Why are spoke nipples called nipples?
In plumbing and piping, a nipple is a fitting, consisting of a short piece of pipe, usually provided with a male pipe thread at each end, for connecting two other fittings.
  
  - http://en.wikipedia.org/wiki/Nipple_%28plumbing%29


This is from the wikipedia entry on the plumbing and piping context of the word Nipple as suggested by Daniel R. Hicks' comment.

Can anyone elaborate on the etymology of this, or why this term for short pieces of pipe was used for spoke ends (perhaps proper piping nipples were used in the past)?