The anal sacs are a pair of structures most of which lie within the fibers of the anal sphincter. The sphincter is a band of muscle encircling the terminal rectum which is responsible for keeping stool (and gas) within the rectum until it is voluntarily passed. The cells lining the sacs produce a thin oily secretion that is normally expelled during defecation or any event associated with a sudden contraction of the sphincter-i.e. fear inducing experiences.

Anal sacs are subject to a variety of medical conditions and I'm not certain which of these afflicts your dog, but by far the most common one is simple impaction. If the ducts through which the secretions empty become narrowed owing to inflammation or scarring, or if the secretion becomes abnormally thickened, the sacs will fail to empty. The distended sacs cause discomfort to the dog or cat who will typically indicate this by 'scooting', that is sitting with legs extended forward as he or she drags his or her itchy anus across the carpet. Affected animals will also lick the area frequently.

It is not uncommon for impacted anal sacs to become abscessed if they are so distended that the lining of the sac which serves as a barrier to infection develops tiny tears which allow bacteria access to the underlying tissues.

Anal sacs can become infected in the absence of impaction, and on occasion tumors will arise from the cells of the sacs.