arrowOn = new Image();
arrowOn.src = "i/ar_on.gif";
arrowOff = new Image();
arrowOff.src = "i/ar_off.gif";

function swapNav(name) {
  document[name].src = (document[name].src.indexOf('_on') != -1) ? 'i/ar_off.gif' : 'i/ar_on.gif';
}