I hadn't heard of Haydn's head but I do know the story of
J.S. Bach's arse.
I was told it when I was in Leipzig in 1977.
When Bach died he was buried in an unmarked grave in the
Alte Johannisfriedhof (then a cemetery as the name suggests, now a park and incidentally where** I spent the very pleasant afternoon of 8th May 1977 (VE Day in the UK but
der Tag der Befreiung vom Faschismus in the DDR)). At the end of the 19th century it was decided that the
Johanniskirche would be a more fitting resting place for the (de)composer, and several bodies were exhumed from the general area where it was thought he'd been interred*. That left the re-interrers with several skeletons, and they hadn't a clue which was that of the great musician. On the basis that he'd spent so much time as an organist in a seated position, rocking from side to side , and reaching for stops, he would have had a well developed posterior, they selected the skeleton with the biggest arse (i.e pelvis). And so those were the bones which were transferred to the vault until they were moved to a grave at the
Thomaskirche where he'd been choirmaster.
That's what I was told in 1977.
This article from 2009 in the Medical Journal of Australia suggests that the bones of the story are more or less correct (at least in some respects), except it uses the phrase 'pevic exostoses' instead of 'biggest arse', but confirms the gist of the story and concludes:
We believe it is unlikely that the skeleton is that of Bach
*
Based on oral tradition, it was in the graveyard surrounding St Johanniskirche in Leipzig, “six paces away from the south portal”.4 This oral tradition apparently originated in 1894 from a 75-year-old man, who in turn was informed about the location 60 years earlier by a 90-year-old gardener employed at the graveyard.
** ETA: or possibly not. I may have confused the old Johannisfriedhof (where Bach was buried) with the new Johannisfriedhof aka Friedenspark (if Brymbo is reading this, the new one is nearer to Tarostrasse).