On the Movement of Animals, Part One

some things must be assumed which the evidence of the senses reveals. That the principle and effective cause of the movement of animals is the soul, no one truly ignores, since living things live through the soul, and while life lasts, motion persists in them; but with the animal extinguished—that is, with the soul no longer operating—the animal machine is left entirely inert and immobile.

That the manifold and numerous motions of an animal occur by election, or by a certain natural appetite of the animal, this also is admitted by all as most evident.

It is also manifest that cognition and appetite by themselves do not move and impel the parts of an animal, but require necessary instruments, without which motions cannot be effected.

They commonly distinguish the instruments of motion, for they hold one to be active, and another to be organic and merely passive. The active instrument of the soul is accustomed to be called the motive virtue or faculty; this, however, is commonly thought to reside in the animal spirits.

The immediate organ, then, by which the motive faculty of the soul moves the parts of the animal, are—according to Aristotle—only the spirits, which, ending and degenerating from the heart through the arteries into the nerve endings, are poured forth as far as the flexures of those same joints; they move the bones by pulling them toward themselves whenever a joint is flexed, or by pushing them outward whenever a joint is extended. But this doctrine is rejected by Galen, and by all others, and by the evidence of the senses itself, by which it is established that muscles are the organs and machines by which the motive faculty of the soul moves the joints and parts of an animal.

For a long time this has been confirmed, because when muscles are cut trans- uer-