My dog always has to have his anal glands squeezed out. What purpose do they serve?

What are popularly termed the anal glands of dogs, but should strictly be called the anal sacs, are two small pouches situated on either side of, and a little below, the anus (i.e. at about the four o’clock and eight o’clock positions). They vary in size from a pea to a walnut, though in a medium sized dog they are about 2/5 inch (1 em) across. A small duct, or tube, leads from each to open at the anus. In their walls are numerous glands producing a secretion (very pungent and unpleasant smelling to us) that is stored in the sacs. This secretion is a thick, grayish liquid which acts as a scent marker. When the dog passes a motion some of this liquid is squeezed out of the sacs to be deposited on the stool so that other dogs will be able to identify it – an important aspect of inter-canine communication. Some dogs will also evacuate the sacs when frightened.

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