Complex sentence: He remained with Reverend Dr. Todd until he was ten, when he was then under the care and instruction of Reverend Dr. Goodrich, of Durham, Connecticut, during which time he was introduced to the practice of medicine.
Split into simpler sentences: He remained with Reverend Dr. Todd until he was ten, when he was then under the care and instruction of Reverend Dr. Goodrich, of Durham, Connecticut. During which time he was introduced to the practice of medicine.