After Mr. Gilmer is finished questioning Sheriff Tate about the events that transpired on the evening of November 21st, Atticus asks him if he called for a doctor. Tate tells Atticus that he felt like there was no need to call a doctor even though Mayella was "mighty banged up." Atticus repeats the question by asking Heck Tate why he didn't call a doctor. Later on, Atticus asks Bob Ewell the same questions concerning why he didn't call a doctor. Atticus was attempting to emphasize the fact that without an examination from a doctor, there is no evidence to suggest that Mayella was a victim of sexual assault, highlighting the fact that the prosecution's arguments are strictly based on the testimonies of the Heck Tate and the Ewells.
Atticus then begins to question Sheriff Tate regarding the specific location of Mayella's injuries and Tate tells Atticus that she was beaten around her head. Sheriff Tate says,
"...she was pretty bruised up when I got there, and she had a black eye comin’" (Lee 104).
Atticus asks Tate which eye was beginning to bruise, and he says,
"It was her right eye, Mr. Finch. I remember now, she was bunged up on that side of her face....” (Lee 104).
The significance of the location of Mayella's bruises suggests that an individual who led predominately with their left hand was responsible for inflicting her injuries.
Atticus then asks Heck Tate to describe Mayella's other injuries. Tate tells Atticus that Mayella had excessive bruises encircling her neck. Atticus then asks, "All around her throat? At the back of her neck?" (Lee 105). Tate confirms that the bruises were around her entire neck. This information is also significant because it reveals that only an individual with two functioning hands would have been capable of choking Mayella's neck all the way around. Atticus understands that Tom Robinson is crippled and is physically not capable of punching Mayella with his left hand, nor is he able to choke her using both hands. His line of questioning reveals that there is essentially no evidence to convict Tom Robinson and that Mayella's injuries were possibility inflicted by a left-handed individual who also had two functioning hands capable of choking her.
As explained in the previous answer, we learn from Tate's evidence that the nature of Mayella's injuries would appear to suggest that she was attacked by someone who mostly used his left hand, and that he also put both hands around her throat. In the light of this evidence, the fact that Bob Ewell, Mayella's father, is left-handed takes on great significance. Atticus emphasizes Bob's left-handedness in court, although Bob, who appears decidedly dim-witted at this point, fails to see the implications.
Jem is excited; he exults that 'we've got him' (referring to Bob Ewell). However Scout at this stage is more sceptical; she thinks Tom might be left-handed too and that 'Jem was counting his chickens'. But then when Tom's cross-examination begins, she sees him clearly for the first time and realizes that he has a badly crippled left arm - as the result of catching it in some machinery when he was young. To any impartial jury, this would rule out Tom Robinson as a suspect. Of course, the point is that it is not an impartial jury, but rather one blighted by racial prejudice who goes on to convict Tom simply because he is black. Had he not been black, he likely would not even have been charged.
Tate's evidence on the nature of Mayella's injuries, then, goes right to the heart of the matter, and appears to point to Mayella's own father Bob as being her attacker; yet this evidence still proves insufficient to acquit Tom Robinson.
Questioned by the prosecutor, Harry Gilmer, Sheriff Tate testified that he was "fetched" by Bob Ewell on the "night of November twenty-first" who claimed that "some nigger'd raped his girl." Heck found Mayella "lying on the floor... pretty well beat up." Mayella told the sheriff that it was Tom Robinson who had beat her and "took advantage of her." Under cross-examination, Atticus determined that no doctor had been called to examine Mayella, and that she had been beaten on the right side of her face. Additionally, Heck claimed that there "were definite finger marks on her gullet... all around" her throat.
The two latter points were important since Tom Robinson's left arm was crippled and useless. The marks on the right side of Mayella's face could only have been made by a left-handed person's punch; likewise, the finger marks that went all the way around her throat could only have been made by a person with two good, strong hands.