206 HUMAN PHYSIOLOGY 



they are mainly composed is absorbed again in nearly 

 equal volume. We probably have little idea of the 

 amount of fluid thus passing in and out. If all the secre- 

 tions could be drained away, and not held for the re- 

 covery of the water, the daily quantity would doubtless 

 be more than a gallon perhaps very much more. So 

 long as absorption fairly balances secretion there is no 

 great draft upon the blood and tissues. Such a with- 

 drawal does occur in diseases like cholera and in any 

 severe catharsis. 



If, on the contrary, absorption were to exceed secre- 

 tion, the result would be the concentration of the in- 

 testinal contents until the residue might be reduced to a 

 crust upon the walls. This does not occur, at least 

 in any such extreme degree. The balance between 

 secretion and absorption seems most of the time to be 

 nicely struck. 



The Pancreatic Juice. This valuable reagent makes 

 its appearance in the duodenum about the time that the 

 first portions of the gastric contents come through the 

 pylorus. The pancreas, like the glands of the stomach, 

 is subject both to Hervous and to chemical influences. 

 But, according to the opinion generally held, the relative 

 importance of the two is contrasted in the two cases: 

 nervous factors are dominant in the stomach while the 

 regulation of the pancreas is more largely chemical. 

 This is rather to be expected, for the activity of the 

 pancreas is not coincident with the pleasure and interest 

 of meals as is the opening work of the gastric glands. 



It is reported that the introduction of acid into the 

 duodenum is followed within a short time by the flow of 

 pancreatic juice. The injection of acid into the blood 

 does not have this effect. Hence, it has been inferred 

 that the acid, striking into the mucous membrane of 

 the duodenum, produces some new agent which passes 

 through the circulation to the pancreas and acts upon its 

 cells after the fashion of a drug. The agent assumed is 

 called secretin. We know that the results are not due to 



