It wasn't the breastbone, at least not in my version:

He's taen three strands o her yellow yellow hair,
Binnorie, O Binnorie,
And he has strung his harp sae rare,
By the bonny mill-dams o Binnorie.

And when he cam to the king's high ha'
Binnorie O Binnorie,
He's set the harp afore them a',
By the bonny mill-dams o Binnorie.

[similarly]

And before that he had touched ae string,
The harp it did baith say and sing.

Fare ye weel, my faither the king,
And fare ye weel, my mither the queen.

Fare ye weel, my sister Ellen,
For the droonin o me in hell ye'll swim.

Cheers
Murray