subNames = new Array('back',
                     'next');

subOvrImgs = new Array('../images/nav-back-o.gif',
                       '../images/nav-readon-o.gif');

subOutImgs = new Array('../images/nav-back.gif',
                       '../images/nav-readon.gif');

subImgCnt = subNames.length;
subOvrPics = new Array(subImgCnt);
subOutPics = new Array(subImgCnt);

for (var i=0 ; i<subImgCnt ; i++)
{
  subOvrPics[i] = new Image;
  subOvrPics[i].src = subOvrImgs[i];
  subOutPics[i] = new Image;
  subOutPics[i].src = subOutImgs[i];
}

function swSub(snum, state)
{
  if (document.images){
    var subImgName = subNames[snum];
  
    if (state == 'b')
    {
      document[subImgName].src = subOvrPics[snum].src;
    }
    else if (state == 'a')
    {
      document[subImgName].src = subOutPics[snum].src;
    }
  }
}