if (document.images) { 

topnav_homeon = new Image();
topnav_homeon.src = "../images/topnav_home_on.gif";

topnav_homeoff = new Image();
topnav_homeoff.src = "../images/topnav_home_off.gif";

topnav_facilitieson = new Image();
topnav_facilitieson.src = "../images/topnav_facilities_on.gif";

topnav_facilitiesoff = new Image();
topnav_facilitiesoff.src = "../images/topnav_facilities_off.gif";

topnav_husbandryon = new Image();
topnav_husbandryon.src = "../images/topnav_husbandry_on.gif";

topnav_husbandryoff = new Image();
topnav_husbandryoff.src = "../images/topnav_husbandry_off.gif";

topnav_availableon = new Image();
topnav_availableon.src = "../images/topnav_available_on.gif";

topnav_availableoff = new Image();
topnav_availableoff.src = "../images/topnav_available_off.gif";

topnav_linkson = new Image();
topnav_linkson.src = "../images/topnav_links_on.gif";

topnav_linksoff = new Image();
topnav_linksoff.src = "../images/topnav_links_off.gif";

topnav_contacton = new Image();
topnav_contacton.src = "../images/topnav_contact_on.gif";

topnav_contactoff = new Image();
topnav_contactoff.src = "../images/topnav_contact_off.gif";

}

function on(imagename, id) {
    if (document.images) {
		document[ imagename ].src = eval( imagename + "on.src" );
    }
  }
function off (imagename, id) {
	if (document.images) {
		document[ imagename ].src = eval( imagename + "off.src" );
	}
}