internal organ vs viscus

internal organ

noun
  • An organ, such as the heart or colon, that is entirely within the confines of the body. 

viscus

noun
  • One of the organs, as the brain, heart, or stomach, in the great cavities of the body of an animal; especially used in the plural, and applied to the organs contained in the abdomen. 

  • The intestines. 

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