THE DIGESTIVE SYSTEM 



327 



centrated near the lumen of the acinus. Secretions pass from the lumina 

 of the glands into fine intercalated ducts, and from these into secretory 

 ducts like those of the parotid. 



Scattered irregularly among the acini of the pancreas are clusters of 

 lightly-staining cells. The area of these clusters in section is considerably 

 greater than that of a single acinus. These are the islands of Langerhans, 

 endocrinal organs which secrete insulin. 



Development of the Pancreas. Like the Hver, the pancreas develops 

 from the endoderm. It is formed by the fusion of two separate out- 

 growths of the intestine, a ventral bilobed outpocketing from the bile- 

 duct, and a dorsal evagination of the intestine slightly anterior to that of 

 the Uver. By the proliferation of the cells of these anlagen, two pan- 

 creases are formed, which secondarily unite, but retain usually the two 



^-STOMACH 



DUCT OF SANTORIN 

 DORSAL RAiNCREAS 



DORSAL PANCREAS 



—VENTRAL RfiiNCREAS' 

 DUODENUM 



BILE DUCT 



WIRSUNG'S DUCT 



A EARLIER STAGE. B. LATER STAGE. 



Fig. 273. — A and B, two stages in the development of the pancreas. The duct of 

 the dorsal pancreas, Santorini's duct, may degenerate in ontogenesis. The two gland 

 anlagen unite into a single organ in the adult. (Redrawn after Broman.) 



primary connexions with the intestine, the ventral becoming Wirsung's 

 duct and the dorsal Santorini's, the two connecting wathin the body of the 

 gland. The dorsal pancreas grows much faster than the ventral, and 

 forms the body and tail of the gland and part of the head. The connec- 

 tive tissue of the gland comes from mesenchymatous cells which penetrate 

 between the acini and lobules. 



History of the Pancreas. The pancreas seems to be an emergent trait 

 of vertebrates, since no comparable structure is found in the invertebrates 

 or even in the lower chordates. In cyclostomes, the pancreatic tissue 

 remains buried in the substance of the hver or in the wall of the small 

 intestine. Since no duct appears in these forms, it is assumed that 

 the pancreas was primarily endocrinal and not digestive. Higher verte- 

 brates, beginning with the elasmobranchs, have both dorsal and ventral 

 pancreases. 



