In A Plea for Captain John Brown, Thoreau writes:
When he was here, some years ago, he showed to a few a little manuscript book,—his “orderly book” I think he called it,—containing the names of his company in Kansas, and the rules by which they bound themselves; and he stated that several of them had already sealed the contract with their blood.
Having done research, methinks bound neither means to surround a territory or other geographical entity; to form the boundary of nor to leap in this sentence.
What does the verb bound mean in this sentence?