In the 1962 movie, Atticus tosses a glass to Tom, who catches it with his right hand. When Atticus asks him to catch it with his left, he explains that he cannot--his left arm got caught in a cotton gin as a child, and the muscles were torn from the bone. His left arm is useless. This scene does not appear in the book. There Atticus simply asks Tom to stand up, and it is revealed that his left arm is shriveled and weak, much shorter than the right. Reverend Sykes explains the origins of Tom's injury to Jem and Scout (and to the reader). In both the book and the film, the point is clear. Atticus has cast serious doubt on the testimony of both Bob and Mayella by showing that the bruises on the right side of Mayella's face almost certainly could not have come from a punch thrown by Tom.