Complex sentence: In 1813 he became head of the mental asylum there, working to reform patient treatment resulting in more humane care and conditions, and spending much of his own salary on improving the lives of individual patients and maintaining the hospital.
Split into simpler sentences: In 1813 he became head of the mental asylum there, and in that capacity was instrumental in modifying patient treatment resulting in more humane care and conditions. He devoted the greater part of his salary in improving the lives of individual patients and in maintaining the hospital.