CAP. II. DE MUSC. DESCRIPT. ET USU.

—the other comes from an externally arriving cause, which uses muscles and organs to lift huge weights; for we see that fibers are not altogether inert, but have some contractive force. For in cadavers, a little while after death, severed muscles contract themselves just as much as in the living, in whom such contraction is exercised elsewhere without any laborious effort. It is similar to the contraction that is effected by the strings of a lyre when pulled, and therefore it depends on the structure of the small machines of which the fibers are composed.

That the degree of such motive virtue of those small machines is slight is clear from the brief contraction of a muscle after the cutting of one of the opposing ligaments, or after the incision or removal of the antagonist a muscle that opposes the action of another, which force, if a violent convulsion does not supervene, can be overcome by the weight of a few pounds.

With the joint flexed as much as possible, the muscle situated in its hollow part remains slack, and therefore can exercise no contractive force. And then the muscle situated in the connected position, without the opposition of the antagonist, could exercise its whole force, and yet we see it to be so weak that it cannot overcome the weight or impediment of the joint itself, since it is not able to direct it spontaneously. This is evidenced by the fact that when the elbows, shins, and remaining joints are totally flexed and perpendicularly erected above the horizon—when, that is, the gravity of the bone and the erected joint does not resist the flexion and traction—then the proper contractive force of the muscles which are destined for the extension of the joints, while they lack the impediment of the antagonists, should act spontaneously and exercise the degree of their natural energy by flexing the joint, whether we are attentive to it or not, or even if we are unwilling. This, however, is so false that we are able to resist the natural action by which the machines of the muscle fibers attempt to contract themselves without any sense of fatigue; nor, furthermore, do we perceive any fatigue from the continued action of muscles against their antagonists, such that they retain the joints in their natural disposition by tonic action.

Besides the slight and weak contraction of the muscle fibers that they exercise against their antagonists, they exercise another very powerful voluntary contraction, by which they suspend huge weights.

Some seem to confuse this action with the former, although they are entirely different. Therefore, for the sake of distinction, I will call the former the proper action of the fiber, and the latter, made by the command of the will or by appetite, I will call the vital action of the muscle.

C A P U T III.

Concerning the degree of the vital motive virtue of muscles according to the ancients.

A large ornamental drop cap "C" begins the chapter.

Since in this first part of the work it is sought how great the vital motive force of muscles is with respect to resistance;