armrest vs ulna

armrest

noun
  • Part of the seat of a chair that is designed to support the arm. 

ulna

noun
  • The bone of the forearm that extends from the elbow to the wrist on the side opposite to the thumb, corresponding to the fibula of the hind limb. Also, the corresponding bone in the forelimb of any vertebrate. 

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