On the Movement of Animals, Part Two

of the juices that were filling the interstices of the fibers, which expulsion was caused by the heat of the ambient water.

And that such an operation cannot similarly be adapted to the vital contraction of muscles can be persuaded in many ways.

First, because it is evident from experience that tendons and membranes are contracted much more by excessive heat through roasting and boiling than are the muscles themselves; but when, by vital action, the fleshy fibers are greatly inflated, tensed, and contracted, the tendons and membranes suffer no contraction whatsoever.

Second, because in muscles, while they are moved most rapidly by contracting themselves, no heat exceeding natural warmth is observed which could produce roasting or a boiling fervor; indeed, in fish and very cold reptiles, motions occur excellently.

Third, to produce that solidity in roasted or boiled nerves, a prolonged action of heat is required, which so alters and transforms their structure and temperament that they cannot reassume their original softness and laxity. This, however, is not verified in the muscles of animals while they move, because they are contracted in a moment and grow slack in the blink of an eye; from which it is clear that the composition and temperament of the muscles are not changed; and therefore their contraction does not occur through an alteration similar to that by which nervous fibers are scorched by fire or corrugated through boiling.

Which is furthermore evident from sense itself: for the hearts and muscles of reptiles, when dissected and removed, are moved for a long time by contracting and relaxing without burning heat, with-