Maxel and Function of the Muscles

Concerning the movement of the muscles, it is the same observation from Steno, that they move when their belly, which is their flesh, becomes shorter and swells, causing the limb, to which the muscle is attached, to be necessarily displaced, particularly when the limb is movable. Most anatomists and philosophers presuppose that the muscle swells during movement when a fluid enters between its fleshy fibers, expanding it, just like a bladder is expanded by inflating wind. This fluid substance most often is considered to be the Animal Spirit, which is brought from the brain along the nerves to the muscle. Others believe that this swelling occurs through the blood, which flows through the arteries into the muscle. Ultimately, some suggest that the blood and animal spirits together would cause this swelling. They assume that the blood is salty, and the Animal Spirit or nerve sap is somewhat acidic, and these two different substances coming together in the muscle cause swelling, similar to what is observed when vitriol oil is poured onto the tartaric salt. However, Steno dismisses all these views and mathematically demonstrates that the muscle can swell without more fluid entering it than was present before this swelling. Therefore, he concludes that the manner and cause of how muscles are moved are not yet known. His further observations can briefly be said to be: namely, that when the muscle moves, meaning it contracts, you then see each fleshy fiber becoming shorter. Another says

Translation Notes

- "Steno": Refers to Niels Stensen, a Danish scientist known for anatomical studies.
- "Animal Spirit": An outdated term referring to a vital force theorized in earlier science, often considered akin to what we understand as nerve impulses today.