function header_menu (menu_id) {
  var p = document.getElementById(menu_id);
  for (var i = 0; i < p.childNodes.length; i++) {
    if(p.childNodes[i].tagName == 'A') {
      var img = p.childNodes[i].firstChild;
      if(img) {
        img.onmouseover = function () {
          this.src = this.src.replace("_a.", "_b.");
        }
        img.onmouseout = function () {
          this.src = this.src.replace("_b.", "_a.");
        }
      }
    }
  }
}