Does the top tube typically open into the head tube? This must be a very basic question, but as it often happens with very basic questions, it is often hard to find the answer by searching.

On a modern bike with metal (welded) frame, is the interior of the top tube (and/or the down tube) accessible through the head tube? I.e does the head tube have holes in it that lead into the other attached tubes of the frame?

From what I could see on the Net, it appears that the head tube is typically a full cylindrical tube, without any holes. Is that so?

Meanwhile, it seems that I have some parts rattling inside the top tube of my bike (Specialized TriCross), which routes the rear brake cable through the top tube. I wonder how they got there, if the interior of the top tube is not accessible (aside from two fairly small cable holes/openings)
Any welded tubes are going to have holes in their ends in order to let the hot gases escape. So, yes, there is virtually always going to be a hole connecting the interior of the top tube and the head tube (along with every other tube on the bike).