Accessing an objects parent object

I have an object, but need to get the handle for that object's parent, for example

<script>
function x(obj){
    obj=obj.parent;
    // do stuff to the parent object
}
</script>
<div><span onmousedown="x(this)">This is a span inside a div</span></div>

In that example, when the function "x" is executed it is passed the handle for the span, but I need the handle for the div that contains the span. Unfortunately what I have does not work, any ideas?