292 



HISTOLOGY 



alveoli appear to be clogged with cells, known as centro-alveolar cells (or 

 centro-acinal cells). Irregularly distributed among the alveoli there are 

 round areas of paler cells, peculiar to the pancreas (Fig. 288). ^ They may 

 be at the center or periphery of the lobule, or occasionally in the inter- 

 lobular connective tissue. These important structures were first described 

 in Langerhans' thesis in 1869 (Inaug. Diss., Berlin), and are known as the 

 pancreatic islands (islands of Langerhans). 



The alveoli are composed chiefly of the secreting pancreatic cells (Fig. 

 289). Toward the lumen their protoplasm contains coarse granules of 

 zymogen, which accumulate while the cell is inactive and are eliminated 

 during secretion. Apparently they are transformed into fluid as they 



Blood 

 capillary. 



Cells oi 

 the al- ^"W Centro-aveolar eel' 



Zymogen granules. 

 A B 



FIG. 289. FROM SECTIONS OF A HUMAN PANCREAS. X 500. 



In ction A the granules are wanting, the centro-alveolar cells are flat and dark; in section B the granules 

 are distinct, the centro-alveolar cells are cuboidal and clear. 



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 reticu- 

 iar. 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 which has coarse masses of chromatin. 

 Within the pancreatic cells there have been found "paranuclei" of un- 

 known 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." 



The centro-alveolar cells may be darker or lighter than the pancreatic 

 cells (Fig. 289), but they are always smaller, and may be readily identified 

 from their central position. They do not contain zymogen granules. 

 The intralobular intercalated ducts, which connect with the alveoli, are 

 very slender, and their walls are formed of flat cells (Fig. 289, A). They 



