internal organ vs viscera

internal organ

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

viscera

noun
  • Collectively, the internal organs of the body, especially those contained within the abdominal and thoracic cavities, such as the liver, heart, or stomach. 

  • The intestines. 

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