REFERENCES FOR DIGESTIVE SYSTEM 1197 



intestinal loop counter-clockwise around the superior mesenteric artery. This accounts for 

 the position and the hook-like form of the processus uncinatus. Following this rotation, the 

 duodenum and the head of the pancreas become pressed backward against the posterior ab- 

 dominal wall, where they become adherent, with fusion and obhteration of the primitive peri- 

 toneum. The body of the pancreas, extending into the dorsal mesogastrium (fig. 900), is simi- 

 larly caught in the pouch-like downgrowth of the latter to form the bursa omentalis (lesser sac), 

 and is thereby carried over to the left side. When the posterior layer of the primitive bursa 

 fold becomes fused with the posterior abdominal wall, the enclosed pancreas is likewise fixed 

 and becomes retroperitoneal. Of these obliterated peritoneal layers of the embryo, only certain 

 layers of fascia remain as their representatives in the adult. From the lower aspect of the pan- 

 creas downward, the posterior layer of the bursa fold becomes fused with the transverse meso- 

 colon, so that in the adult the latter appears to arise from the anterior border of the pancreas 

 (fig. 904). 



Variations. — Aside from minor fluctuations in size and form, the variations of the pancreas 

 are chiefly congenital and of embryonic origin. Cases of accessory or supernumerary pancreas are 

 not rare. They are usually of small size and have separate ducts. They may occur along the 

 wall of the duodenum, or even in the stomach or jejunum. They are perhaps in some way con- 

 nected with the numerous intestinal diverticula which occur in the embryo. Divided pancreas 

 differ from the accessory in that a mass of the pancreas becomes separated from the main gland, 

 connected only by a duct. This occurs oftenest in the region of the tail (sometimes extending 

 into the spleen) or of the processus uncinatus, forming what is termed a 'lesser pancreas.' 

 Sometimes a ring of glandular tissue from the head of the pancreas surrounds the descending 

 duodenum, forming an annular pancreas. Variations in the direction of the body are numerous; 

 it may be horizontal, ascending or bent in various ways. These are doubtless congenital vari- 

 ations, as similar types have been described in the foetus (Jackson). It has been experimentally 

 demonstrated that varying degrees of distention of the stomach and intestines affect profoundly 

 the form of the body of the pancreas. When the stomach alone is distended, the pancreas is 

 flattened antero-posteriorly, the inferior surface being practically obliterated. When both 

 stomach and intestines are distended, the pancreas is flattened from above downward, and 

 extends forward like a sheK, the posterior surface being much reduced (Jackson). Numerous 

 variations in the ducts are easily understood from their complicated development. The acces- 

 sory duct (of Satorini) is in the foetus as large as the main duct (of Wirsung), the preponderance 

 of the latter being estabhshed later. The accessory duct in the adult may be larger than usual, 

 and retain its primitive drainage, or even drain the entire gland in rare cases where the duct 

 of Wirsung is absent. Or the accessory duct may be rudimentary or (rarely) absent. Similar 

 variations occur in the main duct of Wirsung. Rarely the pancreas may open into the duo- 

 denum by three ducts, probably representing three embryonic anlages. Abnormalities of the 

 pancreas are often associated with duodenal diverticula. 



Comparative. — The pancreas, like the liver, is constant throughout the vertebrates. It 

 always arises by budding off from the endodermal epithelium of the intestine, closely associated 

 with the liver. There is typically a triple anlage (rarely multiple, which is perhaps the ancestral 

 type), with one dorsal and two ventral outgrowths. These fuse and form the adult pancreas 

 in a variety of ways. In many of the fishes, the pancreas is very small, diffuse and incon- 

 spicuous, sometimes embedded in the liver or intestinal wall. Of the three primitive ducts, 

 usually only two persist (as in man), but often only one, or all three (in birds). All three types 

 occur in mammals. The islets of Langerhans arise from the epithelial pancreas anlage, and ap- 

 pear to be constantly present, even in the lowest vertebrates. Laguesse even considers that 

 phylogenetically they form the most primitive part of the pancreas, but this is doubtful. 



References for digestive system. — General and Comparative: Quain's Anat- 

 omy, 11th ed.; Poirier-Charpy, Traite d'anatomie; Rauber-Kopsch, Lehrbuch 

 der Anatomie, 9te Aufl.; Oppel, Mikroskopische Anatomie, Bd. 1-3; also 'Ver- 

 dauungsapparat ' in Merkel and Bonnet's 'Ergebnisse'; Wiedersheim, Ban des 

 Menschen. Topography: (adult) Merkel, Topographische Anatomie; (develop- 

 mental) Jackson, Anat. Rec, vol. 3. Development: Keibel and Mall's Manual. 

 Teeth: Tomes, Dental Anatomy. Tonsils: (lingual) Jurisch, Anatomische, 

 Hefte, Bd. 47; (pharyngeal) Symington, Brit. Med. Jour. (Oct., 1910); (palatine) 

 Killian, Archiv f. Laryngol., Bd. 7. (Esophagus: Goetsch, Amer. Jour. Anat., 

 vol. 10. Stomach: (structure), Bensley, Buck's Ref. Handb. Med. Sc, vol. 

 7 (1904); (form) Cunningham, Trans Royal Soc. Edinb., vol. 45; (radiography) 

 Cole, Archives Roentgen Rays, 1911; also Journal Amer. Med. Assn., vol. 59. 

 Duodenum: (diverticula), Baldwin, Anat. Rec, vol. 5. Vermiform process: Berry 

 and Lack, Jour. Anat. and Phys., vol. 40. Rectum: Symington, Jour. Anat. 

 and Phys., vol. 46. Liver: Mall, Amer. Jour. Anat., vol. 5. Pancreas: (islets) 

 Bensley, Amer. Jour. Anat., vol. 12; (ducts) Baldwin, Anat. Rec, vol. 5. 



