How to retrieve the comment for a constraint?

Hallo all,

using at least PostgreSQL 7.4.2, one can use the command
    COMMENT ON CONSTRAINT <constraint_name> ON <table_name>;
to document a constraint defined via
         CONSTRAINT <constraint_name> ...
in the context of a table.

Now, imagine you know the name of a constraint or all of them for a given
table, e.g. destilled via
    SELECT *
      FROM information_schema.table_constraints
     WHERE table_name = '<table_name>';
How can I get the comment for each of these constraints?

I mean, I can query pg_catalog.pg_description with
    SELECT *
           FROM pg_description
          WHERE description ~ '<Begin of comment> *';
and get the tuple
         objoid | classoid | objsubid |                          description
with the string I want to extract.  I can see that it is there.

In short... Given names of schema, table and constraint, how can I get the
description out of pg_catalog.pg_description?
Do I overlook something in the forest of system catalogs, its OIDs, the
information schema...?

Thank you!

Best regards,
    Wolfgang