Here's the quote (from The Tragedy of Macbeth, by William Shakespeare):
This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth? I am thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?
The connotations may have changed since it was written, but doesn't the word "seated" evoke the image of the object under discussion being, to all intents, still? As in pretty much motionless? What am I missing?