function  change_nav(heath_center){
    for(var i=1;i<=6;i++){
        if(i==heath_center){
            document.getElementById("nav_item"+i).className="green2";
			document.getElementById("nav_tab"+i).style.display="";
        }
        else{
            document.getElementById("nav_item"+i).className="";
			document.getElementById("nav_tab"+i).style.display="none";
        }
    }
}