rachis vs rectrix

rachis

noun
  • The central shaft of a feather. 

  • The main shaft of either a compound leaf, head of grain, or fern frond. 

  • An anatomical shaft or axis in a marine invertebrate. 

rectrix

noun
  • A flight feather on the tails of birds, used for directional control. 

  • A governess; a rectoress. 

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