Javascript Callback/Object Conflict

I have the following Javascript code

function Obj()
{
obj.squares = new Array();
}
Obj.prototype.Load= function(xdoc)
{
var goat = "head";
xdoc.ProcessNodes("squares/square", function(node)
{
alert(goat); // <-- this works fine
this.squares.push(
// do processing on node, create a square
);
});
}

Now the xdoc is another object I have, with a method called
ProcessNodes. The only important thing to know about ProcessNodes is
that it does some things, and calls its second argument (which is a
function) successively.

The problem I am running into here is once I get into the callback
function, "this.squares" is returning null ... even though it has been
initialized to an Array. For whatever reason, its like the object loses
scope when I get into the callback function. For all other intents and
purposes however, the callback function has the same scope as the
calling block (i.e. the goat variable is still accessible).