Anal Glands or anal sacs.
The anal glands or anal sacs are small glands near the anus in many mammals, including dogs and cats. They are paired sacs on either side of the anus between the external and internal sphincter muscles. Sebaceous glands within the lining secrete a liquid that is used for identification of members within a species.

How do you know if your dog needs glands expressed?
How can you tell if your dog needs his anal glands expressed? ... Your dog is scooting on the carpet. Your dog is licking his bottom a lot. If your dog's glands are really full, they may leak out a stinky, pungent smell.