deepen vs trephine

deepen

verb
  • To make more sound or heavy. 

  • To make deep or deeper 

  • To become lower in tone 

  • To make darker or more intense; to darken 

  • To become deeper 

  • To become more intimate. 

  • To make lower in tone 

  • To become darker or more intense 

  • To become more thorough or extensive. 

  • To make more thorough or extensive. 

  • To become more sound or heavy. 

  • To make more intimate. 

  • To make more poignant or affecting; to increase in degree 

trephine

verb
  • To perforate with a trephine. 

  • To use a trephine during surgery. 

noun
  • A surgical instrument with a cylindrical blade used to remove a circular section of tissue, f.e. bone or cornea; a trepan. 

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