Atticus asks Mr. Ewell if he called a doctor for Mayella. What point is he trying to make?

This is from Chapter 17 in the
book.

What Atticus is trying to do here is to prove that
Bob Ewell knew exactly how hurt Mayella was.  He knew exactly what had happened to her
because he was the one who beat her.  He beat her after seeing her hugging Tom
Robinson.

If he had really thought that Robinson had beaten
and raped her, he would have called for a doctor to find out how badly she was hurt. 
But he did not need to do that because he already knew.  That is what Atticus is trying
to show.