introitus vs psalter

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. 

psalter

noun
  • Synonym of breviary, which arranges the Psalms for devotional recitation. 

  • Synonym of Book of Common Prayer, which prominently includes the Psalms. 

  • Synonym of Psalms, particularly when printed as a separate work from the Bible. 

  • A rosary with 150 beads, used for reciting the Psalms in Catholic editions of the Bible. 

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