$(document).ready(function () {
  $('#main_menu a').hover(
    function() {
      //$(this).children('.menu_underline').fadeIn(300);
      elems = $(this).children('.menu_underline');
      elems.animate({width: "100%"}, { queue:false, duration:400 });
    }, function() {
      $(this).children('.menu_underline').animate({width: "0"}, { queue:false, duration:400 });
    });
});