The Master Mind of Mars by Edgar Rice Burroughs

Days ran into weeks, weeks into months, as day by day I labored at the side of Ras Thavas, and more and more the old surgeon took me into his confidence, more and more he imparted to me the secrets of his skill and his profession. Gradually he permitted me to perform more and more important functions in the actual practice of his vast laboratory. I started transferring limbs from one subject to another, then internal organs of the digestive tract. Then he entrusted to me a complete operation upon a paying client. I removed the kidneys from a rich old man, replacing them with healthy ones from a young subject. The following day I gave a stunted child new thyroid glands. A week later I transferred two hearts and then, at last, came the great day for me -- unassisted, with Ras Thavas standing silently beside me, I took the brain of an old man and transplanted it within the cranium of a youth.

When I had done Ras Thavas laid a hand upon my shoulder. "I could not have done better myself," he said. He seemed much elated and I could not but wonder at this unusual demonstration of emotion upon his part, he who so prided himself upon his lack of emotionalism. I had often pondered the purpose which influenced Ras Thavas to devote so much time to my training, but never had I hit upon any more satisfactory explanation than that he had need of assistance in his growing practice. Yet when I consulted the records, that were now open to me, I discovered that his practice was no greater than it had been for many years; and even had it been there was really no reason why he should have trained me in preference to one of his red-Martian assistants, his belief in my loyalty not being sufficient warrant, in my mind, for this preferment when he could, as well as not, have kept me for a bodyguard and trained one of his own kind to aid him in his surgical work.