in Thoreau’s Journal:

A closed pitch pine cone, gathered January 22nd, opened last night in my chamber. If you would be convinced how differently armed the squirrel is naturally for dealing with pitch pine cones, just try to get one open with your teeth. He who extracts the seeds from a single closed cone, with the aid of a knife, will be constrained to confess that the squirrel earns his dinner. He has the key to this conical and spiny chest of many apartments. He sits on a post vibrating his tail, and twirls it as a plaything. 

So is a man commonly a locked-up chest to us, to open whom, unless we have the key of sympathy, will make our hearts bleed.