Complex sentence: One possibility is the Roman belief in the Vena amoris, which was believed to be a blood vessel that ran from the fourth finger (ring finger) directly to the heart, Thus when a couple wore rings on this finger, their hearts were connected.
Split into simpler sentences: One possibility is the Roman belief in the Vena amoris, which was believed to be a blood vessel that ran from the fourth finger (ring finger) directly to the heart. Thus, when a couple wore rings on this finger, their hearts were connected.