ascending towards A, and therefore they would be moving with contrary motions.

It would further follow that the length of the muscle, just like a pulled cord, would not be shortened, which is repugnant to the sense; for we see that the interval of the muscle