What does "rowel" mean?

Rowel: A little flat ring or wheel on horses' bits. The iron rowels into frothy foam he bit. Spenser.

Additional senses

  1. 2.(Far.) A roll of hair, silk, etc., passed through the flesh of horses, answering to a seton in human surgery.
  2. 3.To insert a rowel, or roll of hair or silk, into (as the flesh of a horse). Mortimer.

Sources