What happened to Jem when he was nearly thirteen?

The novel begins with Scout stating that when Jem was nearly thirteen, he "got his arm badly broken at the elbow." She goes on to say that his left arm hung slightly shorter than his right and that Jem was seldom self-conscious about his injury. Scout mentions that she and Jem sometimes look back and discuss the events that lead to Jem's injury. The remainder of the novel tells the story leading up to how Jem broke his arm. When Jem's father, Atticus, defends Tom Robinson in court, he destroys Bob Ewell's reputation. Bob Ewell, the novel's antagonist, seeks revenge on Atticus. At the end the novel, Bob Ewell attacks Jem and Scout while they are walking home from the Maycomb Halloween festival. In the midst of the struggle, Jem breaks his arm. Fortunately, the reclusive Boo Radley saves the children by stabbing Bob Ewell and carrying Jem home.