bowstring vs fibril

bowstring

noun
  • The string of an archer's bow. 

  • The string of an archer's bow, as used by the Turks for strangling offenders. 

verb
  • To strangle with a bowstring. 

fibril

noun
  • A fine fibre or filament 

  • Any fine, filamentous structure in animals or plants 

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