function sel_menu(divItem,cat){
      var nAleatorio=Math.floor(Math.random()*1000000000);
      var opt = {
      method: 'post',
      parameters: 'cat=' + cat,
      onSuccess: show,
      onFailure: function(t)  { alert ('Error'); }
      }
      new Ajax.Request('../sel_cat.php?aleatorio='+nAleatorio, opt);
}
function show(obj)
{
$("divcat").innerHTML="";
$("divcat").innerHTML   = obj.responseText;
}