INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 415 



constrictor nerves, for it has the same composition and digestive 

 action as normal succus entericus obtained from a fistula. The 

 secretion begins about four hours after section of the nerves, goes on 

 increasing for about twelve hours, and then rapidly diminishes, so 

 that after about two days the middle loop, as well as the other 

 two, will be found empty. The interpretation usually put upon the 

 experiment is that nerves which normally inhibit the local secre- 

 tory mechanism have been divided. But there is no real proof of 

 the existence of such nerves. 



The same adaptation is seen in the secretion of the succus entericus 

 as in the secretion of the other digestive juices, and the adaptation 

 is naturally most striking in regard to those points in which the 

 intestinal juice is peculiar. While mechanical stimulation of the 

 stomach is ineffective as regards the secretion of gastric juice, 

 mechanical stimulation of the intestine, as by the contact of a 

 cannula, produces a free flow of succus entericus. The reaction is a 

 localized one, the secretion only taking place from the portion of the 

 mucous membrane stimulated. This fact acquires significance when 

 we reflect that the food moves very slowly in the intestine, and a 

 secretion could be of use only at the points where the food happened 

 to be. The juice secreted in response to mechanical stimulation is 

 poor in enterokinase. But if a little pancreatic juice be put into the 

 intestine, and left there for some time, the juice afterwards secreted 

 is rich in enterokinase. 



Summary. Here let us sum up the most important points relat- 

 ing to the secretion of the digestive juices. They are all formed' by 

 the activity of gland-cells originally derived from the epithelial lining 

 of the alimentary canal. The organic constituents or their precursors 

 (including the mother-substances of the ferments) are prepared in the 

 intervals of rest absolute in some glands, relative in others and 

 stored up in the form of granules, which during activity are moved 

 towards the lumen of the gland tubules, and there discharged. 



The nerves of the salivary glands are, as regards their origin, (a) 

 cerebral, (b) sympathetic ; the former group is vaso-dilator, the latter 

 (usually] vaso-constrictor ; both are secretory. Secretion of saliva 

 depends strictly on the nervous system. That nerves influence the 

 gastric and pancreatic secretions is also made out. The psychical secre- 

 tion is of greater importance for the saliva and gastric juice than for 

 the pancreatic juice. The direct action of secretin (produced in the 

 intestinal mucous membrane by the influence of the chyme) is the most 

 characteristic factor in pancreatic secretion. As regards the intestinal 

 glands and the liver, it has not been proved that their secretive activity 

 is under the control of the nervous system, except in so far as the latter 

 may indirectly govern it through the blood-supply, although various 

 circumstances suggest the probability of a more direct action. All the 

 digestive juices show a certain adaptation to the nature of the food, 



