SECRETION OF PANCREATIC JUICE 269 



time a new formation of the substance is going on, as we know from the fact 

 that the percentage of pepsin of the mucosa increases again after the ninth 

 hour of digestion. 



C. WHY DOES THE STOMACH NOT DIGEST ITSELF? 



Several hypotheses have been put forward to account for the fact that the 

 stomach does not digest itself. The mucus of the stomach might act as a kind 

 of varnish to protect the mucosa itself from the action of the gastric juice, or 

 the epithelium of the mucosa mig'ht preserve the underlying parts in some way, 

 or the gastric juice might be neutralized by the alkalinity of the blood, or the 

 mucosa might be absolved from the destructive action of the gastric juice by 

 its absorption. However, many objections can be urged against all of these 

 hypotheses as well as the experimental facts underlying them, and the question 

 was left to a certain extent undecided by simply assuming that the proteolytic 

 enzymes cannot act upon the living cells of the same body. 



A nearer approach to an explanation seems to have been attained in Wein- 

 land's discovery of an antipeptic and antitryptic action of the stomach and intes- 

 tinal mucosa. This action is probably due to antienzymes which are found 

 throughout the whole animal scale, and occur not only in the intestinal tract 

 but also in cells of other organs. As mentioned at page 155, such antienzymes 

 are present also in the blood. 



4. SECRETION OF PANCREATIC JUICE 



A. SECRETORY NERVES 



That the secretion of pancreatic juice is to a certain extent under the 

 control of the central nervous system was rendered very probable by the fact, 

 ascertained by Heidenhain, that it can be started up or accelerated by elec- 

 trical stimulation of the medulla oblongata. Later Pawlow succeeded in com- 

 pleting the evidence of secretory nerves to the pancreas. 



These nerve fibers traverse the vagus. If, with the observance of certain 

 precautions which are necessary to shut out the restraining influence of vari- 

 ous sensory stimuli, the vagus be stimulated, a more or less active secretion 

 of pancreatic juice is plainly demonstrable. The vagus also conveys fibers 

 which inhibit the pancreatic secretion. 



The splanchnic likewise contains secretory fibers for the pancreas, but its 

 action is much less powerful than that of the vagus (Kudrewetsky). 



Secretion of the pancreatic juice in the herbivorous animals is continuous 

 a condition doubtless connected with the continuous character of digestion 

 in these animals. In the carnivora it is intermittent, and (in the dog) the 

 first drops flow from the duct one and one-half to three and one-half minutes 

 after eating. In man also a rise occurs after eating and reaches its maximum 

 in the fourth hour, from which time on it gradually falls (Glaessner; Fig. 

 106). 



To what extent the secretion of pancreatic juice is caused like that of the 



gastric juice by stimuli from the mouth, or what role the psychic factor plays 



in the process must remain for the present undecided. However, still another 



influence is of far-reaching importance here. If an acid (no matter what 



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