How To Separate This String

I have an html form and an icon, if i click on the icon, a new pop-up
window is open and shows a list of numbers with a structure like this
:
x.xx.xxx.xxxx.

Now, this numbers are between an <a href> tag and if i clic on one of
this, i call a javascript function, the idea is to put each one number
separate for the "." in a textbox, so i did it before but putting it
in a single input. Now i need to separate each one of this numbers and
put it in each one of the textbox. If i have to put it in a single
input i´ll do something like this:

<script language="javascript">
function Funcion(val)
{
//val value is 1.12.123.1234
window.parent.opener.document.txtCtaNiv.value=val;
top.parent.window.close();
}
</script>

The code above puts the value (that previosuly i selected in the popup
window) in the input field of the main form. That's easy.

Now if the parameter val has the value 1.12.123.1234 and in the main
form i have four input fields called txtCtaNiv1, txtCtaNiv2,
txtCtaNiv3, txtCtaNiv4, how can i separate the parameter val?, so i
can put the 1 in txtCtaNiv1, the 12 in the input field txtCtaNiv2, the
123 in txtCtaNiv3 and so on?