help with dhtml/javascript statements

Hi,
Could someone be so kind and give me a help with the following:

I want to get some data from a give number of tables.
So I use:

k = indssa.rows[10].cells[i].innerText;
alert(k)

to get the value that I want, from table indssa. For example, Alert 824!.

Problem is, when I change indssa to a varable, 'a' , that was defined as an array, it doesn´t work any more.

var tab = new Array("indssa","indrec")
a = tab[0]
k = a.rows[10].cells[i].innerText;
alert(k)

the error is: 'rows.10' is not an object.
I already tried define 'a' like:

a = document.createElement(tab[0]);

that won´t help. Ther is an small sample of this problem at http://www.smachado.com.br/mis/mis2.htm,
problem is in line 139/140

Tahnks,

Ricardo Machado