GENERAL SURVEY 243 



lined merely to be eliminated from the body. In this section we shall con- 

 sider in the main only the constituents important in digestion. 



In order to study the chemical properties and the action of the different 

 digestive fluids, one may use either extracts of the appropriate glands or the 

 natural secretion collected from their ducts. In the latter case the duct is 

 shifted from its normal connections and is made to open as a fistula on the 

 outer surface of the body, so that it conveys the secretion to the exterior. 



The first fistula of a digestive gland to be the subject of a thoroughly scien- 

 tific investigation was one resulting from a gunshot wound in the stomach of 

 a Canadian hunter. As a consequence of his accident, the hunter had all the 

 rest of his life a stomach fistula opening at the upper part of the abdomen, 

 through which the interior of the stomach could be observed and gastric juice 

 could be obtained. From observations on this man extending over a number of 

 years (1825-1833) Beaumont collected a large number of important facts con- 

 cerning the digestive process in the stomach, and concerning the movements of 

 that organ. Later Bassow and Blondlot (1842) showed how a stomach fistula 

 may be made on an animal. Since that time such fistulse have been made for 

 therapeutic purposes on man himself, and they have been used to good advan- 

 tage for the study of gastric digestion. 



The pancreatic juice is obtained by means of a cannula fastened in the duct 

 of Wirsung, or from the open duct sutured to the abdominal wall, or finally by 

 isolating that portion of the intestine into which the duct opens and bringing 

 it forward to the abdominal wall. 



In order to study the secretion of bile, fistulse are made in the gall bladder. 

 The ductus choledochus can be tied off and the bile can thus be entirely shut out 

 of the intestine, or the duct can be left open, so that the bile flows as usual to 

 the intestine except when the fistula is open (amphibolous biliary fistula). The 

 intestinal loop containing the mouth of the duct can also be resected and brought 

 forward to the abdominal wall. By the last method especially it is possible to 

 observe how the bile flow is affected by different kinds of food. 



To obtain pure intestinal juice a loop of the intestine is isolated, one end 

 of it is sutured to the skin and the other is closed (Thiry's fistula) ; or both 

 ends may be sutured to the skin (Vella's fistula), in which case the intestine is 

 of course more accessible. 



The most important constituents of the digestive fluids are certain en- 

 zymes which may be classified in three groups: proteid dissolving (proteo- 

 lytic), sugar forming (diastatic or amylolytic] and fat splitting (lipolytic). 

 All of the digestive enzymotic processes agree in this, that the organic food- 

 stuffs acted upon absorb the constituents of water and are split into simpler 

 compounds (hydrolytic cleavage). 



The enzymes are formed in the different glands of the alimentary canal; 

 namely, the proteolytic in the glands of the stomach and in the pancreas; 

 amylolytic in the salivary glands, in the pancreas and in the glands of Lieber- 

 kuhn of the small intestine, lipolytic in the mucous membrane of the stomach 

 and in the pancreas. 



Two enzymes which act on the same foodstuffs are not necessarily identical. 

 For example, pepsin from the gastric glands acts on proteid in an acid medium, 

 while trypsin from the pancreas acts on proteid in neutral and alkaline media. 



