DIGESTION. 



199 



the terminal filaments of the pneumogastric nerve ascend to the 

 medulla, whence impulses pass outward through vaso-motor and 

 secretory nerves to the blood-vessels and secreting cells of the glands. 

 Stimulation of the peripheral end of the divided vagus gives rise to 

 increased secretion. Inasmuch as various agents, such as mineral 

 and organic acids, placed on the duodenal mucous membrane excite 

 the flow, it is quite probable that the passage of the acid contents of 

 the stomach through the duodenum also acts as a powerful stimulus 

 to the secretion and discharge of the juice. According to Bayliss 

 and Starling, the -secretory activity of the pancreas is initiated and 

 maintained by the action of a specific substance to which they have 

 given the term secretin. This substance is developed in the duo- 

 denal glands in consequence of the action of the acids in the chyme 

 and then carried by the blood-stream to the pancreas. The total 

 amount of pancreatic juice secreted in twenty-four hours has been 

 only approximately determined ; the estimates based upon the amount 

 obtained from dogs vary from 175 to 800 grams. 



Histologic Changes in the Cells during Secretory Activity. 

 Reference has already been made to the fact that the cells lining the 

 acini consist of two 

 zones: an outer one, 

 clear and homogene- 

 ous; and an inner 

 one, dark and granu- 

 lar. The position of 

 the nucleus of the cell 

 varies, being at one 

 time in the outer, at 

 another time in the 

 inner, zone. If the 

 pancreas be examined 

 microscopically dur- 

 ing the intervals of 

 digestion, it will be 

 observed that the 

 inner zone is broad, 

 highly granular, oc- 

 cupying nearly the 

 entire cell, while the 

 outer zone is narrow 



and clear. If, however, the gland be examined shortly after a 

 period of active secretion, the reverse conditions will be observed; 

 that is, the inner zone will be narrow, containing relatively few 

 granules, while the outer zone will be clear and wide. This 

 change in the cell has been witnessed in the pancreas of the living 



FIG. 84. ONE SACCULE OF THE PANCREAS OF THE 

 RABBIT IN DIFFERENT STATES OF ACTIVITY. A. 

 After a period of rest, in which case the outlines 

 of the cells are indistinct and the inner zone 

 i. e., the part of the cells (a) next the lumen (c) 

 is broad and filled with fine granules. B. After 

 the gland has poured out its secretion, when the 

 cell outlines (d} are clearer, the granular zone (a) 

 is smaller, and the clear outer zone is wider. 

 (Yeo's "Text-book of Physiology" after Kukne 

 and Lea.) 



