The periatrium was the fifth chamber of the heart in the humanoid species native to the unnamed planet where Doctors Julian Bashir and Elizabeth Lense became stranded in late 2376. Doctor Idit Kahayn was surprised to discover, when Bashir was brought to her as an emergency patient, that he did not have a periatrium, and that his heart had only four chambers. (SCE eBook: Wounds, Book 1)