Complex sentence: The use of smoke enemas in Western medicine declined after 1811, when through animal experimentation Benjamin Brodie demonstrated that nicotine--the principal active agent in tobacco smoke--is a cardiac poison that can stop the circulation of blood.
Split into simpler sentences: The use of smoke enemas in Western medicine declined after 1811. In that year, Benjamin Brodie showed through animal experimentation that nicotine, the principal active agent in tobacco smoke, was a cardiac poison prone to stopping the circulation of blood.