In Harper Lee's To Kill a Mockingbird, the very fact that Atticus asks Sheriff Heck Tate, while questioning him on the witness stand, if a doctor had been summoned the evening of the crime indicates that calling a doctor was standard procedure for a crime of that nature, even in that time period. A doctor's evidence would have been necessary to prove the crime actually took place, and it is illegal in the US Court system to try a crime without evidence that a crime actually happened. We call this principle corpus delicti, which translates from the Latin to mean "body of the crime" (West's Encyclopedia of American Law, ed. 2). One example of applying corpus delicti is that a murder cannot be tried unless a body has been found. Hence, the very fact that Bob Ewell did not summon a doctor implies his own guilt and shows Sheriff Tate's incompetency in criminal investigations.
Throughout the trial, Atticus uses his cross-examinations to point to the likelihood of Ewell's guilt in the crime, not Tom Robinson's. Ewell particularly looks guilty when he demonstrates before the court that he is left-handed, whereas Robinson is crippled in both his left arm and hand. Left-handedness is important circumstantial evidence in the case because Sheriff Tate testifies that Mayella had been bruised on the right side of her face, and only a left-handed man facing her could have made those injuries. Furthermore, Robinson testifies that, as he fled from the Ewells' house, he heard Ewell shout to Mayella, "[Y]ou goddamn whore, I'll kill ya" (Ch. 19). Robinson's testimony implies that Ewell attacked Mayella because he had witnessed Mayella try and kiss Robinson, an African-American man. Atticus shows he understands that all evidence points to Ewell's guilt when he asks Mayella during cross-examination, "Who beat you up? Tom Robinson or your father"? (Ch. 18). Hence, as we can see, one reason why Ewell did not call a doctor is because he was guilty of causing Mayella's bruises, not Robinson.
Evidence revealed during Atticus's cross-examination of Mayella and examination of Robinson also implies that Ewell is guilty of sexually assaulting Mayella. Mayella's mother has been deceased for longer than Mayella can remember, yet seven children, some of them very young, are also part of the Ewell clan. In addition, while on the witness stand, Robinson testifies that Mayella said to Robinson that she had never kissed a man before and "what her papa do to her don't count" (Ch. 18). The evidence implies that Mayella is the incestuous mother of at least some of those young children. Therefore, a doctor's examination would not have revealed any injuries or other evidence from recent sexual assault because none took place that evening; however, it would have revealed that Mayella was an illegitimate mother, and Ewell was well aware of those facts.
Hence, it can be said that Ewell did not send for a doctor because he knew a doctor's examination would reveal his own guilt.