jquery 嵌套问题，急！在线等大虾解答

/代码如下：
/ 点击菜单滑动下拉，焦点不在下滑界面自动回收
        $(document).ready(function () {
            $(".TopMenuBar li").hover(function () {
               
                    $(this).children(".SecondMenuBar").slideDown();
                    $("#zg_Iframe").css('display', 'block');
                                  }, function () {
                
                    $(this).children(".SecondMenuBar").slideUp();
                    $("#zg_Iframe").css('display', 'none');
                                        
                }
        )
      })
  貌似hover不只对TopMenuBar li 起作用，连'SecondMenuBar'下面的子li也起作用了，现在只想让这个hover只对外层TopMenuBar li起作用
页面布局如下：<div id='AOPTopMenu' class='TopMenuBar' >
<ul>
   <li>一级菜单
     <div ID='Tb_TopMenu_1' class='SecondMenuBar'>
         <ul>
           <li>二级菜单
            <div  ID='Tb_SecondMenu_1' class='ThirdMenuBar'>
              <ul>
                <li>三级菜单<li>
              </ul>
           </div>
          </li>
         </ul>
    </div>
   </li>
 </ul>
</div>