muse vs rack one’s brains

muse

verb
  • To wonder at. 

  • To say (something) with due consideration or thought. 

  • To become lost in thought, to ponder. 

  • To think on; to meditate on. 

noun
  • A source of inspiration. 

  • A gap or hole in a hedge, fence, etc. through which a wild animal is accustomed to pass; a muset. 

  • An act of musing; a period of thoughtfulness. 

rack one’s brains

How often have the words muse and rack one’s brains occurred in a corpus of books? (source: Google Ngram Viewer )