On the Movement of Animals, Part Two

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

motive force in heavy bodies always flourishes downward and exerts its endeavor, but they rest because their descent is prohibited by the hardness of the pavement or by the force of an underlying hand. It must be shown that this position is impossible and inept.

And in order for the hypothesis of the proposition to be verified—namely, that the muscular chain AD, composed of machines and affixed to the nail A, be able to exert a huge contractive force—it is necessary that it first be distracted by some extrinsic power, such as by the hand M, by elongating it from D to B; for the reason that in the natural and balanced elongation AD, it has no force, and then the same power M, as long as it retains the chain in the distracted position, impedes by its continuous traction the contractive force of the small machines of the chain: we must therefore conceive that the muscle AB is kept perpetually distracted by the power M, and then it acts by lifting the suspended weight R only when the will, or the appetitive force, commands that the power M cease from action and abandon the chain so that it may be able to exercise its contractive nature, and afterwards, for the whole time of life, when the muscle is idle, it is necessary that the power M be present and again distract it and hold it by persistent action. With this posited,

a Par. 1. pr. 31.

I consider that the force of the impediment, or of the power M, must be of such strength that it is able to entirely prohibit the force and contractive operation of the machines of the muscles; therefore a such a force of impediment will be half the energy of the machines, since one does not prevail over the other; but the contractive force of the small machines overcomes the weight of two hundred pounds, which the muscles suspend; therefore, the force of the impediment will be equally as valid as a weight of one hundred pounds.