According to modern textbooks the small bowel is a tube whose structure and physiologic characteristics are much the same from one end to the other; and material is supposed to move aborally because of the myenteric reflex, or Bayliss and Starling's "law of the intestine." According to this law,1 "Excitation at any point of the gut excites contraction above and inhibition below"; and it can easily be seen how this mechanism must favor the aboral progress of material through the intestine.
Cannon2 has warned us, however, that this reflex and normal peristalsis are not the same, nor is it always in control of the mechanism of the bowel. A moment's thought will show that this must be true; otherwise food once introduced into the duodenum would never stop in its rush to the anus. My own graphic records of peristaltic rushes, secured with six or seven enterographs, rarely
ALVAREZ WC. THE MOTOR FUNCTIONS OF THE INTESTINE FROM A NEW POINT OF VIEW. JAMA. 1915;LXV(5):388-394. doi:10.1001/jama.1915.02580050016005