DIGESTION. 197 



which divides the gland tissue into lobules. Each lobule is com- 

 posed of a number of acini or alveoli, more or less elongated or 

 tubular in shape. Each acinus gives origin to a small duct which, 

 uniting with adjoining ducts, forms the lobular duct, which becomes 

 tributary to the main duct. The .acinus is lined by a layer of cylin- 

 dric epithelial cells characterized by a difference in structure be- 

 tween the central and peripheral ends (Fig. 82). The central end, 

 that bordering the lumen of the acinus, is dark in appearance and 

 filled with dark granules, while the peripheral end is clear and homo- 

 geneous. The relative depth of these two zones varies according to 

 the functional activity of the gland. During the intervals of digestion 

 the granular layer is very deep and occupies almost the entire cell; 

 after active digestion the granular layer is very narrow, while the 



FIG. 82. SECTION or HUMAN PAN- FIG. 83. SECTION OF HUMAN PAN- 

 CREAS, INCLUDING SEVERAL ACINI CREAS SHOWING, a, a, ISLANDS 

 AND Two DUCTS. THE CELLS or LANGERHANS, AND b THE 

 PRESENT A CENTRAL GRANULAR USUAL ACINI. (PiersoT). 

 AND A PERIPHERAL CLEAR ZONE. 

 (Piersol.} 



clear zone is largely increased in depth. The blood-vessels of the 

 pancreas are arranged around the acini in a manner similar to that 

 observed in the salivary glands. The ultimate terminations of the 

 nerves in the epithelium are probably by means of the usual end- tufts. 



The Islands of Langerhans. Throughout the body of the pan- 

 creas and especially in the outer extremity there are found between 

 and among the acini collections of globular cells arranged in the 

 form of rods or columns, separated from the acini and from one 

 another by layers of connective tissue in which ramify large tortuous 

 capillary blood-vessels. These columnar bodies, seen in cross- 

 section in Fig. 83, have been named, after their discoverer, the 

 islands of Langerhans. 



Embryologic investigations have shown that these cells are 



