PANCREAS. 



2 33 



have been found in the mucosa. The epithelial cells become lower 

 in the smaller ducts, and are cuboidal or flattened parallel with the 

 long axis in the intercalated ducts. There are no secretory ducts in the 

 pancreas. The long intercalated ducts terminate in the alveoli (or acini) 

 in a peculiar manner. As seen in Fig. 265, the cells of the duct seem 

 prolonged into the center of the alveolus, where they are known as centro- 

 acinal cells. In development the duct is not invaginated into the alveo- 

 lus, but the latter develops so as to consist of two layers, only the outer of 

 which produces the pancreatic secretion. Sometimes the inner cells 

 are lacking. The lumen of the intercalated ducts and alveoli is very 

 small and in many parts of a section it cannot be seen. Intercellular se- 

 cretory capillaries extend from it between the centro-acinal cells to the se- 

 creting cells, as seen in Fig. 266. They may be prolonged between the 

 latter, but they do not reach the basement membrane. 



Inter- 

 calated 

 duct. 



Centroacinal cells. 



Cells of the 

 alveolus. 



Intercellular 

 secretory 

 capillary. 



FIG. 266. A, FROM A SECTION OF THE PANCREAS OP ADULT MAN. X 320; B, AN INTERPRETATION 

 OF THE RIGHT LOWER PORTION OF A. 



The secreting or pancreatic cells are mostly arranged in alveoli but in 

 part they form tubules. Toward the lumen their protoplasm contains a 

 zone of coarse granules of zymogen, which accumulate while the cell is in- 

 active and are eliminated during secretion. Apparently they are trans- 

 formed into fluid as they are discharged, for they are not found free in the 

 intestine. In fresh specimens the granules are refractive and easily seen, 

 but in preserved tissue they are readily destroyed so that the granular zone 

 appears reticular. The granules are soluble in water, and are darkened 

 by osmic acid. The basal protoplasm of the pancreatic cells is vertically 

 striated. It contains the round nucleus with coarse masses of chromatin. 

 Within the pancreatic cells there have been found 'paranuclei' of unknown 

 nature, thought to be functionally important. After the discharge of 

 secretion the cells become smaller and their boundaries more distinct. 

 The pancreatic cells rest upon basement membranes containing 'basket 

 cells.' 



