DIGESTION IN THE SMALL INTESTINE. 415 



as already mentioned, by the fact that the gland itself contains but little 

 ready-formed proteolytic ferment, but a substance termed zymogen, 

 ■which, from exposure to the atmosphere, or under the action of dilute 

 acid, is readily converted into ferment. 



Heidenhain has determined that the amount of zymogen in the pan- 

 creas coincides in amount with the extent of the granular zone ; there- 

 fore, in pancreatic secretion, as in the case of saliva, the act of secretion 

 possesses two phases : the first, the preliminary stage of separation from 

 the blood; the second, the stage of manufacturing of those constituents 

 into the specific ferments of the secretion. 



As regards the action of the nervous system on the secretion of 



Fig. 163.— Pancreas op the Dog in the Second Stage of Digestion. {Heidenhain.) 



pancreatic juice but little is known. Both section and stimulation of 

 the central end of the pneumogastrics temporarily arrest the flow of 

 pancreatic juice ; vomiting also has the same effect, the result being 

 probably due to the stimulation of this nerve. Stimulation of the gland 

 itself by an induction current, as well as stimulation of the medulla 

 oblongata, seems to produce an increase in the secretion, but section of 

 the spinal cord does not raise it. When all the nerves going to the 

 pancreas are divided, a continuous flow of pancreatic juice commences, 

 and under these circumstances the fluid formed has but slight digestive 

 power, and its amount is not influenced by the taking in of food. Injec- 

 tions of ether into the stomach produce an increased flow of pancreatic 

 juice, while the secretion is suppressed in the dog, though not in the 



