Complex sentence: Based on his studies and those of Aristotle, Galen determined that the body had three systems which were the natural spirit (veins and liver which provided nutrition), the vital spirit (heart which gave injury) and the animal spirit.
Split into simpler sentences: Based on his own studies as well as those of Aristotle, Galen determined that the body had three systems. He called them the natural spirit (veins and liver which provided nutrition), the vital spirit (heart) and the animal spirit.