Lee begins chapter 19 with Tom trying to put his arm up to take the oath because this simple action shows that Tom is disabled and therefore unable to use his left arm. This is extremely important in his case as it proves beyond doubt that Tom was unable to beat Mayella due to the bruises being on the right side of her face (her attacker punched with his left arm on to the nearest side of her face, which would be her right side as she was facing her attacker). When Tom is questioned during the trial it becomes extremely obvious that Tom is a truthful, elegant and truly kind soul and everybody knows it. Everybody now realises that Tom has done absolutely nothing wrong, all he did was try to help a lonely girl. Unfortunately this doesn't matter when the simple fact that a young, black man was accused of rape means that he is guilty in the eyes of those who are racist. The beginning of chapter 19 emphasises Tom's absolute innocence and as his hand falls helplessly off the bible, Lee wants us to recognise that Tom could not and would not attack Mayella. It was her father who attacked her.