Hermann Pagenstecher, 1844-1932, ophthalmologist. https://jamanetwork.com/journals/jamaophthalmology/article-abstract/610068
The graph displays the other subjects mentioned on the same pages as the subject “Pagenstecher”. If the same subject occurs on a page with “Pagenstecher” more than once, it appears closer to “Pagenstecher” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.