Life of Commas

While reading from Boswell's Life of Johnson last night, Eric Scheske ran across this:

'But, (said he,) before leaving England I am to take a jaunt to Oxford . . .'

We realize Boswell wrote during the comma's halcyon days (if you want another example of ample comma use in the 18th century, read The Federalist), but we can't even begin to understand the use of the second comma in this phrase. If there are any grammar historians out there who can explain it, we'd love to hear from you (e-mail link on the left).