Sentence:  ...Thou best know'st / What torment I did find thee in. Thy groans / Did make wolves howl, and penetrate the <WSD>breasts</WSD> / Of ever-angry bears— it was a torment / To lay upon the damn'd, which Sycorax / Could not again undo. It was mine art, / When I arrived and heard thee, that made gape / The pine and let thee out. 
Meaning of the word marked with <WSD> tags:  The figurative seat of the emotions, feelings etc.; one's heart or innermost thoughts.