These are literally sacs - about pea sized and attached to a very fine tube.

There’s one on each side of the anus, wedged between the internal and external anal sphincter muscles.

The glandular cells that line the sacs produce a liquid and when the dog defecates the sac full of liquid gets squeezed between the anal sphincter muscles, squirts through the tube, and is deposited into the outside world.

It’s useful for marking territory, having one of the strongest odours known to veterinary science. 

In some unfortunate dogs and for reasons unbeknownst to us, the liquid thickens or becomes gritty and blocks the tube.

When this occurs, the sac over-expands and it’s at about this point that the dog’s bum really starts to bother them.

If they stay blocked, the sac becomes red and sore and bacteria move in.

Very quickly the dog’s bottom goes from being under pressure and annoying to abscessed and excruciatingly painful.  

We can prevent anal sacs becoming infected by ‘expressing’ them.

This involves putting a (gloved) forefinger into the dog’s bottom and squeezing the sac between forefinger and thumb.

This is about as fun as it sounds for both parties, and many dogs need this done so often that we actually opt to surgically remove the glands.

When we do perform this type of surgery, most owners say they wish they’d done it years ago.

If the dogs could speak I suspect they’d say the same.