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

Even that penetralia, the 'Omnibus,' can not compare with the unwalled room outside, with its star-gemmed ceiling, and the cool breeze eddying away the segar-smoke; so its usual occupants are all outside.

From The Continental Monthly, Vol. 2 No 4, October, 1862 Devoted To Literature And National Policy 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

And away through the noiseless glass door buzzed Josephine, on her way to the basement, followed by Isabella on her way to the inner penetralia of the second floor; while Col.

From Shoulder-Straps A Novel of New York and the Army, 1862 by Henry Morford