In Three Men in a Boat by Jerome, what happened during the incident of the narrator's visit to the library where he consulted a medical book?

The narrator, J., tells us this story near the beginning of Chapter I. He had once gone to the British Museum to look up something in a medical reference book. After he found the answer he had been searching for, he began to page through the book at random. To his horror, he discovered that he was suffering from most of the maladies outlined there, from “ague” to “zymosis.” The only ailment for which he demonstrated none of the symptoms was “housemaid’s knee.” He pokes fun at himself, saying that he could be a full-blown study subject for medical students. With this first tale, the narrator shows us that he has a wry sense of humor, and that he may also be quite susceptible to suggestion, to the point of being a hypochondriac.