What are "bone clocks" as presented in the novel, The Bone Clocks?

The name, Bone Clocks, is a reference to the fact that humans age normally as opposed to the Horologists. Horologists, literally translated, are people who study and measure time, and they are reincarnated at the end of each life with memories of their previous lives, meaning they aren't typical children. By contrast, the Anchorites never age. The humans, or Bone Clocks, are the only characters of this story who age normally.