window.onload = function() {
  var navigator = document.getElementById('navigator');
  var subdivs = navigator.getElementsByTagName('div');
  for (var i = 0; i < subdivs.length; i++) {
    if (subdivs[i].className.indexOf('subdivision') >= 0) {
      subdivs[i].display =  subdivs[i].style.display = 'none';
      subdivs[i].parentNode.onmouseover = toggle_menu;
      subdivs[i].parentNode.onmouseout = toggle_menu;
    }
  }
}

function toggle_menu() {
  // Получаем объект Event, либо с помощью DOM, либо с помощью модели событий IE
  var eventObj = arguments[0] || window.event;
  var main_table = document.getElementById('text_container');
  for (var i = 0; i < this.childNodes.length; i++) {
    var child = this.childNodes[i];
    if (child.className && child.className.indexOf('subdivision') >= 0) {
      if (child.display == 'none' && eventObj.type == 'mouseover') {
        child.display = child.style.display = 'block';
        main_table.style.zIndex = "-1";
      }
      else if (child.display == 'block' && eventObj.type == 'mouseout') {
        child.display = child.style.display = 'none';
        main_table.style.zIndex = "1";
      }
    }
  }
}