190 TEXT-BOOK OF PHYSIOLOGY 



a discharge of the juice. Stimulation of the splanchnic nerve under similar 

 conditions also gives rise to a secretion. 



Inasmuch as various agents, such as mineral and organic acids, placed 

 on the duodenal mucous membrane excite the flow, it is quite possible that 

 the passage of the acid contents of the stomach through the duodenum 

 acts as a powerful stimulus to this nerve mechanism. But as the secretion 

 and discharge of the juice is excited by the same conditions after the division 

 of all related nerves, other explanations have been sought for and found 

 in a secondary stimulus discovered by Bayliss and Starling. 



The secondary stimulus is chemic in character and developed in the 

 glands of the mucous membrane of the duodenum by the action of the acids 

 of the chyme, that is, of the digested foods, coming through the pylorus. 



These investigators made the discovery that if an extract of the gland 

 portion of the duodenal mucous membrane, made with hydrochloric acid 0.4 

 per cent, is injected into the blood it evokes a profuse discharge of pancreatic 

 juice. As hydrochloric acid alone will not produce this effect they assumed 

 that the extract contained an agent that excited or aroused the pancreas to 

 secretor activity and to which therefore they gave the name secretin. This 

 agent resists the temperature that usually destroys enzymes and therefore 

 is not regarded as a member of this class of agents. Since hydrochloric acid 

 appears to be necessary to the development of secretin, the further assumption 

 has been made that it is a derivative of a preexisting compound to which the 

 name prosecretin is given. The secretin thus developed is absorbed into the 

 blood and carried eventually to the pancreas and brought into relation with 

 the cells on which it exerts its stimulating action. To an agent of this class 

 Starling has given the name hormone. 



Histologic Changes in the Cells during Secretor Activity. 

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

 consist of two zones: an outer one, clear and homogeneous; and an inner one, 

 dark and granular. 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 during the interv als of digestion, it will be observed 

 that the inner zone is broad, highly granular, occupying 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 animal rabbit by 

 Kuhne and Lea. They observed that as soon as digestion set in, the granules 

 of the broad inner zone began to pass toward the lumen of the acinus and to 

 disappear gradually as the secretion was poured out, while the outer zone 

 increased in width until almost the entire cell became clear and homogeneous. 

 (See Fig. 81.) After secretion ceased the granules again made their appear- 

 ance, the result, in all probability, of metabolic activity. 



Physiologic Action of Pancreatic Juice. Experimental investi- 

 gations have demonstrated the fact that pancreatic juice is the most complex 

 in its physiologic action of all the digestive fluids. By virtue of its contained 

 enzymes, pancreatic juice acts: 



i. On starch. When normal pancreatic juice or a glycerin extract of 

 the gland is added to a solution of hydrated starch, the latter is speedily 



