Pathology of the Brain and the Nervous System

makes for the elucidation of the matter proposed, let it be briefly noted,
How regular motion is performed.

that the animal spirits are the immediate instrument of regular motion, and that their action, or motive force, consists only in this: that, being more densely crowded together in the moving part, and there expanded into a larger space, they inflate and swell the same; which consequently, being contracted as to its length, immediately draws to itself the part attached to it. In our description of the nerves published some time ago, we
Its twofold principle.

intimated that motion of this kind is twofold, namely spontaneous and merely natural; the instinct of the latter from the cerebellum, and of the former from the cerebrum; but that both are conveyed through the ducts of the nerves, as if through channels, as well to the muscles as to the fibers interwoven in the membranes, parenchymata, and other moving parts; finally, that in all these things, various actions are performed so expeditiously—whatever either natural necessity or the command of the will requires—for this reason alone: that among the animal spirits, which constitute the hypostasis of the sensitive soul, within the aforesaid parts disposed in a continuous series, there is everywhere the same communication of office, intimate conjunction, and most rapid commerce.
Likewise a twofold subject.

But indeed, between the motion from a muscle and that produced by other parts, this notable distinction occurs: that in the latter, the action is very often circumscribed within the bounds of the moving body, so that only one part of the same membrane or viscus moves another, and thereafter this is moved by its neighbor: but in the muscular kind, for the most part, the moving part is situated in one limb and the part to be moved in another nearby (although even among muscular parts some, such as the heart and the diaphragm, properly move almost only themselves): Hence membranes and viscera are said to have as it were an internal and vermicular motion; of which kind, wherever it is begun, the spirits being there more densely crowded together and expanded, first cause that part to swell, then proceeding further, they swell another, and soon a further one, and consequently they draw that part which is behind, and so [carry] the swelling, and therefore the motion, from a lo—