Anal sacs are a set of glands that are just under the skin near your pet’s anus. The two glands arelocated at the 4:00 and 8:00 o’clock positions from the anus. The anal sacs fill with a foul-smelling fluid that is normally expressed through a tiny duct when animals defecate. Animals may use their anal glands to mark territory or repell aggressors, although a nervous dog or cat may accidentally express these glands when frightened.