function nav_cat() {
   var w = document.cat.catlist.selectedIndex;
   var url_add = document.cat.catlist.options[w].value;
   window.location.href = url_add;
}

function nav_artist() {
   var w = document.artist.artistlist.selectedIndex;
   var url_add = document.artist.artistlist.options[w].value;
   window.location.href = url_add;
}

function nav_period() {
   var w = document.period.periodlist.selectedIndex;
   var url_add = document.period.periodlist.options[w].value;
   window.location.href = url_add;
}