COSMAS, a distinguished saint and martyr of the early Christian church, was a native of Arabia, where, along with his brother Damianus, he practised medicine gratuitously. The two brothers adopted this profession from the idea that by means of it they might most efficiently benefit their fellow-creatures. After a short but eminently useful career, they both perished in the persecution organized by Diocletian against the Christians, A.D. 303-311.