It starts in his mind and it runs out behind
  To the tip of his tail, and then 
That glad little waggle, that gay little wiggle 
  Begins all over again. 
The day may be sunny or dark with rain, 
  The wiggle is there just the same; 
It needs just a whistle to set it a-wiggle 
  Or the sound of his favorite name. 
No doubt I shall never, in any way ever 
  Find out how that wiggle got there, 
But I'm very sure, while tails shall endure, 
  That tail will wig-wag in the air! 
Arthur Wallace PeachAll the Best Dog Poems