"Brain" as Slang for "Oral Sex": fun with metonymy

[Laurence Horn]

At 2:25 PM -0500 11/7/04, Douglas G. Wilson wrote:
>>what's the motivation for "brains" 'scrotum'?  is it based on the
>>expression "have balls for brains" '[of a man] think with one's
>>genitals'?  or on a perceived physical similarity between a testicle
>>and a brain?  or what?
>
>It seems to me highly probable that originally the human scrotum itself
>(not the testicle) is being likened to a human brain in general external
>appearance. Both scrotum and brain are (approximately) symmetrically
>bilobate (with two hemiscrota [each containing a testis] versus two
>cerebral hemispheres) and corrugated (from dartos muscle contractions
>versus cortical gyri and sulci). The other member of the set is the walnut,
>maybe.
>
>-- Doug Wilson

Interestingly, transitivity doesn't work here:

(kick someone in the) nuts = balls [I have indeed always assumed walnuts here]
balls/scrotum = brain (as we've seen), but
nuts =/= brain.
Unless when we say someone is nuts we're really saying their brain
has reverted to its primitive walnuttian state.

larry