Muscles and Their Function:

Compared to before, the one that was stretched out then tightens opposite the influencing spirits and relaxes another previously contracted one: thus, one must note the movement of the muscles in three phases; first, when a muscle is pulled inward towards the head; and then its counter-muscle rests. Second, when it remains contracted for a long time, it appears thicker in that action. Third, as it is released again, and returned by its counter-muscle to its previous rest and position; whereupon it becomes slender again. From this, one can deduce that the expansion of one corresponds with the contraction of the other; and therefore they must work alternately, and in no form or posture can both be seen or shown equally.

Through this reciprocal exchange, the limbs are moved, and humans perform through their will, which they have previously considered and decided upon. The great philosopher Descartes, speaking on these matters, attempted to explain: It is known (he said) that all movement of the limbs depends on the muscles, and that they are set up against each other in such a way that when one contracts, it draws towards itself that part of the body to which it is attached, and when it happens at another time, the other contracts, causing the first to stretch out, pulling the limb to which it is attached toward itself.

Furthermore, it should be known that all muscle movements depend on the nerves, which are small threads one might call tiny pipes.

Translation Notes:

• The term "spirits" in this context may refer to what was historically thought of as vital forces or energies influencing muscle movement.
• "Wijsgeer" refers to a philosopher, in this case, Descartes, applying logic to explain bodily functions.
• The "threads" and "pipes" refer to the nerves, related to how they transmit movement signals.