Tail of the dog

The old dog I think is gradually losing control of her rear end - her legs are bad, her bowels are unpredictable, and her tail has gone crazy.

It is the strangest thing to see - she wakes up, staggers into an upright position (if she can), takes a stretch (hopefully without falling back down), gives us a grin, and then her tail goes off at 100 miles an hour, like windshield wipers at top speed.