Insert JavaScript & Function Call via DOM

I am attempting to insert a Javascript source via the DOM (which I've figured out), and then insert a Javascript tag to call a function in that JavaScript source.

I'm finding that when I do this via the DOM, the function fails because the source is not fully loaded.

How can I fix this?  Do I need to wait till the source is fully loaded before I inject the function call?

var _script = document.createElement("script");
_script.src = "http://cluster.pppp.com/includes/z.js";
(document.getElementsByTagName("head")[0] || document.documentElement).appendChild(_script);
var headID = document.getElementsByTagName('body')[0];
var _script2 = document.createElement("script");
_script2.innerHTML = 'aarun(22664,4870);';
_script2.type = 'text/javascript';
headID.appendChild(_script2);

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