800 PHYSIOLOGY 



continued into the secretin part, where they lie internal to the 



g 



secreting cells, as the so-called centro-acinar cells. The secreting 

 cells themselves present two well-marked zones, a narrow peripheral 

 zone in which the nucleus is embedded, which is strongly basophile, 

 and a central part which is turned towards the lumen, occupying 

 two-thirds or three-quarters of the cell, and is closely packed with 

 highly refractive granules strongly acidophile and presumably con- 

 taining or composed of the precursors of the various constituents of the 

 pancreatic juice (Fig. 339). If the activity of the gland be aroused by 



A B 



FIG. 339. Alveoli of dog's pancreas. (SCHAFER.) 

 A, resting ; B, after moderate secretion with discharge of granules. 



injection of secretin and the injection be continued until the rate of 

 secretion evoked by each injection diminishes considerably, i.e. the 

 gland shows signs of fatigue, marked changes are observed both 

 macroscopically and under the microscope. The gland is now pink 

 and transparent in appearance, moist and soft in consistence. On 

 section the lumen of each alveolus is enlarged, the cells are shrunken, 

 and the granules are found to he only along the border of the cell 

 turned towards the lumen, the rest of the cell, which is much reduced 

 in size, being made up of the basophile protoplasm. So far the changes 

 resemble those already described for the salivary glands. If, however, 

 the injection of secretin be continued until no more secretion is 

 evoked, the histological changes in the cells will be found still more 

 marked. We have long known of the existence in the pancreas 



