introit vs introitus

introit

noun
  • A composition of vocal music sung at the opening of a church service. 

  • A prayer, typically part of a psalm or other portion of the Bible, read or sung at the start of Mass while or immediately after the priest ascends to the altar. 

  • An anthem or psalm sung before a Communion service 

introitus

noun
  • A piece of music played before a mass; a musical introduction of any sort. 

  • The entrance to a hollow organ or canal; often specifically the entrance to the vagina. 

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