Definition of "narrowness"

presently, then, I felt the stiff insertion between the yielding, divided lips of the wound, now open for life; where the narrowness no longer put me to intolerable pain, and afforded my lover no more difficulty than what heighten'd his pleasure, in the strict embrace of that tender, warm sheath, round the instrument it was so delicately adjusted to

1749, [John Cleland], “(Please specify the letter or volume)”, in Memoirs of a Woman of Pleasure [Fanny Hill], London: […] G. Fenton [i.e., Fenton and Ralph Griffiths] […]