DIGESTION 



347 



of the pancreatic cell grows broader and broader at the expense 

 of the inner granular zone, until at last the granular zone may in 

 its turn be reduced to a narrow contour line around the lumen 

 (Fig. 136) . In the uniformly clouded parotid cell a similar change 

 takes place ; a transparent outer zone arises ; and, after prolonged 

 secretion, only a thin edging of granules may remain at the inner 

 portion of the cell (Fig. 137). In both glands the outlines of the 

 cells become more clearly indicated, and a distinct lumen can now 

 be recognised. The cells are smaller than they are during rest. 



The disappearance of granules from without inwards during 

 activity suggests that these are manufactured products eliminated 

 in the secretion. 



In one respect the changes in the pancreas differ remarkably 

 from those in the salivary glands. The ' islets of Langerhans,' 

 those characteristic groups of small polygonal cells, richly sup- 

 plied with bloodvessels, but not arranged in the form of alveoli 



FIG. 137. ALVEOLI OF PAROTID GLAND : A, AT REST ; B, AFTER A SHORT PERIOD 

 OF ACTIVITY; C, AFTER A PROLONGED PERIOD OF ACTIVITY (FRESH PRE- 

 PARATIONS) (LANGLEY). 



In A and B the nuclei are obscured by the granules of zymogen. 



and unprovided with ducts, which are scattered here and there 

 among the alveoli, are markedly increased in size and, it is said, 

 in number when the pancreas is caused to secrete actively by 

 repeated injections of secretin, and also in starvation. Some 

 observers consider that they are derived from the ordinary 

 secreting cells, which when exhausted undergo rearrangement, 

 and that they can, in turn, give rise to new alveoli by a process 

 of proliferation. Others look upon them as independent struc- 

 tures, with a different function from the pancreatic alveoli 

 (p. 554). The discussion of this question is assuredly not yet 

 closed. 



Changes in the Glands of the Stomach during Secretion. The 

 mucous membrane of the stomach is covered with a single layer of 

 columnar epithelium, largely consisting of mucigenous goblet-cells. 

 It is studded with minute pits, into which open the ducts of the 

 peptic and pyloric glands, the ducts being lined with cells just like 

 those of the general gastric surface. Three varieties of gastric 



