...on a lumpy leg. It is, rather, a guilloche pattern, typically seen on banknotes but also incorporated into other works of art and design. In the past such patterns might have been generated by a geometric lathe, but nowadays they can be created mathematically. The one above is generated by:
Found at The Ministry of Type, via Proof Math is Beautiful.
http://www.subblue.com/projects/guilloche
It's pretty fun to play with.
Now let me please continue on the formula to get drunk and wasted before I looked to long at that pattern....