integument vs sheathing

integument

noun
  • An outer protective covering such as the feathers or skin of an animal, a rind or shell. 

  • The outer layer of an ovule, which develops into the seed coat. 

  • A shell or other outer protective layer. 

sheathing

noun
  • Something that wraps around or surrounds something, as a sheath encases its blade. 

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