How does the theme of obsession arise within the story about Verheyen?

Verheyen's leg must be amputated after a scratch from a nail becomes infected. After his recovery, Verheyen returns to his studies, showing a remarkable talent for sketching human anatomy. Soon he is instructing students, including Van Horssen, as they perform autopsies. During the visits, however, Van Horssen notices that Verheyen is becoming reclusive and “increasingly obsessed” with his amputated leg. He has “crossed over an invisible boundary, a point of no return” (196).