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

[Douglas G. Wilson]

>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