The sonic separator was a medical device in use in the 2260s. It was found in the emergency surgical kit of a medical tricorder. Per Spock's suggestion, Dr. McCoy used the device in 2268 during an operation to return the Vulcan's brain to his body. McCoy used it to stimulate Spock's nerve endings in order to determine which nerves to join together. (TOS: "Spock's Brain" )