Cannot read property 'childNodes' of undefined

I am running a request to an xml file like this :

function load()
{

var request=new XMLHttpRequest();
request.open("GET","response.xml",true);
request.send();

request.onreadystatechange=function()

{
if(request.readyState==4)
{
var x=request.responseXML; 
var y=x.getElementsByTagName("Limits")[0].childNodes[1];
var z=y.getElementsByTagName("Limits")[0].childNodes[0];
document.write(z)


}

}


}

The xml response.xml is this:

<Factory>
<Limits>
  <Point X="92" Y="489">hahahahff</Point>
  <Point X="570" Y="487">1111</Point>
  <Point X="570" Y="138">33333</Point>
  <Point X="92" Y="140">44444444</Point>
  <Point X="92" Y="139">5555555</Point>
</Limits>
<Cells>
  <Cell>
    <Point X="358" Y="138"/>
    <Point X="361" Y="487"/>
    <Point X="570" Y="487"/>
    <Point X="570" Y="138"/>
    <Point X="358" Y="138"/>
  </Cell>
  <Cell>
    <Point X="311" Y="139"/>
    <Point X="311" Y="488"/>
    <Point X="92" Y="489"/>
    <Point X="92" Y="140"/>
    <Point X="311" Y="139"/>
  </Cell>
</Cells> </Factory>

I get this error :

Cannot read property 'childNodes' of undefined

why is this happening ? what i am trying to achieve is print the value of the 2nd 'POINT' node WITHOUT using getElementsByTagName("POINT")[1]. Basically I want to 'enter' the Limits node, and print the 2nd value.