Doggy Tails

So, the other day, I was petting one of my poochies, Lizzie. Her tail was going a mile a minute...and the thought occured to me... "Do dogs wag their tail on purpose or do they have a happy sensor in their tails?". You know what I'm saying? Like is it like a human's smile, completely voluntary, or is it like an involuntary movement? Is there a sensor in our doggie's tails that knows when the dog is happy...so the tail wags? This is a question I ask myself constantly...does anyone have any thoughts on this? Happy Sensors In Dog Tails...the world may never know.