bursa vs geode

bursa

noun
  • Any of the many small fluid-filled sacs located at the point where a muscle or tendon slides across bone. These sacs serve to reduce friction between the two moving surfaces. 

geode

noun
  • A nodule of stone having a cavity lined with mineral or crystal matter on the inside wall. 

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