fixed header,sidebar, footer with scrollable center for text, using menu link to load an outside html file into scrollable arae only

Question

I like the structure used in another article fixed header, footer and sidebars with scrolling content area in the middle. I want to put a menu with links in the left sidebar that load other html pages into the scrollable content area. My links aim at the name of the center region but the load replaces the entire page rather than the subsection. Now the sections are all defined as classes in the css and I don't know how to identify it correctly to the link.

This is my menu `` `` Page 1 Page 2 my scrollable section content1
content2
content1
I expected when I clicked a menu item it would load the html file into scrollable area instead it replaced whole page.