Why did Huck go to fetch a doctor?

Huck fetched a doctor because Tom had been shot in the leg. 

I knowed he was white inside, and I reckoned he'd say what he did say—so it was all right now, and I told Tom I was a-going for a doctor.  He raised considerable row about it, but me and Jim stuck to it and wouldn't budge; so he was for crawling out and setting the raft loose himself; but we wouldn't let him.  Then he give us a piece of his mind, but it didn't do no good.