The text discusses the composition and service of muscles, explaining them as instruments of voluntary movement controlled by animal spirits and influenced by the will. It describes four factors that allow limb movement: the soul, animal spirits, muscles with nerves and tendons, and coordinated action. The coordination leads to contraction and motion of limbs, a concept rooted in historical anatomical theories.
Muscle Composition and Service
This involves their substance, shape or figure: in size, number, position and initially their emergence and implantation: above all, they influence power.
Muscles should be further observed as instruments of voluntary movements; for they receive only the influences, or possibly the impulses of the moving animal spirits, which mostly obey our will: Therefore, we can manage or delay them, or completely stop them as we wish. However, in some, there is also a natural movement, which moves some parts against our will, such as the chest, the eyes, and also some parts we share with animals; about which we will talk later.
The means by which the limbs can be moved should be noted in four aspects: first, the thinking soul as the commander. Secondly, the animal spirits as the helpers and servants, thirdly, the muscles, nerves, and tendons as the laborers and tools; and fourthly, the action itself by the limb or limbs being steered; the nature of the action depends on this. The manner and order of motion is generally perceived in this way: the animal spirits influence the will in the muscles through threads and influence by the nerves (others believe this is done by separation of impulse) which distance and shorten them, in essence contracting; whereby the limb attached by its sinews is necessarily raised, and another muscle of opposing action extends and flattens.
Translation Notes: "Dierlijke Geesten" translates to "animal spirits," a concept from historical anatomy describing forces or spirits believed to enable movement.