190 TEXT-BOOK OF PHYSIOLOGY 



motor nerve impulses reach the blood-vessel supplied to the gland, by 

 way of the great splanchnic nerve and the post-ganglionic fibers from the 

 semilunar ganglion. That the vagus nerve contains secretor fibers for the 

 pancreas has been established by Pavlov. This investigator states indeed 

 that the vagus nerve contains two classes of fibers for the pancreas, secreto- 

 motor and secreto-inhibitor, as well as vaso-dilatator fibers for the blood- 

 vessels, and therefore the effects of stimulation are often contradictory and 

 confused, but if the nerve be divided and time given for the degeneration of 

 the secreto-inhibitor and vaso-dilatator nerves, usually a period of four or 

 five days, then stimulation of the peripheral end of the nerve with induced 

 electric currents is followed after a latent period of two to three minutes by 

 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 was thought for 

 a long time that the passage of the acid contents of the stomach through 

 the duodenum acted as a stimulus to this nerve mechanism. But as the 

 secretion and discharge of the juice is excited by the same agents after 

 the division of all related nerves, other explanations were 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 discover}' 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 secret in. 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 intervals 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 



