Final answer:
Peristalsis is the mechanism that allows food to pass into the duodenum in the small intestine. Hormone motilin initiates peristalsis in the form of a migrating motility complex, which forces the chyme into the duodenum.
Step-by-step explanation:
In the small intestine, the mechanism that allows food to pass into the duodenum is peristalsis. Peristalsis is a series of wave-like muscle contractions that push the food forward through the digestive tract. In the duodenum, the hormone motilin initiates peristalsis in the form of a migrating motility complex. These complexes force the chyme, the partially digested food, through a short section of the small intestine and into the duodenum. This process is repeated until the chyme reaches the end of the ileum.