These observations place the development of the human pancreas in line with the triple pancreatic outgrowths, two ventral and one dorsal characteristic of the majority of the lower vertebrates, which have been hitherto carefully examined. The ventral or distal bud is probably double in the majority of vertebrates. The two segments fuse, however, so early that the derivation of the pancreas from a double outgrowth, as described above for the human embryo, practically obtains. In forms in which the adult gland presents a number of separate openings into the duodenum (cf. [p. 118]), the development would probably show multiple embryonic outgrowths from the intestinal hypoblast.

In any case the dorsal pancreatic bud appears to have developed in the vertebrate series before the ventral outgrowth and to be hence phylogenetically the older structure.

COMPARATIVE ANATOMY OF THE PANCREAS.

With the exception of Amphioxus and probably also of the Cyclostomata, the gland appears to be present in all vertebrates, varying, however, much in size, shape and relation to the intestinal tube. Usually it appears as an elongated, flattened, more or less distinctly lobulated organ, in close apposition to the duodenum between the layers of the mesoduodenum. In all forms in which the gland is found it is connected with the post-gastric intestine and marks the beginning of the midgut. In structure the gland is usually acinous, resembling the salivary glands. It is well developed in the selachians, forming a triangular body connected with the beginning of the midgut ([Fig. 202]). In some instances the gland elements do not extend beyond the intestine itself, but remain imbedded in the wall of the midgut, as in Protopterus. In certain adult teleosts the pancreas is surrounded by the liver (Fig. 196), in others it does not appear as a compact gland but is distributed in the form of finely scattered lobules throughout the mesentery between the two layers of this membrane. On account of this concealed position of the gland it was formerly believed that the adult teleosts did not possess a pancreas. The pyloric cæca (cf. [p. 119]) found in these forms were consequently considered to be homologous with the pancreas of the higher vertebrates.

Fig. 196.—A portion of alimentary canal of Pleuronectes maculatus, the flounder, with pancreas attached to biliary duct and concealed in the substance of the liver, which has been removed. (Columbia University Museum, No. 1491.)   Fig. 197.—Pancreas and biliary ducts of Rana esculenta, frog. (Wiedersheim, after Parker; both from Ecker.)

Fig. 198.—Necturus maculatus, mud puppy. Dissection of intestinalcanal, liver, pancreas, and spleen, with blood-vessels injected. (ColumbiaUniversity Museum, No. 1863.)

 

Fig. 199.—Pancreas and pancreatic ducts of rabbit. (Nuhn.)

Fig. 200.—Abdominal viscera of dog, showing arrangement ofpancreatic ducts. (Nuhn.)

In Myxinoids a peculiar lobulated glandular organ is found imbedded in the peritoneal coat of the intestine near the entrance of the bile-duct, into which its lobules open separately. This organ possibly corresponds to the higher vertebrate pancreas.

An organ which may represent a dorsal pancreas is also developed in Ammocœtes (larva of Petromyzon), but its exact homology is still doubtful. It is possible that a true pancreas has not yet developed in the cyclostomata. In Amphioxus no trace of a pancreas is found. In all other vertebrates the gland is present. In certain amphibians, as the frog, the single pancreatic duct opens into the common bile duct ([Fig. 197]).

In lacertilians and in some chelonians a lateral offshoot of the pancreas is directed transversely and is adherent to the spleen. [Fig. 113] shows the gland in Chelydra serpentaria. While the gland usually has a single duct, yet two ducts are found in a number of animals (many mammals, birds, chelonians and crocodiles). At times three ducts are encountered, as in the chicken and pigeon.

The arrangement of the pancreatic duct system among mammalia presents the following variations: