With two external displays in Mountain Lion how do I specify where the content of the Desktop is shown? I have connected two external displays to a Macbook and have deselected 'Mirror Displays'. The lid of the Macbook is closed, therefore I am using just the larger screens. The contents of my desktop are currently only shown on screen 2.

However, I would like to tell the OS to not show the contents of the Desktop on screen 2, but on screen 1. How can I achieve that?

Sorry, if this is a duplicate, I couldn't find any question about the desktop contents.
In my experience for Mountain Lion, determining which display is the primary display is usually achieved by dragging the white Menu Bar (circled in screenshot below) onto the graphic of another display, thus setting the other display as the primary monitor.