19. Describe the functions of the large intestine. 20. Describe the five principal gastrointestinal hormones by listing for each (a) the site of secretion, (b) the stimuli for secretion, and (c) the action of the target cells receiving the hormone. The hormones are (1) gastrin, (2) cholecystokinin (CCK), (3) secretin, (4) gastric inhibitory peptide (GIP), and (5) motilin. .