388 PRACTICAL PHYSIOLOGY 



again becomes acid in reaction towards litmus. The mucosa of the 

 large intestine does not secrete any digestive juices, its sole function 

 being one of absorption. In its passage along it the fluid of the 

 intestinal contents becomes gradually absorbed, and the unabsorbed 

 residue forms the faeces. 



It will be seen, therefore, that there are four distinct digestive 

 agencies at work in the intestine, and we will now study the action of 

 each of these separately. 



The Pancreatic Juice. Composition. This can be collected by pro- 

 ducing a fistula of the pancreatic duct. The juice is strongly alkaline 

 in reaction, gives a coagulum of protein on boiling, and contains, besides 

 protein, a considerable amount of organic matter. 



Its percentage composition varies very much with the method 

 adopted for collecting it, that obtained immediately after the establish- 

 ment of the fistula being very much richer in solids than that secreted 

 a few days later. 



Directly after Permanent 

 operation. fistula. 



Water, - - 90 '08 97 '68 



Total solids, 9'92 2-32 



Organic, 9 '04 1'64 



Inorganic, - 0'88 0'68 



In studying its digestive action we may employ, as in the case of 

 gastric digestion, an extract of the gland. This extract may be made 

 with glycerine, after treating the minced gland with weak acid, or 

 allowing it to stand some time, so as to convert the zymogens into the 

 active ferments. Glycerine does not extract all the ferments, however, 

 so that it is more usual to employ the minced gland itself, or a watery 

 extract of it. 



The secretion of pancreatic juice is stimulated by the presence in the 

 blood of a substance called secretin. Secretin is produced in the epi- 

 thelial lining of the small intestine by the action of weak acids on it. 

 Thus, if some of the inner lining of the small intestine be scraped off 

 and ground in a mortar with fine sand and 0*4 % HC1, and the resulting 

 mixture boiled, neutralised and filtered, a solution is obtained which, 

 when injected intravenously into an anaesthetised animal with a pan- 

 creatic fistula, causes an immediate and abundant secretion of pancreatic 

 juice. The secretin does not exist preformed in the intestinal epi- 

 thelium, for a saline extract of this, when similarly injected, has no 

 effect on pancreatic secretion. 



The pancreatic juice thus secreted differs in its digestive properties 

 from an extract of pancreas. The chief difference lies in the fact that 



