THE PANCREATIC JUICE 801 



of islets of cells known as < Langerhans' islets/ composed of epithelial 

 cens, of which the individual members take any stain with gre 

 difficulty, so that the islets appear as unstained areas in the middle 

 the darkly stained secreting substance. These falete have been 

 looked upon as structures altogether apart from the secreting tissue 

 of the pancreas, and have been endowed with functions, e.g con- 

 nected with the metabolism of carbohydrates, quit,* independent 

 of the role of the pancreas in digestion. Dale has found, however, 

 that these islets are more numerous and larger in a gland which Ims 



FIG. 340. Formation of islet of Langerhans from secretory alveoli. Part of the 

 pancreas of a toad in which active secretion had been excited by injection of 

 secretin. The islet of Langerhans with its unstained hyaline cells presents 

 a marked contrast to the secretory alveoli with their basophile protoplasm 

 and deeply stained zymogen granules. The islet, however, is increasing in size 

 at the expense of the secreting tissue, which in many places is losing all its 

 chromophile elements. In the middle of the islet is some of the secretory 

 tissue where the change is not yet complete. (Drawn from a micro-photograph 

 of a specimen by H. H. DALE.) 



been actively secreting. In the completely exhausted gland large 

 tracts of the secreting tissue are found to have lost not only their 

 acidophile granules but also the whole of the basophile substance 

 from their protoplasm, so that they are indistinguishable from the 

 cells making up the cells of the islets of Langerhans. Moreover in such 

 specimens the islets are observed to be actively increasing in circum- 

 ference (Fig. 340), taking in one alveolus after another of the secreting 

 tissue, and the growth of the islet may be noted often by the fact 

 that it includes portions of the alveoli in which the stage of exhaustion 

 has not proceeded to such a great extent, so that the cells still contain 



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