The secondary pancreatic duct (of Santorini) of the adult corresponds to that section of the proximal or larger embryonic outgrowth situated between the intestine and the point where the two glandular diverticula fuse with each other. Hence the canal of Wirsung in the adult is a compound product. It includes the duct system developed, in connection with the bile duct, in the head of the gland, forming the intestinal termination of the main duct. Its distal body portion on the other hand is derived from the duct system of the originally larger proximal outgrowth, including the entire peripheral portion which has become secondarily added to the duct of the ventral outgrowth to form together with it the canal of Wirsung. On the other hand the proximal portion of the duct system of this originally larger part becomes secondarily differentiated as the duct of Santorini.

Fig. 188 shows the normal adult arrangement of the pancreatic and biliary ducts in a corrosion preparation of the canal.

The duct of Santorini in this case opened by a separate orifice into the duodenum above the common opening of the biliary and pancreatic ducts (cf. [p. 113]).

Explanation of Adult Arrangement of Human Pancreatic Ducts and Their Variations Dependent Upon the Embryonic Development.—The smaller distal embryonic outgrowth is, as we have seen, from its inception in close connection with the duodenal end of the common bile-duct ([Fig. 185]).

The proximal outgrowth, situated nearer to pylorus and derived directly from the duodenal epithelium, is the larger and forms the greater part of the bulk of the adult pancreas ([Figs. 186], [187]).

Figs. 189-192.—Series of schemata showing normal and variant adult types of biliary and pancreatic ducts.
Fig. 189.—Usual human adult type.
Fig. 190.—Persistence of early embryonal type.
Fig. 191.—Duct of Santorini has no duodenal orifice.
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.

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.

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.