If, notwithstanding this primitive arrangement, the distal duct (canal of Wirsung) appears as the main pancreatic duct in the adult, while the proximal (duct of Santorini) is secondary, this depends upon a union of the products of the two outgrowths in such a manner that the greater part of the duct system of the proximal and larger portion is transferred to the distal duct to form the adult canal of Wirsung, while the smaller segment of the proximal duct, between its opening into the duodenum and the point of fusion of the two outgrowths, forms the adult secondary duct of Santorini. This duct opens usually into the duodenum upon a small papilla situated about 2.5 cm. above the common duodenal termination of the bile-duct and canal of Wirsung (papilla Vateri) (Fig. 193). The duct of Santorini usually tapers toward the duodenal opening from its point of departure from the main duct, its caliber gradually diminishing in the direction indicated, so that it is smaller at the duodenal opening than at the point of confluence with the main duct (Fig. 189). Hence the secretion from the proximal head portion of the pancreas, conveyed by this duct and its tributaries, passes usually into the main pancreatic duct and not directly into the intestine through the duodenal opening of the duct of Santorini. The latter is, however, thus enabled to vicariously take upon itself the conduct of the pancreatic secretion in cases of obstruction or obliteration of the main duct (calculi, ulcers, cicatrices, etc.). In these cases of obstruction of the main duct the duct of Santorini enlarges and performs its functions. [Illustration: FIGS. 189-192.--Series of schemata showing normal and variant adult types of biliary and pancreatic ducts.] [Illustration: FIG. 189.--Usual human adult type.] [Illustration: FIG. 190.--Persistence of early embryonal type.] [Illustration: FIG. 191.--Duct of Santorini has no duodenal orifice.] [Illustration: FIG. 192.--Duct of Santorini forms the only pancreatic duct. Separate duodenal openings of biliary and pancreatic ducts, resulting from failure of development of distal embryonal pancreatic bud.] [Illustration: FIG. 193.--Mucous surface of human duodenum, showing entrance of biliary and pancreatic ducts and diverticulum Vateri. (Columbia University Museum, No. 1842.)] Occasionally, without obstruction of the main duct, the duodenal opening of the duct of Santorini is large, and the flow of secretion evidently the reverse of the usual, _i. e._, directly into the intestine.