498 DIGESTION IN THE INTESTINES [CH. XXXII. 



or lymph-stream ; they must therefore ba resynthesised into proteids 

 during the process of absorption. 



The bile, as we shall find, has little or no digestive action by 

 itself, but combined with pancreatic juice it assists the latter in all 

 its actions. This is true for the digestion of starch and of proteid, 

 but most markedly so for the digestion of fat. Occlusion of the bile- 

 duct by a gall-stone or by inflammation prevents bile entering the 

 duodenum. Under these circumstances the faeces contain a large 

 amount of undigested fat. 



The importance of the work of Pawlow, and the other physi- 

 ologists whose names have been mentioned, arises from the entirely 

 new light thrown upon the digestion process as a whole. We have 

 been too apt to think of the occurrences in the alimentary canal as a 

 series of isolated phenomena. We now see that not only is there a 

 beautiful adjustment in the quantity and composition of the various 

 juices to the kind of work they have to do, but each step follows in 

 an orderly manner as the result of the previous steps. For example, 

 the acid gastric juice reaches the small intestine, and there produces 

 secretin from its forerunner ; the secretin is taken by the blood -stream 

 to the pancreas, where it excites a flow of pancreatic juice; this juice 

 arrives in the duodenum ready to act on starchy substances and on 

 fat. With the assistance of the bile fatty acid is liberated which in 

 its turn forms more secretin, and so more pancreatic juice. The 

 pancreatic juice, however, cannot act on proteids without enterokinase, 

 which is supplied by the succus entericus ; * this sets free the trypsin ; 

 and trypsin effectively carries out digestive proteolysis. 



Bacterial Action. 



The gastric juice is an antiseptic; the pancreatic juice is not. 

 An alkaline fluid like pancreatic juice is just the most suitable medium 

 for bacteria to flourish in. Even in an artificial digestion the fluid 

 is very soon putrid, unless special precautions to exclude or kill 

 bacteria are taken. It is often difficult to say where pancreatic 

 action ends and bacterial action begins, as many of the bacteria that 

 grow in the intestinal contents (having reached that situation in 

 spite of the gastric juice) act in the same way as the pancreatic juice. 

 Some form sugar from starch, others peptone, leucine, and tyrosine 

 from proteids, while others, again, break up fats. There are, how- 

 ever, certain actions that are entirely due to these putrefactive 

 organisms. 



i. On carbohydrates. The most frequent fermentation they set 



* The mixture of pancreatic and intestinal juice is extraordinarily powerful. 

 If secretin is administered to a fasting animal the juice secreted, having no food to 

 act upon, will produce erosion and inflammation of ths intestinal wall. (Starling.) 



