Place the numbers

The numbers 1-9 are placed in a 3 by 3 grid. For any such arrangement, let the medians of the three rows be $a_1$ , $a_2$ and $a_3$ , and let $m$ be the median of $a_1$ , $a_2$ and $a_3$ . If $m=5$ , then how many possible arrangements of the numbers are there?