The Collaborative International Dictionary of English v.0.48:

Rowel \Row"el\, n. [OF. roele, rouele, properly, a little wheel,
   F. rouelle collop, slice, LL. rotella a little wheel, dim. of
   L. rota a wheel. See Roll, and cf. Rota.]
   1. The little wheel of a spur, with sharp points.
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            With sounding whip, and rowels dyed in blood.
                                                  --Cowper.
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   2. A little flat ring or wheel on horses' bits.
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            The iron rowels into frothy foam he bit. --Spenser.
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   3. (Far.) A roll of hair, silk, etc., passed through the
      flesh of horses, answering to a seton in human surgery.
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The Collaborative International Dictionary of English v.0.48:

Rowel \Row"el\, v. t. [imp. & p. p. Roweledor Rowelled; p.
   pr. & vb. n. Roweling or Rowelling.] (Far.)
   To insert a rowel, or roll of hair or silk, into (as the
   flesh of a horse). --Mortimer.
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WordNet (r) 3.0 (2006):

rowel
    n 1: a small spiked wheel at the end of a spur