How to get info out of DOM tree?

Now that I have read the following XML document into memory.  What DOM API
can I use to parse out the content of the ProcessingInstruction and the
DocumentType?

thanks

<?xml version="1.0"?>
<!DOCTYPE doc SYSTEM "some.dtd">
<doc>
  <Title>
   Document Title
  </Title>
  <Name>
   Name
  </Name>
</doc>