The novel opens with the injury Jem suffers to his arm. The incident does not occur till the end of the novel (Chap 28 , P 289).
This gives the whole story a circular feeling, and establishes the novel as one dealing with memory, in particular , childhood memories.The mention of Jem s injury prefigures other "injuries" and handicaps in the novel:
Atticus is blind in one eye
Calpurnia is short sighted
Tom Robinson has one arm shorter than the