if (document.images) {
   headeron=new Image();
   headeron.src="images/header-anim.gif";
   homeon=new Image();
   homeon.src="images/tab-home-on.gif";
   prologueon=new Image();
   prologueon.src="images/tab-prologue-on.gif";
   earthon=new Image();
   earthon.src="images/tab-earth-on.gif";
   airon=new Image();
   airon.src="images/tab-air-on.gif";
   fireon=new Image();
   fireon.src="images/tab-fire-on.gif";
   wateron=new Image();
   wateron.src="images/tab-water-on.gif";
   epilogueon=new Image();
   epilogueon.src="images/tab-epilogue-on.gif";
   sequelaeon=new Image();
   sequelaeon.src="images/tab-sequelae-on.gif";
   nexton=new Image();
   nexton.src="images/next-on.gif";

   headeroff=new Image();
   headeroff.src="images/header-off.gif"
   homeoff=new Image();
   homeoff.src="images/tab-home-off.gif";;
   prologueoff=new Image();
   prologueoff.src="images/tab-prologue-off.gif";
   earthoff=new Image();
   earthoff.src="images/tab-earth-off.gif";
   airoff=new Image();
   airoff.src="images/tab-air-off.gif";
   fireoff=new Image();
   fireoff.src="images/tab-fire-off.gif";
   wateroff=new Image();
   wateroff.src="images/tab-water-off.gif";
   epilogueoff=new Image();
   epilogueoff.src="images/tab-epilogue-off.gif";
   sequelaeoff=new Image();
   sequelaeoff.src="images/tab-sequelae-off.gif";
   nextoff=new Image();
   nextoff.src="images/next-off.gif";
   }

function imgOn(imgName) {
   if (document.images) {
   document[imgName].src=eval(imgName + "on.src");
   }
}

function imgOff(imgName) {
   if (document.images) {
   document[imgName].src=eval(imgName + "off.src");
   }
}