Gee, maybe I've been looking at the wrong anatomy charts over the years, but I thought I saw the heart between the lungs and low. This leads me to believe that in order to get the heart you have to shoot through the lungs (on a broadside shot). Of course, I'm a double lung kinda guy. As I always say "Stay away from the Bone". The heart lies very tight to the leg bones. Arrows don't penetrate bones well.

Of course I get the message as read. Less trauma means probably less of a trailing job.