buttock vs popliteus

buttock

noun
  • Each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs. 

  • The convexity of a ship behind, under the stern. 

popliteus

noun
  • A small muscle at the back of the knee that aids in bending the knee and in the rotation of the lower leg. 

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