caudal vs retral

caudal

adj
  • Pertaining to the tail or posterior or hind part of a body. 

  • Toward the tail end (hind end) of the body; in bipeds such as humans, this direction corresponds to inferior. 

noun
  • A caudal vertebra. 

retral

adj
  • back; posterior 

How often have the words caudal and retral occurred in a corpus of books? (source: Google Ngram Viewer )