DE GRAD. VIRT. MOTIV. VITAL. &c. original: "On the Degree of Vital Motive Virtue, etc."

We must see, in the first place, what our predecessors have handed down regarding such a subject.

P R O P O S I T I O VIII.

We must now seek the measure and quantity of that virtue, or vital strength, of the muscles, by which not only the arm or the leg, but the entire animal machine—as great as it is—is sustained, lifted, and propelled even to the point of dancing. Furthermore, besides the bulk of the animal, which is sufficiently heavy in itself, it carries, pulls, and pushes huge weights.

This matter causes the greatest difficulty for Aristotle, who did not recognize muscles, but only fashioned spirits gaseous or subtle breaths that would pull and push the joints. And truly, as that insightful man had observed, since it would be difficult for the vast mass of an elephant to be moved, agitated, and pushed by a thin spirit or breath, he meets the difficulty by saying that Nature does not move the joints and parts of the animal with equal force, but with the least and smallest strength. He says that she achieves this by means of a machine whereby small motions made at the beginning become great and manifold afterwards; just as from a small and easy motion of the tiller and rudder, a great transport of the prow and ship is effected. Later, in his mechanical questions, he considers the nature of the tiller and says that it exercises its operation by means of a lever, in which, indeed, it is not a wonder that immense weights can be moved and pushed with small virtue. Lucretius uses the same example:

And one hand guides it, however great the force with which it goes,
And twists the rudder, one man alone.

And subsequently, employing other instruments, he says:

And through pulleys and drums, the machine
Moves a great weight, and lifts it with light effort.

Gassendi also admits that, by means of a machine, the animal faculty can move great weights with small forces, but he doubts that levers, pulleys, and drums can be found in an animal, since only muscles are observed, which are better likened to the ropes of pulleys. He imagines, however, a mechanical instrument in the muscle composed of pulleys and ropes, and that the same number of corrugations and wrappings, by which the fibers of the muscles are twisted when the length of the muscle is shortened and contracted, supply the place of pulleys, and from these a certain series of pulleys is made, as in a block and tackle, whose force is almost immense.

Galen also says that a tendon is like a lever, whence he believes that great weights can be pulled and moved by a small virtue of the animal faculty.

Others also assert the same, but for a different reason, that muscles, by means of a machine, lift great weights with a small virtue of the spirits.