On the Movement of Animals, Part Two

Chap. 1. Concerning the modes, and mechanical operations, by which the contraction of muscles can occur.

is not impeded when we rest; and therefore that large weights are not lifted by the force of the machine when the impediment ceases.

Furthermore, if some cause were to impede the contractive force of a muscle, such a cause would either act by continuous action against that contractive faculty perpetually flourishing, just as a weight balanced on a scale always endeavors to descend, although it is not moved; or it would entirely destroy it and render it torpid. If the first, then the muscle, even when it does not act by pulling a resistance, would persist as turgid, tense, and hard: because the action of the machines is exercised by pulling and shortening themselves as much as they can and as much as the impediments permit. If the second, by a more lengthy and laborious action, it would achieve the same thing that could have been effected more compendiously. For two most powerful causes would be posited, not existing at the same time, but following each other in turn, of which one—namely, the contractive force of the machines—would exist in the muscles precisely at that time when huge weights are lifted at the command of the appetitive faculty, and then the force of the impediment would not exist. Conversely, with the former extinguished, the latter would have to be posited for the whole time during which the muscles are idle.

Now, if nature seeks nothing else but that weights be suspended at stated times, and this can be obtained more compendiously by a single cause—namely, by the force contracting the muscles—to what end is the superfluous positing of another cause impeding that action, namely a cause destined for doing nothing, that is, for bringing about the rest and idleness of the muscles?

Furthermore, because from a defect of appetite, as it is a defect,