Complex sentence: Afterwards he attended the lycée Louis Barthou in Pau (Aquitaine) in 1863, where he attended classes in rhetoric and philosophy (under and uppergreat), did well in arithmetic and drawing, and already showed extravagance in thinking and style.
Split into simpler sentences: Afterwards he attended the lycée Louis Barthou in Pau (Aquitaine) in 1863. He attended classes in rhetoric and philosophy (under and uppergreat), did well in arithmetic and drawing, and already showed extravagance in thinking and style.