Movements in the Digestive Tract
(a)  Peristalsis.  A wave of relaxation of the circular muscles is followed by a wave of strong contraction of the circular muscles, which propels the bolus of food through the digestive tract.  (b) Segmental contractions.  Each section of the digestive tract involved in segmental contractions alternates between contraction and relaxation.  The series of figures from top to bottom depicts a temporal sequence for one part of the small intestine.  The arrows indicate the direction material in a given part of the intestine moves with each contraction.  Material (brown) introduced at the beginning of the sequence (top figure) is spread out and becomes more diffuse (lighter color) through time.