Definition of "os"

I was once, I remember, called to a patient who had received a violent contusion in his tibia, by which the exterior cutis was lacerated, so that there was a profuse sanguinary discharge; and the interior membranes were so divellicated, that the os or bone very plainly appeared through the aperture of the vulnus or wound.

1749, Henry Fielding, The History of Tom Jones, a Foundling, volume III, London: A[ndrew] Millar, […]

The instrument closed, as seen in Fig. 1, is then passed along the finger to the os, in and through the cervix up to the fundus of the uterus, which may be determined both by the distance and the resistance to the broad rounded head of the Capiat.

1891, Texas Medical Association, Transactions, volume 23, page 175