Replacing A Div Content With Content From Another File

I am working on a page, which has menus on the left side in one div, that, when clicked should display a proper content on the right side, in another div. Since the content is rather long I have it in a separate file, so I am trying to come up with a function that will replace the right div contect with that external file content. I believe the function i should use looks something like this:

function ReplaceDiv( DivId, NewData )
{
var data_div = document.getElementById( DivId);
if( data_div )
{
data_div.innerHTML = NewData;
}
}

I tried to use this in my page as following: <a href="" onclick="ReplaceDiv(MyDiv, NewDataFile.php )">Menu Item #1</a> But it does not do what I need it to do. The menu link disappears and the right div content stays the same