I just wanted to start this deep thread regarding reference of hirda many times in Gurbani.
My take on Hirda in gurbani have multiple meaning depending on the context its not referred to as physical heart. Yes only few times according to my understanding- refered to anhat-reference point/pointer from shabad surat perspective only few times but for most part its refered to jot saroop where our jot/atma resides and its not location either..i found very deep and interesting article on hirda /heart references in Gurbani but that too looks like all conceptual real understanding will probably come from expereince within:
http://www.gurbani.org/articles/webart68.htm