THE PANCEEAS 403 



The tubules (intercalary ducts) are said to branch and anastomose 

 freely and to be capable of differentiating either into acini, or islets, or 

 both. They may produce also small mucous glands which open into the 

 tubules ; and single islet cells may be formed along the ducts. Notwith- 

 standing their intimate developmental relationship the islets remain 

 isolated from the general exocrin parenchyma, since the lumen of the 

 connecting tubules does not penetrate its substance. There is no satis- 

 factory evidence to show that islet tissue may be increased or diminished, 

 or that acinus tissue may change into islet tissue, or vice versa, concom- 

 itant with experimentally induced alterations in nutritive and func- 

 tional conditions, as has been repeatedly claimed ; nor is there evidence of 

 a transition between islet and acinar cells. Islets and acini have a com- 

 mon embryonic origin, but once differentiated they are not capable of 

 transformation one into the other (Bensley). The islets appear in the 

 human pancreas about the time the embryo attains a length of 50 milli- 

 meters (Lewis). 



The islets are believed to constitute a group of internally secreting 

 organs. The evidence for such conclusion is derived from a series of 

 experiments by Opie and many others. When the pancreas is removed, 

 a form of diabetes follows, characterized by the appearance of sugar in 

 the urine. When the pancreatic duct is simply ligated, the flow of pan- 

 creatic juice is checked and atrophy of the acinar tissue ensues, but no 

 disturbance in carbohydrate metabolism results, nor is any alteration 

 produced in the islet tissue. Moreover, in cases of death following 

 diabetes mellitus, Opie demonstrated gross degenerative changes. Ho- 

 mans' recent experiments (Jour. Med. Eesearch, March, 1914) have ex- 

 tended the evidence in support of the conclusion that the islets are con- 

 cerned in the metabolism of sugar. When more than three-fourths of 

 the pancreas was removed in cats, the main duct being left intact, fatal 

 diabetes occasionally followed. Microscopic examination of the islets 

 revealed the following conditions : ( 1 ) in those instances where diabetes 

 did not follow the operation, the islet cells showed signs of over-activity 

 indicated in part by a disappearance of secretory granules; (2) in the 

 subjects which died from diabetes following the operation, the islet cells 

 showed degenerative changes, the acinous tissue having remained un- 

 altered. 



Blood Supply. The large blood-vessels of the pancreas accompany 



the interlobular ducts, but after repeated subdivision these vessels part 



company, and the smaller arteries pursue a separate course through the 



interlobular connective tissue. Thus they reach all portions of the 



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