How to extract numbers from the middle of a string of numbers ?

I have a string of 19 digits but when I enter the 19 digits into a form in access I want the field to display only the 9 digits located in the middle of the string which means that the nine digits are bound by 7 digits on the left and 2 digits on the right.
I think I need a VB code in order to make this work.
Any help will be appreciated.