What is Mr. Tate's story in Tom's trial in To Kill a Mockingbird?

In Chapter 17, Mr. Heck Tate, the sheriff, has been called as the first witness in Tom Robinson's trial. He is questioned by Mr. Horace Gilmer, the prosecuting attorney.

Accordingly, he testifies that he was summoned by an agitated Bob Ewell to the family residence on the night of November twenty-first. When he gets to the house, he finds Mayella Ewell on the floor, in the middle of the living room. He helps Mayella up, and she washes her face. When questioned, she tells the sheriff that Tom Robinson beat her up and raped her. Because of her accusations, Mr. Tate arrests Tom.

After Mr. Gilmer finishes with Mr. Tate, the sheriff is questioned by Atticus, who wants to know whether anyone bothered to call the doctor that night. Mr. Tate admits that no one did, as her injuries were pretty self-evident. However, this seems to be a poor answer. Atticus asks the question two more times, making Mr. Tate a little irritated in the process. It appears that Atticus' aim is to get the jury to question whether there was possible negligence on someone's part on the night of November twenty-first. Also, if there was negligence, since no one called the doctor, the next question would hinge on the reasons why.

Atticus then asks Mr. Tate to describe Mayella's injuries. The sheriff testifies that Mayella was badly beaten up; she had injuries on her arms, her head, and her neck. By the time he saw her, a black eye was already forming. Atticus asks Mr. Tate which eye was bruised, and Mr. Tate initially states that it was Mayella's left eye. However, Atticus wants to know whether the eye he is referencing was the one facing his own left eye or Mayella's left from her position. In the end, Mr. Tate testifies that it was actually Mayella's right eye which was bruised.

Mr. Tate is then anxious to describe Mayella's other injuries in fuller detail, and Atticus lets him. However, Mr. Tate's testimony about Mayella's bruised right eye raises other pertinent questions: as he instinctively looks at Tom Robinson, Harper Lee tells us that 'something had suddenly been made plain to him.' This fitting moment foreshadows a future moment when we are led to question Tom Robinson's supposed guilt in Mayella's abuse.