wikipedia shows the same rise of doctor per capita that look suspiciously the same as the lawyer rise: https://en.wikipedia.org/wiki/File:Physicians_in_the_United_States_per_10,000_people_(1850-2009).svg
While in both cases the growth of the profession started at similar times, doctors seem to have doubled while lawyers quadrupled in the same timeframe.
Hm. Very interesting.
EDIT: I just remembered that I think this is mentioned in The Rise and Fall of American Growth and that it was attributed to an increase in specialization