As the previous educator mentions, Atticus discredits Bob Ewell's testimony in court by proving that a right-handed man like Tom (who also has a disabled left arm) physically could not have hit Mayella on the right side of her face. She also had marks all around her neck, not just on one side, which shows that someone other than Tom must have wrapped both hands around her neck to choke her. The only other person with Mayella that night was her father before the sheriff came to investigate, so that implies that Mr. Ewell probably beat up his own daughter. That's pretty shameful!
Another sad fact that Atticus brings to light is that Bob Ewell's first reaction wasn't for the safety of his daughter, but for "justice" for what he supposedly saw happening between Mayella and Tom. When Atticus asks Bob why he didn't get immediate medical attention for Mayella, Scout summarizes what Bob says in the following quote:
The witness said he never thought of it, he had never called a doctor to any of his'n...
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