a vesicular cutaneous invagination opening by a duct on each side of the anal canal in carnivores (best developed in skunks, but absent in some bears, the raccoon, kinkajou, coati, and sea otter), each lying between the external and internal anal sphincter muscles, which aid in emptying the contents. The sac stores odoriferous scent markers produced by glands that line its wall or duct; frequently the sac becomes impacted in the dog or cat, requiring manual emptying.