converting datum to numeric

Hi,
I've got a couple of questions.

I was using numeric_out like this:

DatumGetCString(DirectFunctionCall1(numeric_out, d));

Why do I have to use DirectFunctionCall1 instead of calling numeric_out?

I was suggested to use numeric_send instead of numeric_out, however when
changing the function names in the above example, the whole expression
returns 8. Always 8. I check with:

char *x = DatumGetCString(DirectFunctionCall1(numeric_send, d));
PLy_elog(NOTICE, x).

And my main question: is there any documentation about those internal
functions, which use and when etc?

thanks
Szymon