nombre = navigator.appName;
version = parseInt(navigator.appVersion);

if ((nombre == "Netscape" && version >= 3) || (nombre == "Microsoft Internet Explorer" && version > 3)) 
ver = "correcta";
        else ver = "incorrecta";

if (ver == "correcta") {

tra = new Image();
tra.src = "images/includes/bot_tr_in.gif";
trb = new Image();
trb.src = "images/includes/bot_tr.gif";


tr1a = new Image();
tr1a.src = "images/includes/bot_tr1_in.gif";
tr1b = new Image();
tr1b.src = "images/includes/bot_tr1.gif";


alqa = new Image();
alqa.src = "images/includes/bot_alqui_in.gif";
alqb = new Image();
alqb.src = "images/includes/bot_alqui.gif";


}

function abajo(imgName) {
        if (ver == "correcta") {
        document[imgName].src = eval(imgName + "a.src");
}
}
function arriba(imgName) {
        if (ver == "correcta") {
        document[imgName].src = eval(imgName + "b.src");
}
}