recursive queries for subdomains

[saruman7]

We are trying to setup bind so that our company's external facing dns
servers will do recursive lookups to internal subdomains that have
their own dns servers that are not world accessible, but we do not want
the external dns servers to be able to do recursive lookups for other
domains (i.e. yahoo.com, cnn.com, etc.)  Can someone tell me how to set
up recursive lookups in this manner?

Currently our external dns servers have all recursion turned off with
this setting in the named.conf file:

allow-recursion { none; };