How to add an element in the middle of content

I am trying to add a new element (HTML tag) into the content of a page via java script DOM, the tricky thing is that i would like to add it into the middle of content not just append it before or after a existing node.

for example, I would like to change this

<div>hello this is some sample text to illustrate a point</div>

to this using the DOM

<div>hello this is some <b>sample text</b> to illustrate a point</div>

any ideas?
Thanks

 

use this;
https://developer.mozilla.org/en/DOM/element.insertBefore

as to finding the middle;
var c = containerHTMLelement;
var middle = c.childNodes[Math.round(c.childNodes.length/2)];
c.insertBefore (newElement, middle);