How to extend a bash built-in but still use it within the new function?

[Chris Green]

I want to extend the 'cd' command slightly so that I can execute some
specific actions when entering (and leaving) a particular directory
hierarchy.

So I need to create a bash function called 'cd' to do this, but I
still need to call the underlying bash built-in cd.  Is there any way
of doing this?

-- 
Chris Green