What does Atticus attempt to establish through Heck Tate's testimony?

Atticus wants to establish there was never a doctor called to verify Mayella's injuries and to get Heck's description of her injuries (on which side of her body were the majority of the bruises). Later he proves that Tom Robinson couldn't of assaulted Mayella because the side of her face that the bruises were on (the right) would of required Tom to hit her with the arm that he disabled years ago (the left), and could no longer use. After that he proves that Bob Ewell is left handed, and he was far more likely to of been the one to hit her.