Complex sentence: John Elliotson was a brilliant but erratic heart specialist became a phrenologist in the 1840s, he was also a mesmerist and combined the two into something he called phrenomesmerism or phrenomegnatism.
Split into simpler sentences: John Elliotson was a brilliant but erratic heart specialist who became a phrenologist in the 1840s. He was also a mesmerist and combined the two into something he called phrenomesmerism or phrenomegnatism.