A portion of each pubis (front portion of the coxal bone) passes down and in the back to join an ischium (lower portion of the coxal bone). Between the bodies of these bones, on either side, there is an gap, called the "obturator foramen," which is the largest foramen (passage in the bone for fibers to pass through) in the skeleton. In life, the foramen is covered over and nearly closed by an "obturator membrane."