function MausDrauf(Nummer) {
var oldPic = document.images[Nummer].src;
var off = oldPic.lastIndexOf("off");
if (off != -1) { /* Sicherheitsabfrage */
var newPic = oldPic.substring(0,off) + "on";
document.images[Nummer].src = newPic + ".gif";}}

function MausWeg(Nummer) {
var Bild = document.images[Nummer].src;
var on = Bild.lastIndexOf("on");
if (on != -1) { /* Sicherheitsabfrage */
var newPic = Bild.substring(0,on) + "off" + Bild.substring(on + 2, Bild.length);
document.images[Nummer].src = newPic;}}