DIGESTION 387 



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 

 secretory 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. 



Effect of Certain Drugs on the Digestive Secretions. A small dose 

 of atropine, as has been said, abolishes the secretory action of the 

 chorda tympani. This it does by paralyzing the nerve-endings or 

 ' receptive ' substances in the gland-cells through which the nerve- 

 impulses excite secretion. The gland-cells are not completely 

 paralyzed, for the sympathetic can still cause secretion. The 

 nerve-fibres are not paralyzed, because the direct application of 

 atropine does not affect them ; nor is the seat of the paralysis the 

 ganglion-cells on the course of the fibres, for stimulation between 

 those cells and the gland-cells is ineffective. Pilocarpine is the 

 physiological antagonist of atropine, and restores the secretion which 

 atropine has abolished. In small doses it causes a rapid flow of 

 saliva, its action being certainly a peripheral action, and probably 

 an action on the nerve-endings (or receptive substances), for it 

 persists after all the nerves going to the salivary glands have been 

 divided, and after the ganglion-cells have been paralyzed by nico- 

 tine. Atropine and pilocarpine act similarly on some of the other 

 digestive glands, atropine paralyzing the pancreatic secretion elicited 

 by stimulation of the vagus, although not that obtained by the 

 introduction of acid into the intestine. Pilocarpine seems only to 

 cause a secretion of pancreatic juice when other stimuli are already 

 acting, especially the stimulus determined by the presence of acid 

 in the duodenum. Pilocarpine increases the secretion of gastric, 

 and probably of intestinal juice, but atropine does not stop the 

 secretion caused by division of the intestinal nerves. Physostigmine 

 and muscarine act on the whole like pilocarpine, but physostigmine 

 in small doses gives rise to an abundant flow of pancreatic juice, even 

 when the intestine is empty, probably by stimulating the endings 

 of the secretory fibres in the vagus, 



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