Green’s Dictionary of Slang

central furrow n.

the vagina.

1748–49		Cleland Memoirs of a Woman of Pleasure (1985) 115: At the bottom of the smoothest, roundest belly, by that central furrow which nature had sunk there.
1787		Harris’s List of Covent-Garden Ladies 132: This truly amorous girl [...] confines her whole pleasure to the central spot of nature.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 254: Tonsure, f. The female pudendum; ‘the central office’.