Preface vs introit

Preface

noun
  • The part of the liturgy that precedes the main part of the Eucharist 

introit

noun
  • 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. 

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

  • An anthem or psalm sung before a Communion service 

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