backbone vs caudal

backbone

noun
  • The series of vertebrae, separated by disks, that encloses and protects the spinal cord, and runs down the middle of the back in vertebrate animals. 

  • Courage, fortitude, or strength. 

  • Any fundamental support, structure, or infrastructure. 

caudal

noun
  • A caudal vertebra. 

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

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

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