Dr. McCoy uses the Great Teacher.
The Great Teacher is a machine, part of the ancient civilization located deep underground on the planet Sigma Draconis VI. It consists of a complex computer interface and a large, clear helmet containing many metallic appliances. These feeder circuits connect directly to the brain of anyone who placed their head under this helmet. The Teacher automatically activated whenever someone placed their head in the helmet, but according to Spock, its use was strictly predetermined by the builders, suggesting that even the leader of the Eymorgs would only use the Teacher when somehow instructed to do so. In less than a minute, the Teacher transfered vast amounts of knowledge to the user, but this knowledge did not last. According to the Eymorg Kara the knowledge persisted for about three hours.
The Teacher's feeder circuits were optimized for the configurations of an Eymorg brain. When Dr. McCoy used the device in 2267, he experienced great pain. Spock initially warned him against using the device, concerned about the possibility of brain damage, but McCoy felt he had no choice as there was no other chance of restoring Spock's brain to his body before the body failed. McCoy hoped to retain the knowledge and share it with the galaxy, but could not do so. Reuse of the Teacher would likely have caused brain damage. (TOS: "Spock's Brain")
Background
An often discussed plot error is why the Enteprise landing team did not simply cycle each member through the Great Teacher in order to have enough time to finish Spock's operation. The novelization of the episode explains this by mentioning that McCoy's existing medical knowledge was merely augmented by the Great Teacher, meaning that someone who was not a doctor to begin with would have gained the medical knowledge to treat Spock even with the Great Techer's help.