penetralia

Like Kahn’s buildings, too, Lesser’s book has its penetralia, core elements to which one is only gradually led.

From New York Times ● Mar. 7, 2017

The book is littered with show-off phrases such as "alembicated piety" and "the penetralia of one's self-regard."

From Time Magazine Archive

John Ayliffe had a strong inclination to knock him down, but he restrained himself, and at length a large plated iron door admitted the two gentlemen into the penetralia of the temple.

From The International Monthly, Volume 3, No. 2, May, 1851 by Various

He seemed to feel pleasure in the opportunity of baring to the view of a stranger the very penetralia 448of his beautiful abode.

From Narrative of the Circumnavigation of the Globe by the Austrian Frigate Novara, Volume II (Commodore B. Von Wullerstorf-Urbair,) Undertaken by Order of the Imperial Government in the Years 1857, 1858, & 1859, Under the Immediate Auspices of His I. and R. Highness the Archduke Ferdinand Maximilian, Commander-In-Chief of the Austrian Navy. by Karl Ritter von Scherzer

An arrased doorway right leads thro a passage to the street gate, and one left to the penetralia of the house, from which Marina enters deeply troubled.

From Porzia by Cale Young Rice