See the documentation for jQuery Event Target. Using the target property of the event object, you can detect where the click originated within the #menu_content element and, if so, terminate the click handler early. You will have to use .closest() to handle cases where the click originated in a descendant of #menu_content.
$(document).click(function(e){
// Check if click was triggered on or within #menu_content
if( $(e.target).closest("#menu_content").length > 0 ) {
return false;
}
// Otherwise
// trigger your click function
});