$$('.nav_button').each(function(el)
{
	Event.observe(el, 'mouseover', function(event)
	{ 
		buttonId = el.id.replace('nav_', '');
		$('button_left_' + buttonId).className = 'button_left_over';
		$('button_right_' + buttonId).className = 'button_right_over';

	});
	Event.observe(el, 'mouseout', function(event)
	{ 
		buttonId = el.id.replace('nav_', '');
		$('button_left_' + buttonId).className = 'button_left';
		$('button_right_' + buttonId).className = 'button_right';
		
	});	
});