if (document.images) {

var menu1on = new Image();
menu1on.src = "menu/1b.gif";
var menu1off = new Image();
menu1off.src = "menu/1a.gif";

var menu2on = new Image();
menu2on.src = "menu/2b.gif";
var menu2off = new Image();
menu2off.src = "menu/2a.gif";

var menu3on = new Image();
menu3on.src = "menu/3b.gif";
var menu3off = new Image();
menu3off.src = "menu/3a.gif";

var menu4on = new Image();
menu4on.src = "menu/4b.gif";
var menu4off = new Image();
menu4off.src = "menu/4a.gif";

var menu5on = new Image();
menu5on.src = "menu/5b.gif";
var menu5off = new Image();
menu5off.src = "menu/5a.gif";

}

function On(name) {
  if (document.images) {
    document.images[name].src = eval(name + 'on.src');
  }
}

function Off(name) {
  if (document.images) {
    document.images[name].src = eval(name + 'off.src');
  }
}