On his return he entered the Union Army as Surgeon of the 18th Massachusetts Infantry, but was soon made Medical Director of General George H. Thomas' division.
After the Peninsula campaign, he was placed in charge of the hospital at Fairfax Seminary, near Alexandria, Va., and while there rendered most valuable and conspicuous service, and laid the foundation for his subsequent eminence and success in difficult surgical operations.
After resuming practice in Springfield at the close of the American Civil War, he rapidly advanced in professional standing, and when in 1873 he was elected to the chair originally held by his grandfather in the Yale Medical School, he was the acknowledged head of his profession in the region of his residence.
His laborious practice and the intense energy with which he gave himself to it, had unfitted him for resisting disease, and death resulted from a chill contracted in a drive to a neighboring town and neglected until too late.
By his will his professional library and his valuable collections of medical and surgical instruments were given to Yale College, and the proceeds of two-fifths of his estate were used for the endowment of the chair of the Theory and Practice of Medicine.