On the Movement of Animals, Part Two

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

happens in the masseter when the jaws are brought together and tightened, then said masseter ought to contract, tense, and harden of its own accord; but this is repugnant to sense, for it remains lax and soft until it swells at the command of the will: therefore, the contraction and tension is not proper, nor does it depend on the structure of the machine of the fibers themselves, but on another cause, far different.

Notwithstanding these things, one might say that that degree of vigor which muscles exert by vital motion is proper to the small machines of which the fibers are composed, which therefore is not continuously exerted because the contraction of the machines is impeded until the appetitive faculty permits them to exercise the operation, not otherwise than as the violence of a crossbow is impeded by the notch or bond of the nerve when it is pulled; which, once removed or released, follows a most violent distraction and projection of the missile; therefore, we shall show that

P R O P O S. V I.

The huge, vital, contractive force of the machines of the muscles does not rest, nor does it cease from operation because it is impeded by an obstacle, nor does it act when that impediment is commanded by the animal faculty to be removed. Tab. 15. Fig. 4.

Let us suppose that that great vital force, which muscles exert at the command of the motive faculty, is natural and inherent to the muscles, so that they are always apt and disposed to exercise that operation; and when they are idle, they do not rest due to a defect of strength, but because its action is impeded by some external cause: just as