The symptoms described in the question, such as myotic pupils, sleepiness, urinary retention, and slowing-down of heart rate, are consistent with the effects of morphine poisoning. Atropine, on the other hand, would cause the opposite effects, such as dilated pupils and increased heart rate. Phosphacole, caffeine, and barbital do not typically cause these specific symptoms. Therefore, the substance that caused the poisoning in this case is most likely morphine.