THE DIGESTIVE ORGANS. 



151 



salivary glands, but its lobules are separated and held in plaee by 

 a rather more considerable amount of loose areolar tissue, in which 

 there are occasional groups of cells of uncertain nature, but cer- 

 tainly distinct from those lining the glandular acini. They are 

 called the " interalveolar cell-islets," and may, perhaps, be of the 

 nature of ductless glands (q. v.). These structures, also known as 

 the islands of Langerhans, have no secretory ducts, and are com- 

 posed of cells of epithelial nature which have the same origin as the 

 ordinary secreting cells of the pancreas. They are not grouped so 

 as to form acini, but are arranged in anastomosing columns, like the 

 cells of the liver, with an abundant capillary network situated in the 

 spaces between these columns of cells. The cells are devoid of 

 zymogen granules, but are of two kinds : 1, smaller cells with faintly 

 staining cytoplasm and a vesicular nucleus in which the chromatin 



Fig. 130. 



< V X ■ 



175 



Island of Langerhans, guinea-pis- (Schulze.) a and b, isolated cells; c, injection of blood- 

 vessels showing the abundant, capillary network within the island. 



is more or less massed into granules ; and, 2, larger cells with a 

 cytoplasm that stains more deeply (Fig. 130). The islands of 

 Langerhans have of late received much attention as possibly being 

 the structures elaborating an internal secretion of importance in the 

 general carbohydrate metabolism of the bodv. 



As the pancreas exercises its secretory function the granules 

 within its cells move toward the lumina of the acini and successively 

 disappear, the attached ends of the cells becoming clearer and the 

 whole cell diminishing somewhat in size during the process. 



The nerves of the stomach and intestinal tract form two gan- 

 glionated plexuses, the plexus of Auerbach, which lies between the 

 two layers of the muscular coat, and the plexus of Meissner, situ- 

 ated in the submucous coat. From these plexuses fibres are dis- 



