How did Rab's death lead to the discevery of a new hand for Johnny?

As Johnny holds the musket, Dr. Warren gets a good look at his hand for the first time. He explains that he can operate on the hand to give Johnny back the use of his thumb. However, Johnny will have to be brave enough to allow Dr. Warren to cut away the scar tissue. Johnny agrees and Dr. Warren says he has time to do the operation now, if only Johnny will take a walk and get some fresh air while he gets his instruments ready. As Johnny walks around the Lexington countryside, he sees the women cleaning up the mess left behind by the battle, and some haggard but determined Minute Men marching toward Charlestown. He feels proud of his country and its people.