In Richter's hernia only part of the circumference of the bowel wall (the antimesenteric border) is trapped in the sac, so it can strangulate and become gangrenous without causing complete obstruction; it is common in femoral hernia. Two loops of bowel in the sac is Maydl's hernia; bladder as part of the sac is a sliding hernia; omentum is an omentocele.