294 



HISTOLOGY 



embryo, as in the adult (Fig. 292), they consist of coiled anastomosing 

 cords of cells, or irregular masses, which are in close relation with the 

 endothelium of dilated capillary blood vessels. The islands are composed 

 of pale cells with very delicate cell walls, and they contain finer granules 

 than those in the pancreatic cells. In fresh preparations Bensley ob- 

 served that these granules exhibit the Brownian movement, and that 

 colorless spaces occur among them, representing the canals of Holmgren's 

 trophospongium. When preserved by special methods, two forms of 

 island-cells may be distinguished by the staining reactions of their gran- 



Alveoli 



Tubule. 

 FIG. 291. DIAGRAM OF THE PANCREAS. 



FIG. 292. AN ISLAND OF THE PANCREAS WITH THB SUR 

 ROUNDING ALVEOLI, FROM AN ADULT, x 400. 



ules. In one type of cell the nucleus is oval, with finely granular chro- 

 matin; and in the other it is round, with large chromatin granules. Having 

 neither ducts nor lumen, the islands produce an internal secretion, which 

 is received by the blood vessels. There is evidence that this secretion 

 plays an important part in carbohydrate metabolism. If the pancreas 

 is removed, sugar appears in the urine; but if the ducts of the pancreas 

 are tied, the pancreatic alveoli degenerate, leaving the islands functional, 

 and sugar is not found in the urine. Thus the islands are regarded as 

 physiologically distinct from the remainder of the pancreas. 



Morphologically the islands are likewise distinct, and Bensley finds 

 that the possibility of the transformation of alveolar tissue into island 

 tissue, or conversely of island tissue into alveolar tissue, "has not a single 

 well-established fact to support it" (Amer. Journ. Anat., 1911, vol. 12, pp. 

 297-388). The number of islands, however, is subject to great variation, 



