JS部分代码：

    tabChange()
    function tabChange(){
      var Nav = document.getElementsByClassName("main_nav")[0].getElementsByTagName("div");
      var Content =  document.getElementsByClassName("content")[0].getElementsByTagName("div");
        for(var i=0;i<Nav.length;i++)
        {
            Nav[i].index = i ;         //赋值序号
            tabclick(Nav[i]);      //执行点击方法
            ContentClick(Content[i]);   //执行内容
        }
       //最外面的序号
        function tabclick(obj) {
        obj.onmouseover = function(){
            for(var m=0;m<Content.length;m++)
            {
                Content[m].style["display"] = "none";
                Nav[m].className = "";
            }
            this.className = "active";
            Content[this.index].style["display"] = "block";
        }
        }
        //最外面结束
        //最里面的
        function ContentClick(obj)
        {
                for(var m=0;m<Content.length;m++)
                {
                    obj.getElementsByClassName("content_nav")[0].getElementsByTagName("li")[m].index = m ;
                    obj.getElementsByClassName("content_nav")[0].getElementsByTagName("li")[m].onmouseover = function(){
                        for(var i=0;i<obj.getElementsByClassName("content_content")[0].getElementsByTagName("li").length;i++)
                        {
                            obj.getElementsByClassName("content_content")[0].getElementsByTagName("li")[i].style["display"] = "none";
                            obj.getElementsByClassName("content_nav")[0].getElementsByTagName("li")[i].className = "";
                        }
                        this.className = "active";
                        obj.getElementsByClassName("content_content")[0].getElementsByTagName("li")[this.index].style["display"] = "block";
                }
            }
        }
        //最里面的结束
    }

问 代码中的 obj是怎么来的？